Method and apparatus for investigating earth formations by emitting survey and auxiliary currents from the same electrode

Claim 2

Apparatus for measuring a characteristic of earth formations traversed by a borehole, comprising: a plurality of electrodes; carrying means for mounting said electrodes for passage through a borehole; means for supplying both survey and auxiliary currents to one and only one of said electrodes for emission into the media surrounding the carrying means, said survey current returning to a first return electrode for return to the current supplying means and said auxiliary current returning to at least two auxiliary current return electrodes located relatively near said current emission electrode for return to the current supplying means, one each of said auxiliary current return electrodes being located on each side of said current emission electrode; means for measuring the potential at a plurality of points along said mounting means; and means operative in response to the measured potentials for adjusting one of the survey or auxiliary currents to set up a given potential distribution in at least a portion of a borehole whereby the auxiliary current will tend to force the survey current into a formation to thereby obtain a measure of a formation charcteristic.

Claim 3

Apparatus for measuring a characteristic of earth formations traversed by a borehole, comprising: a plurality of electrodes; carrying means for mounting said electrodes for passage through a borehole; means for supplying both survey and auxiliary currents to one of said electrodes for emission into the media surrounding the carrying means where the survey current can return to a first return electrode for return to the current supplying means and where the auxiliary current can return to at least two auxiliary current return electrodes located relatively near said current emission electrode for return to the current supplying means, one each of said auxiliary current return electrodes being located on each side of said current emission electrode; means for measuring the potential at a plurality of points along said mounting means; means operative in response to the measured potentials for adjusting one of the survey or auxiliary currents to set up a given potential distribution in at least a portion of a borehole; and means for measuring the survey current being emitted from said current emission electrode to obtain a measure of a formation characteristic.

Claim 4

Apparatus for measuring a characteristic of earth formations traversed by a borehole, comprising: a plurality of electrodes; carrying means for mounting said electrodes for passage through a borehole; means for supplying both survey and auxiliary currents to one of said electrodes for emission into the media surrounding the carrying means where the survey current can return to a first return electrode for return to the current supplying means and where the auxiliary current can return to at least two auxiliary current return electrodes located relatively near said current emission electrode for return to the current supplying means, one each of said auxiliary current return electrodes being located on each side of said current emission electrode; means for measuring the potential at a plurality of points along said mounting means; means operative in response to the measured potentials for adjusting one of the survey or auxiliary currents to set up a given potential distribution in at least a portion of a borehole; and means for measuring the survey current being emitted from said current emission electrode to obtain a measure of a formation characteristic.

Claim 5

Apparatus for measuring a characteristic of earth formations traversed by a borehole, comprising: a plurality of electrodes; means for mounting electrodes for passage through a borehole, said electrodes including one current-emitting electrode for emitting survey and auxiliary currents into the media surrounding the mounting means, and a measuring electrode pair spaced apart from said one current-emitting electrode, a pair of return electrodes, one each located on either side of said one current-emitting electrode and one located between said one current-emitting electrode and said measuring electrode pair, a third return electrode located at a greater distance from said one current-emitting electrode than either electrode of the return electrode pair; means for supplying all of said survey and auxiliary currents to said one current-emitting electrode, all of the auxiliary current returning to the return electrode pair for return to the current supplying means and the survey current returning to the third return electrode for return to the current supplying means; and means responsive to the potentials on said measuring electrodes for adjusting one of the auxiliary of survey currents to maintain a desired potential relationship between the potentials of said measuring electrode pair whereby said survey current will tend to be forced into a formation to thereby obtain a measure of a formation characteristic.

Claim 6

Apparatus for measuring a characteristic of earth formations traversed by a borehole, comprising: a plurality of electrodes; means for mounting electrodes for passage through a borehole, said electrodes including one current-emitting electrode for emitting survey and auxiliary currents into the media surrounding the mounting means, and a measuring electrode pair spaced apart from said one current-emitting electrode, a pair of return electrodes, one each located on either side of said one current-emitting electrode and one located between said one current-emitting electrode and said measuring electrode pair, a third return electrode located at a greater distance from said one current-emitting electrode than either electrode of the return electrode pair; means for supplying all of said survey and auxiliary currents to said one current-emitting electrode, all of the auxiliary current returning to the return electrode pair for return to the current supplying means and the survey current returning to the third return electrode for return to the current supplying means; and means responsive to the potentials on said measuring electrodes for adjusting one of the auxiliary of survey currents to maintain a desired potential relationship between the potentials of said measuring electrode pair whereby said survey current will tend to be forced into a formation to thereby obtain a measure of a formation characteristic.

Claim 7

Apparatus for measuring a characteristic of earth formations traversed by a borehole, comprising: a plurality of electrodes; means for mounting said electrodes for passage through a borehole; means for supplying survey and auxiliary currents to at least one of said electrodes for emission into the media surrounding the carrying means, said survey current returning to a first electrode for return to the current supplying means and said auxiliary current returning to a second electrode for return to the current supplying means; and means responsive to the potential gradient at two locations in a borehole for adjusting one of said survey or auxiliary currents to maintain a fixed relationship between said two potential gradients whereby said survey current will tend to be forced into a formation to thereby obtain a measure of a formation characteristic.

Claim 8

Apparatus for measuring a characteristic of earth formations traversed by a borehole, comprising: a plurality of electrodes; means for mounting said electrodes for passage through a borehole; means for supplying survey and auxiliary currents to at least one of said electrodes for emission into the media surrounding the carrying means, said survey current returning to a first electrode for return to the current supplying means and said auxiliary current returning to a second electrode for return to the current supplying means; and means responsive to the potential gradient at two locations in a borehole for adjusting one of said survey or auxiliary currents to maintain a fixed relationship between said two potential gradients whereby said survey current will tend to be forced into a formation to thereby obtain a measure of a formation characteristic.

Claim 9

Apparatus for measuring a characteristic of earth formations traversed by a borehole, comprising: a plurality of electrodes; means for mounting said electrodes for passage through the borehole; means for supplying survey and auxiliary currents to at least one of said electrodes for emission into the media surrounding the mounting means, said survey current returning to a first electrode for return to the current supply means and said auxiliary current returning to a second electrode for return to the current supply means; and means responsive to the potential gradient at two locations differently spaced from said current emission electrode in the borehole for adjusting one of said survey or auxiliary currents to maintain a fixed relationship between said two potential gradients whereby said survey current will tend to be forced into the formation to thereby obtain a measure of a formation characteristic.

Claim 10

Apparatus for measuring a characteristic of earth formations traversed by a borehole, comprising: a plurality of electrodes; means for mounting said electrodes for passage through the borehole; means for supplying survey and auxiliary currents to at least one of said electrodes for emission into the media surrounding the mounting means, said survey current returning to a first electrode for return to the current supply means and said auxiliary current returning to a second electrode for return to the current supply means; and means responsive to the potential gradient at two locations differently spaced from said current emission electrode in the borehole for adjusting one of said survey or auxiliary currents to maintain a fixed relationship between said two potential gradients whereby said survey current will tend to be forced into the formation to thereby obtain a measure of a formation characteristic.

Claim 11

Apparatus for measuring a characteristic of earth formations traversed by a borehole, comprising: a plurality of electrodes; means for mounting said electrodes for passage through a borehole; means for supplying survey and auxiliary currents to at least one of said electrodes for emission into the media surrounding the mounting means, said survey current returning to a relatively remote electrode for return to the current supplying means and said auxiliary current returning to at least one electrode which is relatively near said at least one current-emitting electrode for return to the current supplying means; means for measuring the potential gradient at two points in a borehole and generating a control signal representative of the relationship of the two gradients to one another; and means responsive to the control signal for adjusting one of said survey and auxiliary currents to maintain said control signal at a substantially constant level whereby said survey current will tend to be forced into a formation to thereby obtain a measure of a formation characteristic.

Claim 12

Apparatus for measuring a characteristic of earth formations traversed by a borehole, comprising: a plurality of electrodes; means for mounting said electrodes for passage through a borehole; means for supplying survey and auxiliary currents to at least one of said electrodes for emission into the media surrounding the mounting means, said survey current returning to a relatively remote electrode for return to the current supplying means and said auxiliary current returning to at least one electrode which is relatively near said at least one current-emitting electrode for return to the current supplying means; means for measuring the potential gradient at two points in a borehole and generating a control signal representative of the relationship of the two gradients to one another; and means responsive to the control signal for adjusting one of said survey and auxiliary currents to maintain said control signal at a substantially constant level whereby said survey current will tend to be forced into a formation to thereby obtain a measure of a formation characteristic.

Claim 13

Apparatus for measuring a characteristic of earth formations traversed by a borehole comprising: a plurality of electrodes; means for mounting said electrodes for passage through the borehole; means for supplying survey and auxiliary currents to at least one of said electrodes for emission into the media surrounding the mounting means, said survey current returning to a relatively remote electrode for return to the current supplying means and said auxiliary current returning to at least one electrode which is relatively near said at least one current-emitting electrode for return to the current supplying means; means for measuring the potential gradient at two points differently spaced from said current emitting electrode in the borehole and generating a control signal representative of the relationship of the two gradients to one another; and means responsive to the control signal for adjusting one of said survey and auxiliary currents to maintain said control signal at a substanially constant level whereby said survey current will tend to be forced into a formation to thereby obtain a measure of a formation characteristic.

Claim 14

Apparatus for measuring a characteristic of earth formations traversed by a borehole comprising: a plurality of electrodes; means for mounting said electrodes for passage through the borehole; means for supplying survey and auxiliary currents to at least one of said electrodes for emission into the media surrounding the mounting means, said survey current returning to a relatively remote electrode for return to the current supplying means and said auxiliary current returning to at least one electrode which is relatively near said at least one current-emitting electrode for return to the current supplying means; means for measuring the potential gradient at two points differently spaced from said current emitting electrode in the borehole and generating a control signal representative of the relationship of the two gradients to one another; and means responsive to the control signal for adjusting one of said survey and auxiliary currents to maintain said control signal at a substanially constant level whereby said survey current will tend to be forced into a formation to thereby obtain a measure of a formation characteristic.

Claim 15

Apparatus for measuring a characteristic of earth formations traversed by a borehole, comprising: a mounting means; a plurality of electrodes mounted on said mounting means for passage through a borehole, Said electrodes including at least one current-emitting electrode for emitting survey and auxiliary currents into the media surrounding the mounting means, a first return electrode located relatively near said at least one current-emitting electrode, a second return electrode located at a greater distance from said at least one current-emitting electrode than the first return electrode, and a plurality of measuring electrodes located at various distances from said at least one current-emitting electrode; means for supplying survey and auxiliary currents to said at least one current-emitting electrode, the auxiliary current returning to the first electrode for return to the current supplying means and the survey current returning to the second electrode for return to the current supplying means; and means responsive to the potential on a plurality of the measuring electrodes for adjusting one of the auxiliary or survey currents to set up a potential distribution which decreases substantially as a function of 1/d where d is the distance from said at least one current-emitting electrode, whereby the survey current will tend to be forced into a formation to thereby obtain a measure of a formation characteristic.

Claim 16

Apparatus for measuring a characteristic of earth formations traversed by a borehole, comprising: a mounting means; a plurality of electrodes mounted on said mounting means for passage through a borehole, Said electrodes including at least one current-emitting electrode for emitting survey and auxiliary currents into the media surrounding the mounting means, a first return electrode located relatively near said at least one current-emitting electrode, a second return electrode located at a greater distance from said at least one current-emitting electrode than the first return electrode, and a plurality of measuring electrodes located at various distances from said at least one current-emitting electrode; means for supplying survey and auxiliary currents to said at least one current-emitting electrode, the auxiliary current returning to the first electrode for return to the current supplying means and the survey current returning to the second electrode for return to the current supplying means; and means responsive to the potential on a plurality of the measuring electrodes for adjusting one of the auxiliary or survey currents to set up a potential distribution which decreases substantially as a function of 1/d where d is the distance from said at least one current-emitting electrode, whereby the survey current will tend to be forced into a formation to thereby obtain a measure of a formation characteristic.

Claim 17

Apparatus for measuring a characteristic of earth formations traversed by a borehole, comprising: a mounting means; a plurality of electrodes mounted on said mounting means for passage through a borehole, said electrodes including at least one current-emitting electrode for emitting survey and auxiliary currents into the media surrounding the mounting means, a first return electrode located relatively near said at least one current-emitting electrode, a second return electrode located at a greater distance from said at least one current-emitting electrode than the first return electrode, and two measuring electrodes located at various distances from said at least one current-emitting electrode; means for supplying survey and auxiliary currents to said at least one current-emitting electrode, the auxiliary current returning to the first electrode for return to the current supplying means and the survey current returning to the second electrode for return to the current supplying means; and means responsive to the potential on said two measuring electrodes for adjusting one of the auxiliary or survey currents to maintain the potential of one of the measuring electrodes substantially equal to d1/d2 of the potential on the other measuring electrode where d1 and d2 are the distance of said two measuring electrodes from said at least one current-emitting electrode, whereby the survey current will tend to be forced into a formation to thereby obtain a measure of a formation characteristic.

Claim 18

Apparatus for measuring a characteristic of earth formations traversed by a borehole, comprising: a mounting means; a plurality of electrodes mounted on said mounting means for passage through a borehole, said electrodes including at least one current-emitting electrode for emitting survey and auxiliary currents into the media surrounding the mounting means, a first return electrode located relatively near said at least one current-emitting electrode, a second return electrode located at a greater distance from said at least one current-emitting electrode than the first return electrode, and two measuring electrodes located at various distances from said at least one current-emitting electrode; means for supplying survey and auxiliary currents to said at least one current-emitting electrode, the auxiliary current returning to the first electrode for return to the current supplying means and the survey current returning to the second electrode for return to the current supplying means; and means responsive to the potential on said two measuring electrodes for adjusting one of the auxiliary or survey currents to maintain the potential of one of the measuring electrodes substantially equal to d1/d2 of the potential on the other measuring electrode where d1 and d2 are the distance of said two measuring electrodes from said at least one current-emitting electrode, whereby the survey current will tend to be forced into a formation to thereby obtain a measure of a formation characteristic.

Claim 19

Apparatus for measuring a characteristic of earth formations traversed by a borehole, comprising: a plurality of electrodes; means for mounting said electrodes for passage through a borehole; means for supplying survey and auxiliary currents to at least one of said electrodes for emission into the media surrounding the mounting means, said survey current returning to a first return electrode for return to the current supplying means and said auxiliary current returning to at least one electrode which is relatively near said at least one current emission electrode for return to the current supplying means; and means responsive to the potential gradient at a given location in the borehole for adjusting one of the survey or auxiliary currents to set up a potential distribution which decreases substantially as a function of 1/d where d is the distance from said at least one current emission electrode, whereby the survey current will tend to be forced into a formation to thereby obtain a measure of a formation characteristic.

Claim 20

Apparatus for measuring a characteristic of earth formations traversed by a borehole, comprising: a plurality of electrodes; means for mounting said electrodes for passage through a borehole; means for supplying survey and auxiliary currents to at least one of said electrodes for emission into the media surrounding the mounting means, said survey current returning to a first return electrode for return to the current supplying means and said auxiliary current returning to at least one electrode which is relatively near said at least one current emission electrode for return to the current supplying means; and means responsive to the potential gradient at a given location in the borehole for adjusting one of the survey or auxiliary currents to set up a potential distribution which decreases substantially as a function of 1/d where d is the distance from said at least one current emission electrode, whereby the survey current will tend to be forced into a formation to thereby obtain a measure of a formation characteristic.

Claim 21

Apparatus for measuring a characteristic of earth formations traversed by a borehole, comprising: a plurality of electrodes including at least one current-emitting electrode, a first return electrode located relatively near said at least one current-emitting electrode, a second return electrode located at a greater distance from said at least one current-emitting electrode than the first return electrode, and a plurality of measuring electrodes located at various distances from said at least one current-emitting electrode; means for mounting said electrodes for passage through a borehole; means for supplying a first current to said at least one current-emitting electrode, said first current returning to one of the return electrodes for return to the current supplying means; means responsive to the potentials on a plurality of measuring electrodes for supplying a second current between said at least one current-emitting electrode and the other of said return electrodes to set up a potential distribution which decreases substantially as a function of 1/d where d is the distance from said at least one current-emitting electrode; and means responsive to the potentials on a plurality of said electrodes for obtaining a measure of a formation characteristic.

Claim 22

Apparatus for measuring a characteristic of earth formations traversed by a borehole, comprising: a plurality of electrodes including at least one current-emitting electrode, a first return electrode located relatively near said at least one current-emitting electrode, a second return electrode located at a greater distance from said at least one current-emitting electrode than the first return electrode, and a plurality of measuring electrodes located at various distances from said at least one current-emitting electrode; means for mounting said electrodes for passage through a borehole; means for supplying a first current to said at least one current-emitting electrode, said first current returning to one of the return electrodes for return to the current supplying means; means responsive to the potentials on a plurality of measuring electrodes for supplying a second current between said at least one current-emitting electrode and the other of said return electrodes to set up a potential distribution which decreases substantially as a function of 1/d where d is the distance from said at least one current-emitting electrode; and means responsive to the potentials on a plurality of said electrodes for obtaining a measure of a formation characteristic.

Claim 23

Apparatus for measuring a characteristic of earth formations traversed by a borehole, comprising: a plurality of electrodes; means, including a borehole wall-engaging pad member, for mounting said electrodes for passage through a borehole, said electrodes including at least one pad mounted current-emitting electrode, a first pad mounted return electrode concentrically surrounding said at least one current-emitting electrode, a pair of pad mounted measuring electrodes concentrically surrounding said first return electrode, a second return electrode located at a greater distance from said at least one current-emitting electrode than said first return electrode; means for supplying survey and auxiliary currents to said at least one current-emitting electrode, the auxiliary current returning to the first electrode for return to the current supplying means and the survey current returning to the second electrode for return to the current supplying means; and means responsive to the potentials of the measuring electrodes for adjusting one of the survey or auxiliary currents to maintain a given potential difference between said two measuring electrodes whereby said survey current will tend to be forced into a formation to thereby obtain a measure of a formation characteristic.

Claim 24

Apparatus for measuring a characteristic of earth formations traversed by a borehole, comprising: a plurality of electrodes; means, including a borehole wall-engaging pad member, for mounting said electrodes for passage through a borehole, said electrodes including at least one pad mounted current-emitting electrode, a first pad mounted return electrode concentrically surrounding said at least one current-emitting electrode, a pair of pad mounted measuring electrodes concentrically surrounding said first return electrode, a second return electrode located at a greater distance from said at least one current-emitting electrode than said first return electrode; means for supplying survey and auxiliary currents to said at least one current-emitting electrode, the auxiliary current returning to the first electrode for return to the current supplying means and the survey current returning to the second electrode for return to the current supplying means; and means responsive to the potentials of the measuring electrodes for adjusting one of the survey or auxiliary currents to maintain a given potential difference between said two measuring electrodes whereby said survey current will tend to be forced into a formation to thereby obtain a measure of a formation characteristic.

Claim 25

Apparatus for measuring a characteristic of earth formations traversed by a borehole, comprising: a plurality of electrodes; means, including a borehole wall-engaging pad member, for mounting said electrodes for passage through a borehole, said electrodes including at least one central pad mounted current-emitting electrode, a first pad mounted return electrode concentrically surrounding said at least one current-emitting electrode, a pair of pad mounted measuring electrodes concentrically surrounding said first return electrode, a second return electrode located at a greater distance from said at least one current-emitting electrode than said first return electrode; means for supplying survey and auxiliary currents to said central current-emitting electrode, the auxiliary current returning to the first electrode for return to the current supplying means and the survey current returning to the second electrode for return to the current supplying means; means responsive to the potentials of the measuring electrodes for adjusting one of the survey or auxiliary currents to maintain a given potential difference between said two measuring electrodes; and means for measuring said survey current to obtain a measure of a formation characteristic.

Claim 26

Apparatus for measuring a characteristic of earth formations traversed by a borehole, comprising: a plurality of electrodes; means, including a borehole wall-engaging pad member, for mounting said electrodes for passage through a borehole, said electrodes including at least one central pad mounted current-emitting electrode, a first pad mounted return electrode concentrically surrounding said at least one current-emitting electrode, a pair of pad mounted measuring electrodes concentrically surrounding said first return electrode, a second return electrode located at a greater distance from said at least one current-emitting electrode than said first return electrode; means for supplying survey and auxiliary currents to said central current-emitting electrode, the auxiliary current returning to the first electrode for return to the current supplying means and the survey current returning to the second electrode for return to the current supplying means; means responsive to the potentials of the measuring electrodes for adjusting one of the survey or auxiliary currents to maintain a given potential difference between said two measuring electrodes; and means for measuring said survey current to obtain a measure of a formation characteristic.

Claim 27

Apparatus for measuring a characteristic of earth formations traversed by a borehole, comprising: a plurality of potential and current electrodes including electrodes for emitting survey and auxilary currents in a localized region of a borehole, at least one current return electrode spaced apart from said region, and electrodes for measuring potentials at two points differently spaced on either side of said region; support means for movement of said electrodes in spaced relation through a borehole; means for passing all of said survey current and at least part of said auxiliary current between a current electrode in said region and at least one electrode outside of said region; means responsive to the potential difference measured between said two points for varying one of the auxiliary current or survey current to maintain said potential difference substantially at a selected value; and means responsive to the electric field proximate to said region for providing a measure of a formation characteristic.

Claim 28

Apparatus for measuring a characteristic of earth formations traversed by a borehole, comprising: a plurality of potential and current electrodes including electrodes for emitting survey and auxilary currents in a localized region of a borehole, at least one current return electrode spaced apart from said region, and electrodes for measuring potentials at two points differently spaced on either side of said region; support means for movement of said electrodes in spaced relation through a borehole; means for passing all of said survey current and at least part of said auxiliary current between a current electrode in said region and at least one electrode outside of said region; means responsive to the potential difference measured between said two points for varying one of the auxiliary current or survey current to maintain said potential difference substantially at a selected value; and means responsive to the electric field proximate to said region for providing a measure of a formation characteristic.

Claim 29

In a system for investigating earth formations traversed by a borehole, apparatus for establishing a given potential distribution comprising: a plurality of electrodes; means for mounting said electrodes for passage through a borehole; means for supplying first and second currents to at least one of said electrodes for emission into the media surrounding the mounting means, said first current returning to a first return electrode for return to the current supplying means and said second current returning to at least one electrode which is relatively near said current emission electrodes for return to the current supplying means; and means responsive to the potentials at a plurality of points in a borehole for adjusting one of said first or second currents to establish a potential distribution which descreases as a function of 1/d where d is the distance from said at least one current-emitting electrode.

Claim 30

In a system for investigating earth formations traversed by a borehole, apparatus for establishing a given potential distribution comprising: a plurality of electrodes; means for mounting said electrodes for passage through a borehole; means for supplying first and second currents to at least one of said electrodes for emission into the media surrounding the mounting means, said first current returning to a first return electrode for return to the current supplying means and said second current returning to at least one electrode which is relatively near said current emission electrodes for return to the current supplying means; and means responsive to the potentials at a plurality of points in a borehole for adjusting one of said first or second currents to establish a potential distribution which descreases as a function of 1/d where d is the distance from said at least one current-emitting electrode.

Claim 31

In a system for investigating earth formations traversed by a borehole, apparatus for forcing current into a formation comprising: a plurality of electrodes; means for mounting said electrodes for passage through a borehole; means for supplying first and second currents to at least one of said electrodes for emission into the media surrounding the mounting means, said first current returning to a first electrode for return to the current supplying means and said second current returning to a second electrode for return to the current supplying means; and means responsive to the potential gradient at two locations in a borehole for adjusting one of said first or second currents to maintain a fixed relationship between said two potential gradients whereby said first current will tend to be forced into a formation.

Claim 32

In a system for investigating earth formations traversed by a borehole, apparatus for forcing current into a formation comprising: a plurality of electrodes; means for mounting said electrodes for passage through a borehole; means for supplying first and second currents to at least one of said electrodes for emission into the media surrounding the mounting means, said first current returning to a first electrode for return to the current supplying means and said second current returning to a second electrode for return to the current supplying means; and means responsive to the potential gradient at two locations in a borehole for adjusting one of said first or second currents to maintain a fixed relationship between said two potential gradients whereby said first current will tend to be forced into a formation.

Claim 33

In a system for investigating earth formations traversed by a borehole, apparatus for forcing current into a formation, comprising: a plurality of electrodes; means for mounting said electrodes for passage through the borehole; means for supplying first and second currents to at least one of said electrodes for emission into the media surrounding the mounting means, said first current returning to a first electrode for return to the current supply means, and said second current returning to a second electrode for return to the current supply means; and means responsive to the potential gradient at two locations differently spaced from said current supply electrode in the borehole for adjusting one of said first or second currents to maintain a fixed relationship between said two potential gradients whereby said first current will tend to be forced into a formation.

Claim 34

In a system for investigating earth formations traversed by a borehole, apparatus for forcing current into a formation, comprising: a plurality of electrodes; means for mounting said electrodes for passage through the borehole; means for supplying first and second currents to at least one of said electrodes for emission into the media surrounding the mounting means, said first current returning to a first electrode for return to the current supply means, and said second current returning to a second electrode for return to the current supply means; and means responsive to the potential gradient at two locations differently spaced from said current supply electrode in the borehole for adjusting one of said first or second currents to maintain a fixed relationship between said two potential gradients whereby said first current will tend to be forced into a formation.

Claim 51

In a system for investigating earth formations traversed by a borehole, apparatus for forcing cuRrent into a formation comprising: a plurality of electrodes; means for mounting said electrodes for passage through a borehole, said electrodes incuding a current-emitting electrode for emitting first and second currents into a media surrounding the mounting means, a measuring electrode pair spaced apart from said at least one current-emitting electrode, a pair of return electrodes having one each located on either side of said at least one current-emitting electrode and one of said return electrodes located between said at least one current-emitting electrode and said measuring electrode pair, a third return electrode located at a greater distance from said at least one current-emitting electrode than either electrode of the return electrode pair; means for supplying all of said first and second currents to said current-emitting electrode, all of the second current returning to the return electrode pair for return to the current supplying means and the first current returning to the third return electrode for return to the current supplying means; and means responsive to the potential of said measuring electrodes for adjusting one of the first or second currents to maintain a desired potential relationship between the potentials of said measuring electrode pair whereby said first current will tend to be forced into a formation.

Claim 52

In a system for investigating earth formations traversed by a borehole, apparatus for forcing cuRrent into a formation comprising: a plurality of electrodes; means for mounting said electrodes for passage through a borehole, said electrodes incuding a current-emitting electrode for emitting first and second currents into a media surrounding the mounting means, a measuring electrode pair spaced apart from said at least one current-emitting electrode, a pair of return electrodes having one each located on either side of said at least one current-emitting electrode and one of said return electrodes located between said at least one current-emitting electrode and said measuring electrode pair, a third return electrode located at a greater distance from said at least one current-emitting electrode than either electrode of the return electrode pair; means for supplying all of said first and second currents to said current-emitting electrode, all of the second current returning to the return electrode pair for return to the current supplying means and the first current returning to the third return electrode for return to the current supplying means; and means responsive to the potential of said measuring electrodes for adjusting one of the first or second currents to maintain a desired potential relationship between the potentials of said measuring electrode pair whereby said first current will tend to be forced into a formation.

Claim 53

Apparatus for measuring a characteristic of earth formations traversed by a borehole, comprising: a plurality of electrodes; means for mounting said electrodes for passage through a borehole, said electrodes including at least one current-emitting electrode for emitting survey and auxiliary currents into the media surrounding the mounting means, two pairs of measuring electrodes located one each on each side of said at least one current-emitting electrode, a current return electrode pair having one electrode each located between said at least one current-emitting electrode and each of said measuring electrode pairs, and a third current return electrode spaced apart from said other electrodes; means for supplying all of the survey and auxiliary currents to said at least one current-emitting electrode for emission into the media surrounding the mounting means, said survey current returning to said third current return electrode for return to the current supplying means and said auxiliary current returning to said current return electrode pair for return to the current supplying means; and means responsive to the potentials on said measuring electrodes for adjusting one of said survey or auxiliary currents to maintain a given potential distribution in the vicinity of said measuring electrodes whereby said survey current will tend to be forced into a formation to thereby obtain a measure of a formation characteristic.

Claim 54

Apparatus for measuring a characteristic of earth formations traversed by a borehole, comprising: a plurality of electrodes; means for mounting said electrodes for passage through a borehole, said electrodes including at least one current-emitting electrode for emitting survey and auxiliary currents into the media surrounding the mounting means, two pairs of measuring electrodes located one each on each side of said at least one current-emitting electrode, a current return electrode pair having one electrode each located between said at least one current-emitting electrode and each of said measuring electrode pairs, and a third current return electrode spaced apart from said other electrodes; means for supplying all of the survey and auxiliary currents to said at least one current-emitting electrode for emission into the media surrounding the mounting means, said survey current returning to said third current return electrode for return to the current supplying means and said auxiliary current returning to said current return electrode pair for return to the current supplying means; and means responsive to the potentials on said measuring electrodes for adjusting one of said survey or auxiliary currents to maintain a given potential distribution in the vicinity of said measuring electrodes whereby said survey current will tend to be forced into a formation to thereby obtain a measure of a formation characteristic.

Claim 55

In a system for investigating earth formation traversed by a borehole, apparatus for forcing current into a formation, comprising: a plurality of electrodes; means for mounting sai electrodes for passage through a borehole; means for supplying first and second currents to at least one of said electrodes for emission into the media surrounding the mounting means, said first current returning to a first return electrode for return to the current supplying means and said second current returning to at least one electrode which is relatively near said at least one current emission electrode for return to the current supplying means; means for measuring the difference between the potentials at two points in a borehole and generating a first output signal representative of said potential difference; means for measuring the difference between the potential existing at a point in the vicinity of said two points and a remote point and generating a second output signal representative thereof; and means responsive to the ratio of said first signal to said second signal for adjusting one of said first or second currents to maintain a given potential gradient in the vicinity oF said two borehole points whereby said first current will tend to be forced into a formation.

Claim 56

In a system for investigating earth formation traversed by a borehole, apparatus for forcing current into a formation, comprising: a plurality of electrodes; means for mounting sai electrodes for passage through a borehole; means for supplying first and second currents to at least one of said electrodes for emission into the media surrounding the mounting means, said first current returning to a first return electrode for return to the current supplying means and said second current returning to at least one electrode which is relatively near said at least one current emission electrode for return to the current supplying means; means for measuring the difference between the potentials at two points in a borehole and generating a first output signal representative of said potential difference; means for measuring the difference between the potential existing at a point in the vicinity of said two points and a remote point and generating a second output signal representative thereof; and means responsive to the ratio of said first signal to said second signal for adjusting one of said first or second currents to maintain a given potential gradient in the vicinity oF said two borehole points whereby said first current will tend to be forced into a formation.

Claim 57

In a system for investigating earth formations traversed by a borehole, apparatus for forcing current into a formation comprising: a plurality of electrodes including at least one current-emitting electrode, a first pair of measuring electrodes, a first return electrode located between said at least one current-emitting electrode and said first pair of measuring electrodes, a second pair of measuring electrodes, a second current-emitting electrode located between said first and second pairs of measuring electrodes, and a second current return electrode located between said second current-emitting electrode and said second pair of measuring electrodes; means for mounting said electrodes for passage through a borehole; means for supplying a first current to said at least one current-emitting electrode for emission into the media surrounding said mounting means; means responsive to the potentials on said first pair of measuring electrodes for supplying a first auxiliary current between said at least one current-emitting electrode and said first return electrode of sufficient magnitude to maintain a given potential relationship between the electrodes of said first measuring electrode pairs; and means responsive to the potentials on said second pair of measuring electrodes for supplying a second auxiliary current between said second current-emitting electrode and said second current return electrode of sufficient magnitude to maintain a given relationship between the potentials of said second measuring electrode pair whereby said first current will be forced into a formation.

Claim 58

In a system for investigating earth formations traversed by a borehole, apparatus for forcing current into a formation comprising: a plurality of electrodes including at least one current-emitting electrode, a first pair of measuring electrodes, a first return electrode located between said at least one current-emitting electrode and said first pair of measuring electrodes, a second pair of measuring electrodes, a second current-emitting electrode located between said first and second pairs of measuring electrodes, and a second current return electrode located between said second current-emitting electrode and said second pair of measuring electrodes; means for mounting said electrodes for passage through a borehole; means for supplying a first current to said at least one current-emitting electrode for emission into the media surrounding said mounting means; means responsive to the potentials on said first pair of measuring electrodes for supplying a first auxiliary current between said at least one current-emitting electrode and said first return electrode of sufficient magnitude to maintain a given potential relationship between the electrodes of said first measuring electrode pairs; and means responsive to the potentials on said second pair of measuring electrodes for supplying a second auxiliary current between said second current-emitting electrode and said second current return electrode of sufficient magnitude to maintain a given relationship between the potentials of said second measuring electrode pair whereby said first current will be forced into a formation.

Claim 59

Apparatus for measuring a characteristic of earth formations traversed by a borehole, comprising: a plurality of electrodes; means for mounting said electrodes for passage through a borehole, said electrodes inlcuding one current-emitting electrode for emitting survey and auxiliary currents into the media surrounding the mounting means, two pair of measuring electrodes located one each on each side of said current-emitting electrode, a current return electrode pair having one electrode each located between said current-emitting electrode and each of said measuring electrode pairs, a pair of measuring electrodes located one each between said current emitting electrode and each of said electrodes of said current return electrode pair, and a third current return electrode spaced apart from said other electrodes; means for supplying all of the survey and auxiliary currents to said current-emitting electrode for emission into the media surrounding the mounting means, said survey current returning to said third current return electrode for return to the current supplying means and said auxiliary current returning to said current return electrode pair for return to the current supplying means; and means responsive to the potentials on said measuring electrodes for adjusting one of said survey or auxiliary currents to maintain a given potential distribution in the vicinity of said two pair of measuring electrodes and obtaining an indication of a formation characteristic.

Claim 60

Apparatus for measuring a characteristic of earth formations traversed by a borehole, comprising: a plurality of electrodes; means for mounting said electrodes for passage through a borehole, said electrodes inlcuding one current-emitting electrode for emitting survey and auxiliary currents into the media surrounding the mounting means, two pair of measuring electrodes located one each on each side of said current-emitting electrode, a current return electrode pair having one electrode each located between said current-emitting electrode and each of said measuring electrode pairs, a pair of measuring electrodes located one each between said current emitting electrode and each of said electrodes of said current return electrode pair, and a third current return electrode spaced apart from said other electrodes; means for supplying all of the survey and auxiliary currents to said current-emitting electrode for emission into the media surrounding the mounting means, said survey current returning to said third current return electrode for return to the current supplying means and said auxiliary current returning to said current return electrode pair for return to the current supplying means; and means responsive to the potentials on said measuring electrodes for adjusting one of said survey or auxiliary currents to maintain a given potential distribution in the vicinity of said two pair of measuring electrodes and obtaining an indication of a formation characteristic.

Claim 61

Apparatus for measuring a characteristic of earth formations traversed by a borehole, comprising: a plurality of electrodes; means for mounting said electrodes for passage through a borehole, said electrodes including at least one current-emitting electrode for emitting survey and auxiliary currents into the media surrounding the mounting means, two pair of measuring electrodes located one each on each side of said at least one current-emitting electrode, a current return electrode pair hving one electrode each located between said at least one current-emitting electrode and each of said measuring electrode piars, one pair of measuring electrodes located one each between said current emitting electrode and each of said electrodes of said current return electrode pair, and a third cUrrent return electrode spaced apart from said other electrodes; means for supplying all of the survey and auxiliary currents to said at least one current-emitting electrode for emission into the media surrounding the mounting means, said survey current returning to said third current return electrode for return to the current supplying means and said auxiliary current returning to said current return electrode pair for return to the current supplying means; means responsive to the potentials on said two pair of measuring electrodes for adjusting one of said survey or auxiliary currents to maintain a given potential distribution in the vicinity of said measuring electrodes whereby said survey current will tend to be forced into a formation; and means responsive to the potentials on said one pair of measuring electrodes for obtaining an indication of formation conductivity or resistivity.

Claim 62

Apparatus for measuring a characteristic of earth formations traversed by a borehole, comprising: a plurality of electrodes; means for mounting said electrodes for passage through a borehole, said electrodes including at least one current-emitting electrode for emitting survey and auxiliary currents into the media surrounding the mounting means, two pair of measuring electrodes located one each on each side of said at least one current-emitting electrode, a current return electrode pair hving one electrode each located between said at least one current-emitting electrode and each of said measuring electrode piars, one pair of measuring electrodes located one each between said current emitting electrode and each of said electrodes of said current return electrode pair, and a third cUrrent return electrode spaced apart from said other electrodes; means for supplying all of the survey and auxiliary currents to said at least one current-emitting electrode for emission into the media surrounding the mounting means, said survey current returning to said third current return electrode for return to the current supplying means and said auxiliary current returning to said current return electrode pair for return to the current supplying means; means responsive to the potentials on said two pair of measuring electrodes for adjusting one of said survey or auxiliary currents to maintain a given potential distribution in the vicinity of said measuring electrodes whereby said survey current will tend to be forced into a formation; and means responsive to the potentials on said one pair of measuring electrodes for obtaining an indication of formation conductivity or resistivity.

Claim 63

Apparatus for measuring a characteristic of earth formations traversed by a borehole, comprising: a plurality of electrodes; means, including a borehole wall-engaging pad member, for mounting said electrodes for passage through a borehole, said electrodes including one centrally located pad mounted current-emitting electrode, a first pad mounted measure electrode concentrically surrounding said current emitting electrode, a first pad mounted return electrode concentrically surrounding first measured electrode, second and third pad mounted measuring electrodes conecentrally surrounding said first return electrode, a second return eelctrode located at a greater distance from said at least one current-emitting electrode than said first return electrode; means for supplying survey and auxiliary currents to said current-emitting electrode, the auxiliary current returning to the first return electrode for return to the current supplying means and the survey current returning to the second return electrode for return to the current supplying means; and means responsive to the potentials of the measuring electrodes for adjusting one of the survey or auxiliary currents to maintain a given potential difference between said second and third measuring electrodes and obtaining a measure of a formation characteristic.

Claim 64

Apparatus for measuring a characteristic of earth formations traversed by a borehole, comprising: a plurality of electrodes; means, including a borehole wall-engaging pad member, for mounting said electrodes for passage through a borehole, said electrodes including one centrally located pad mounted current-emitting electrode, a first pad mounted measure electrode concentrically surrounding said current emitting electrode, a first pad mounted return electrode concentrically surrounding first measured electrode, second and third pad mounted measuring electrodes conecentrally surrounding said first return electrode, a second return eelctrode located at a greater distance from said at least one current-emitting electrode than said first return electrode; means for supplying survey and auxiliary currents to said current-emitting electrode, the auxiliary current returning to the first return electrode for return to the current supplying means and the survey current returning to the second return electrode for return to the current supplying means; and means responsive to the potentials of the measuring electrodes for adjusting one of the survey or auxiliary currents to maintain a given potential difference between said second and third measuring electrodes and obtaining a measure of a formation characteristic.

Claim 65

Apparatus for measuring a characteristic of earth formations traversed by a borehole, comprising: a plurality of electrodes; means, including a borehole wall-engaging pad member, for mounting said electrodes for passage through a borehole, said electrodes including one centrally located pad mounted current-emitting electrode, a first pad mounted measure electrode concentrically surrounding said current emitting electrode, a first pad mounted return electrode concentrically surrounding first measure electrode, second and third pad mounted measuring electrodes concentrically surrounding said first return electrode, a second return electrode located at a greater distance from said at least one current-emitting electrode than said first return electrode; means for supplying survey and auxiliary currents to said current-emitting electrode, the auxiliary current returning to the first return electrode for return to the current supplying means and the survey current returning to the second return electrode for return to the current supplying means; and means responsive to the potentials of the second and third measuring electrodes for adjusting one of the survey or auxiliary currents to maintain a given potential difference between said second and third measuring electrodes whreby said survey current will tend to be forced into a formation; means responsive to the potential on said first measure electrode for obtaining a measure of formation conductivity or resistivity.

Claim 66

Apparatus for measuring a characteristic of earth formations traversed by a borehole, comprising: a plurality of electrodes; means, including a borehole wall-engaging pad member, for mounting said electrodes for passage through a borehole, said electrodes including one centrally located pad mounted current-emitting electrode, a first pad mounted measure electrode concentrically surrounding said current emitting electrode, a first pad mounted return electrode concentrically surrounding first measure electrode, second and third pad mounted measuring electrodes concentrically surrounding said first return electrode, a second return electrode located at a greater distance from said at least one current-emitting electrode than said first return electrode; means for supplying survey and auxiliary currents to said current-emitting electrode, the auxiliary current returning to the first return electrode for return to the current supplying means and the survey current returning to the second return electrode for return to the current supplying means; and means responsive to the potentials of the second and third measuring electrodes for adjusting one of the survey or auxiliary currents to maintain a given potential difference between said second and third measuring electrodes whreby said survey current will tend to be forced into a formation; means responsive to the potential on said first measure electrode for obtaining a measure of formation conductivity or resistivity.

Claim 67

Apparatus for investigating earth formations traversed by a borehole, comprising: a support means for carrying a plurality of electrodes and adapted for movement through a borehole, the electrodes include a current-emitting electrode array having aT least a central current-emitting electrode, two pairs of potential monitoring electrodes, one each located on opposite sides of said central electrode, and an auxiliary electrode located between said central current-emitting electrode and each of said monitoring elecrode pairs; means for supplying survey current to said central current-emitting electrode for obtaining a measure of a formation characteristic, all of said survey current being emitted from said central electrode; means for measuring the potential difference between the potentials on said potential monitoring electrodes; and means responsive to said measured potential difference for supplying current between at least one of said current-emitting electrodes and said auxiliary electrodes to maintain said potential difference at a substantially fixed value.

Claim 68

Apparatus for investigating earth formations traversed by a borehole, comprising: a support means for carrying a plurality of electrodes and adapted for movement through a borehole, the electrodes include a current-emitting electrode array having aT least a central current-emitting electrode, two pairs of potential monitoring electrodes, one each located on opposite sides of said central electrode, and an auxiliary electrode located between said central current-emitting electrode and each of said monitoring elecrode pairs; means for supplying survey current to said central current-emitting electrode for obtaining a measure of a formation characteristic, all of said survey current being emitted from said central electrode; means for measuring the potential difference between the potentials on said potential monitoring electrodes; and means responsive to said measured potential difference for supplying current between at least one of said current-emitting electrodes and said auxiliary electrodes to maintain said potential difference at a substantially fixed value.