Method and combination well-logging apparatus for conducting both deep and shallow investigation of the formations surrounding a borehole

Claim 2

A logging apparatus for measuring characteristics of earth formations surrounding a borehole, comprising: an elongated instrument adapted to traverse an earth borehole; means for passing a first electrical current from a first electrode mounted on said instrument through said formations to a remote point; first guard means mounted on said instrument for causing said first current to travel substantially laterally from said first electrode before traveling to said remote point; means for passing a second electrical current from a second electrode connected to said instrument through said formations to said first guard means; second guard means connected to said instrument for causing said second current to travel substantially laterally from said second electrode before traveling to said first guard means; and means for providing indications of the electrical potential between said first guard means and said remote point and between said first and second guard means, said electrical potential related to the characteristics of earth formations surrounding the borehole.

Claim 3

A logging apparatus according to claim 1 comprising in addition thereto an armor-plated cable for supporting said instrument during traversal of the borehole and for permitting transmission of said indications of electrical potential to the earth''s surface.

Claim 4

A logging apparatus according to claim 2 wherein said remote point comprises the armor on said cable.

Claim 5

A logging apparatus for measuring characteristics of earth formations surrounding a borehole, comprising: an elongated instrument adapted to traverse an earth borehole; a first electrode mounted on said instrument; a first electric current source connected to said first electrode for passing current from said first electrode through the formations surrounding the borehole to a first point remote from said first electrode; a first pair of guard electrodes on opposite sides of said first electrode, each of said first pair of guard electrodes having the same electrical potential; means for maintaining the same electrical potential on said pair of guard electrodes as is on said first electrode; a pad member on said instrument arranged to be held in contact with the borehole wall during traversal by said instrument; a second electrode mounted on said pad; a second Electric current source connected to said second electrode for passing current from said second electrode through the formations surrounding the borehole to a second point remote from said second electrode; a third guard electrode mounted around said second electrode; means for maintaining the same electrical potential on said third guard electrode as is on said second electrode; means for providing indications of the electrical potential between said first pair of guard electrodes and said first remote point, said electrical potential related to the characteristics of earth formations surrounding the borehole; and means for providing indications of the electrical potential between said third guard electrode and said second remote point wherein said second remote point comprises one of said first pair of guard electrodes and said electrical potential is related to the characteristics of earth formations surrounding the borehole.

Claim 6

A logging apparatus for measuring characteristics of earth formations surrounding a borehole, comprising: an elongated instrument adapted to traverse an earth borehole; a first electrode mounted on said instrument; a first electric current source connected to said first electrode for passing current from said first electrode through the formations surrounding the borehole to a first point remote from said first electrode; a first pair of guard electrodes on opposite sides of said first electrode, each of said first pair of guard electrodes having the same electrical potential; means for maintaining the same electrical potential on said pair of guard electrodes as is on said first electrode; a pad member on said instrument arranged to be held in contact with the borehole wall during traversal by said instrument; a second electrode mounted on said pad; a second Electric current source connected to said second electrode for passing current from said second electrode through the formations surrounding the borehole to a second point remote from said second electrode; a third guard electrode mounted around said second electrode; means for maintaining the same electrical potential on said third guard electrode as is on said second electrode; means for providing indications of the electrical potential between said first pair of guard electrodes and said first remote point, said electrical potential related to the characteristics of earth formations surrounding the borehole; and means for providing indications of the electrical potential between said third guard electrode and said second remote point wherein said second remote point comprises one of said first pair of guard electrodes and said electrical potential is related to the characteristics of earth formations surrounding the borehole.

Claim 7

A logging apparatus according to claim 4 comprising in addition thereto an armor-plated cable for supporting said instrument during traversal of the borehole and for permitting transmission of said indications of electrical potential to the earth''s surface.

Claim 8

A logging apparatus according to claim 5 wherein said first remote point comprises the armor on said cable.

Claim 9

In a method for simultaneously investigating the deep and shallow electrical resistivities of earth formations traversed by a borehole, the steps of passing a first focused electrical current having a given lateral movement into the formations from a first electrode to a first point remote from said first electrode; passing a second focused electrical current having a lateral movement into the formations reduced from said given lateral movement from a second electrode to a second point remote from said second electrode; measuring the deep electrical resistivity of the formations as a function of the electrical potential between said first electrode and said first remote point; and measuring the shallow electrical resistivity of the formations as a function of the electrical potential between said first and second electrodes.

Claim 10

In a method for simultaneously investigating the deep and shallow electrical resistivities of earth formations traversed by a borehole, the steps of passing a first focused electrical current having a given lateral movement into the formations from a first electrode to a first point remote from said first electrode; passing a second focused electrical current having a lateral movement into the formations reduced from said given lateral movement from a second electrode to a second point remote from said second electrode; measuring the deep electrical resistivity of the formations as a function of the electrical potential between said first electrode and said first remote point; and measuring the shallow electrical resistivity of the formations as a function of the electrical potential between said first and second electrodes.

Claim 11

The method according to claim 7 wherein each of the steps are repeated at different depths in the borehole.

Claim 12

In a method for simultaneously investigating the electrical resistivities of earth formations traversed by a borehole with a pair of focused electrical resistivity sections in the same borehole instrument, the steps of passing a first focused electrical current into the formations from a first electrode to a first point remote from said first electrode; passing a second focused electrical current into the formations from a second electrode to a second point remote from said second electrode, said second remote point having the same electrical potential as the potential of said first electrode; providing an indication of the electrical potential between said first electrode and said first remote point, said electrical potential related to the characteristics of earth formations traversed by the borehole; and providing an indication of the electrical potential between said second electrode and said second remote point, said electrical potential related to the characteristics of earth formations traversed by the borehole.

Claim 13

In a method for simultaneously investigating the electrical resistivities of earth formations traversed by a borehole with a pair of focused electrical resistivity sections in the same borehole instrument, the steps of passing a first focused electrical current into the formations from a first electrode to a first point remote from said first electrode; passing a second focused electrical current into the formations from a second electrode to a second point remote from said second electrode, said second remote point having the same electrical potential as the potential of said first electrode; providing an indication of the electrical potential between said first electrode and said first remote point, said electrical potential related to the characteristics of earth formations traversed by the borehole; and providing an indication of the electrical potential between said second electrode and said second remote point, said electrical potential related to the characteristics of earth formations traversed by the borehole.