Electromagnetic logging device and method utilizing three mutually perpendicular coils for determining the dip of discontinuities in the electrical resistivity of underground formations

Claim 2

Logging device for detecting and determining electrical discontinuities in the areas of the earth surrounding a borehole by means of a sonde lowered into the borehole, said device including means having an axis of symmetry for transmitting into the ground electric currents, means for measuring the magnetic field produced by said currents at a measuring point in spaced relationship to said transmitting means and located within the borehole, said measuring means including a first electric coil means placed in a direction parallel to the borehole axis, a second and a third coil means, the axes of which are perpendicular to each other and to the axis of said first coil and means for determining the dip of the electrical discontinuities as a function of the ratio of the amplitude of the components of the magnetic field depicted by said coil means.

Claim 3

Logging device according to claim 1, further including means for rotating said sonde about its axis.

Claim 4

A logging device according to claim 1, wherein the electric currents are transmitted at a frequency between 1,000 and 500,000 Hz.

Claim 5

A logging device according to claim 3, wherein the electric currents are transmitted at a frequency between 10,000 and 50,000 Hz.

Claim 6

Logging device for detecting and determining electrical discontinuities in the areas of the earth surrounded by a borehole, by means of a sonde lowered into the borehole, said device including means having an axis of symmetry for transmitting into the ground alternating electric currents, means for measuring the magnetic field produced by these currents at a measuring point in spaced relationship to said transmitting means and located inside said borehole, said means including a first electric coil means placed in a direction parallel to, said axis of symmetry, a second and a third coil means, the axes of which are perpendicular to each other and to the axis of said first coil, all of these coils belonging respectively to three detecting circuits tuned to the transmitting frequency, and means for determining the dip of the electrical discontinuity as a function of the ratio of the amplitude of the signal received by said first coil to the square root of the sum of the squares of the respective amplitudes of the signals which are picked-up by said second and said third coil means.

Claim 7

Logging device for detecting and determining electrical discontinuities in the areas of the earth surrounded by a borehole, by means of a sonde lowered into the borehole, said device including means having an axis of symmetry for transmitting into the ground alternating electric currents, means for measuring the magnetic field produced by these currents at a measuring point in spaced relationship to said transmitting means and located inside said borehole, said means including a first electric coil means placed in a direction parallel to, said axis of symmetry, a second and a third coil means, the axes of which are perpendicular to each other and to the axis of said first coil, all of these coils belonging respectively to three detecting circuits tuned to the transmitting frequency, and means for determining the dip of the electrical discontinuity as a function of the ratio of the amplitude of the signal received by said first coil to the square root of the sum of the squares of the respective amplitudes of the signals which are picked-up by said second and said third coil means.

Claim 8

Method for detecting and determining electrical discontinuities in the areas of the earth surrounding a borehole, by using a sonde located in the borehole, said sonde including a first electrical coil means having an axis placed in a direction parallel to the borehole axis, a second and a third coil means, the axes of which are both perpendicular to the axis of the first coil means, comprising the steps of transmitting electrical currents into the ground from a transmitter means located in the borehole, said transmitter means having an axis of symmetry parallel to the axis of the borehole, detecting three electrical signals received by said first, second and third coil means respectively in response to the transmission of said electrical currents through the earth, measuring, as a function of said three electrical signals, the magnetic field at a point located inside the borehole, in spaced relationship to the point where said currents are transmitted, in the form of the components of said field respectively along the axis of symmetry and in a plane perpendicular to said axis and determining the ratio of the amplitudes of said components, said ratio having a finite value different from zero wHen the emission of the currents if effected in the vicinity of an electrical discontinuity in the ground surrounding the borehole and deriving from the value of said ratio the dip of said discontinuity.

Claim 9

Method for detecting and determining electrical discontinuities in the areas of the earth surrounding a borehole, by using a sonde located in the borehole, said sonde including a first electrical coil means having an axis placed in a direction parallel to the borehole axis, a second and a third coil means, the axes of which are both perpendicular to the axis of the first coil means, comprising the steps of transmitting electrical currents into the ground from a transmitter means located in the borehole, said transmitter means having an axis of symmetry parallel to the axis of the borehole, detecting three electrical signals received by said first, second and third coil means respectively in response to the transmission of said electrical currents through the earth, measuring, as a function of said three electrical signals, the magnetic field at a point located inside the borehole, in spaced relationship to the point where said currents are transmitted, in the form of the components of said field respectively along the axis of symmetry and in a plane perpendicular to said axis and determining the ratio of the amplitudes of said components, said ratio having a finite value different from zero wHen the emission of the currents if effected in the vicinity of an electrical discontinuity in the ground surrounding the borehole and deriving from the value of said ratio the dip of said discontinuity.

Claim 10

A method according to claim 6, wherein the electrical currents are transmitted at a frequency between 1,000 and 500,000 Hz.

Claim 11

The method according to claim 7, wherein the electrical currents are transmitted at a frequency between 10,000 and 50,000 Hz.