Nuclear magnetism well logging method and apparatus

Claim 2

In the method of identifying in an earth formation traversed by a well bore, nuclear magnetism signals indicative of a characteristic of said earth formation by (i) positioning a polarizing coil in said well bore adjacent said formation, (ii) applying a magnetic polarizing field of magnitude (Hp) to a portion of said formation by flowing a direct current in a forward direct current biasing mode through said polarizing coil for a preselected polarizing cycle and then (iii) after interruption of said forward current biasing mode, detecting said nuclear magnetism signals due to inphase precession of protons in said earth formation under the influence of the earth''s magnetic field over a preselected detection cycle, the improvement for reducing the effect of magnetic materials in said formation on said nuclear magnetism signals which comprises a substep occurring after step (i) but before step (ii), of flowing direct current to said polarizing coil in a reverse direct current biasing mode to generate a reverse polarizing field in said earth formation of a preselected magnitude and after said preselected reverse polarizing field has been attained, then reversing the amplitude of said reverse current flow to zero.

Claim 3

The improvement of claim 1 in whicH said attained preselected reverse polarizing field is equal to (-KH''p) and is related to said magnetic polarizing field (Hp) of step (ii) in accordance with (-H''p) K''(+Hp) where K and K'' are members of a set of all positive numbers.

Claim 4

The improvement of claim 2 where K is equal to 1.

Claim 9

The method of claim 5 in which said digital time source code in the steps (ii) through step (vi) constitutes the digital signals representative of multibit digital timing words and a plurality of clock pulses cooperatively functioning to provide sequential control of said indicated process steps (ii) through (vi).

Claim 10

The method of claim 5 wherein step (i) is further characterized by the sub-step of generating a multi-bit digital code indicative of the depth of said polarizing Coil within said well bore.

Claim 11

The method of claim 5 wherein step (iii) is further characterized by the sub-step of generating a multi-bit digital code indicative of the amplitude of direct current in said polarizing coil during said reverse direct current biasing mode.

Claim 12

The method of claim 8 in which said multibit digital code indicative of said amplitude of said direct current during said reverse current biasing mode in said polarizing coil, is recorded on magnetic tape in a 9-bit word.

Claim 13

A digitally controlled nuclear magnetism logging apparatus adapted to digitally reproduce nuclear magnetism signals induced by protons within an earth formation penetrated by a well bore precessing about the earth''s magnetic field while simultaneously minimizing the effect of sources of magnetic field inhomogenieties within said earth formation as said precession occurs, comprising (i) a polarizing coil adapted to be positioned within said well bore at a selected depth (ii) means for detecting said nuclear magnetism signals in said earth formation due to the precession of protons of said formation under the influence of the earth''s magnetic field over a preselected detection cycle, (iii) a digital logging system means operatively connected to said polarizing coil and said detecting means, said digital logging system including a direct current supply means disconnectably connected to said polarizing coil (iv) cooperative switch means disconnectably connected intermediate said polarizing coil and said direct current supply means of said digital logging system, (v) said digital logging system also including a digital signal generating and control means connected so as to control operation of said cooperative switching means in accordance with a digital time source code as to thereby provide a series of digital control signals for controlling the direction and time of occurrence of direct current flow between said polarizing coil and said direct current supply means, (vi) said cooperative switch means also including a reversing mode polarity switch means controlled by said digital generating and control means of said digital logging system adapted to be enabled so as to flow said current to said polarizing coil in a reverse direct current direct current biasing mode during a selected time increment determined by said digital time source code so as to generate a reverse polarizing field of magnitude (KHp) in said earth formation prior to allowing current flow to said polarizing coil in a normal forward direct current biasing mode (vii) said cooperative switch means also including a two-state switch means in circuit with reverse mode switch means having an enabled state to allow flow of current in said forward direct current biasing mode to said polarizing coil to generate a magnetic polarizing field of magnitude (Hp) in said earth formation for preselected polarizing cycle measured by and in accordance with said digital time source code to thereby polarize said protons in said formation, said two-state switch means also causing interruption of said forward current flow after said polarizing cycle has been completed so as to cause said magnetic polarizing field (Hp) to fall to zero rapidly.

Claim 14

A digitally controlled nuclear magnetism logging apparatus adapted to digitally reproduce nuclear magnetism signals induced by protons within an earth formation penetrated by a well bore precessing about the earth''s magnetic field while simultaneously minimizing the effect of sources of magnetic field inhomogenieties within said earth formation as said precession occurs, comprising (i) a polarizing coil adapted to be positioned within said well bore at a selected depth (ii) means for detecting said nuclear magnetism signals in said earth formation due to the precession of protons of said formation under the influence of the earth''s magnetic field over a preselected detection cycle, (iii) a digital logging system means operatively connected to said polarizing coil and said detecting means, said digital logging system including a direct current supply means disconnectably connected to said polarizing coil (iv) cooperative switch means disconnectably connected intermediate said polarizing coil and said direct current supply means of said digital logging system, (v) said digital logging system also including a digital signal generating and control means connected so as to control operation of said cooperative switching means in accordance with a digital time source code as to thereby provide a series of digital control signals for controlling the direction and time of occurrence of direct current flow between said polarizing coil and said direct current supply means, (vi) said cooperative switch means also including a reversing mode polarity switch means controlled by said digital generating and control means of said digital logging system adapted to be enabled so as to flow said current to said polarizing coil in a reverse direct current direct current biasing mode during a selected time increment determined by said digital time source code so as to generate a reverse polarizing field of magnitude (KHp) in said earth formation prior to allowing current flow to said polarizing coil in a normal forward direct current biasing mode (vii) said cooperative switch means also including a two-state switch means in circuit with reverse mode switch means having an enabled state to allow flow of current in said forward direct current biasing mode to said polarizing coil to generate a magnetic polarizing field of magnitude (Hp) in said earth formation for preselected polarizing cycle measured by and in accordance with said digital time source code to thereby polarize said protons in said formation, said two-state switch means also causing interruption of said forward current flow after said polarizing cycle has been completed so as to cause said magnetic polarizing field (Hp) to fall to zero rapidly.

Claim 15

The apparatus of claim 10 wherein said two-2-state switch means is in circuit with said digital logging system so as to be under control of and operating in accordance with said digital source time code becoming enabled during said polarizing cycle to connects in circuit said polarizing coil and said direct urrent supply means during both said reverse and forward current biasing modes and then becoming disabled during said detection cycle whereby said nuclear magnetism signals are detected by said detecting means.

Claim 16

The apparatus of claim 11 wherein the digital logging system includes a digital recording means operatively connected and controlled by said generating and control means for recording said nuclear magnetism signals characteristic of said preCessing protons in said earth formation on magnetic tape in a digital multibit format.

Claim 17

The apparatus of claim 12 with the addition of a reverse current digital encoder means in circuit with said cooperative switch means, said polarizing coil and said direct current supply means for generating a first multibit digital code indicative of the amplitude of said reverse current flowing to said polarizing coil during said reverse current biasing mode, said encoder means being also in series with said digital recording means so as to allow recording, on said magnetic tape, of said first multibit digital code.

Claim 18

The apparatus of claim 13 with the addition of depth encoding means in circuit with said generating and control means of said digital logging system and operative thereby for generating a second multibit digital code indicative of the depth of said polarizing coil within said well bore, said depth encoding means being in series with said digital recording means so as to also allow recording of said second multibit digital code onto said magnetic tape.