Methods and apparatus for measurement of low frequency a.c. magnetic fields within cased boreholes to determine properties of geological formations

The method of measurement of geological formation properties in a formation penetrated by two current conducting electrodes and a conductive and magnetic steel borehole casing which comprises the steps of conducting A.C. current from a first current conducting electrode in electrical contact with the surface of the earth to a second current conducting electrode in electrical contact with the surface of the earth which is spaced apart from said first electrode, wherein the frequency of the current is between 0.001 Hz, wherein said current selectively conducts through relatively less conductive geological formations between said current conducting electrodes which therefore generates and applies a primary A.C. magnetic field to the subterranean formation to the depth of measurement thereby resulting in the production of eddy currents in the geological formation which in turn produce secondary A.C. magnetic fields, said primary and secondary A.C. magnetic fields having longitudinal components at the casing which subsequently penetrate the casing to the interior of the casing, measuring the longitudinal components of said primary and said secondary A.C. magnetic fields on the interior of the casing, repetitively performing said measurements at many vertical positions within the borehole casing and determining from said measured longitudinal components characteristic parameters for the formation through said borehole casing including the resistivity and changes in the resistivity of the geological formations.

An apparatus for measurement of the properties of a geological formation penetrated by two current conducting electrodes and a conductive and magnetic steel borehole casing comprising:

• a means of conducting A.C. current from a first current conducting electrode in electrical contact with the surface of the earth to a second current conducting electrode in electrical contact with the surface of the earth, which is spaced apart from said first current conducting electrode, thereby causing flowing A.C. currents which selectively follow geological formations which are relatively less resistive than other adjacent geological formations, whereby said flowing A.C. currents generate primary A.C. magnetic fields in the formation which in turn produce eddy currents which generate secondary A.C. magnetic fields in the formation, said primary and secondary A.C. magnetic fields having longitudinal components at the casing which subsequently penetrate the casing to the interior of the casing;
• one or more magnetic field measurement means sensitive to the longitudinal components of the A.C. magnetic fields on the interior of the casing at the depth of measurement for repetitively measuring the longitudinal components of both the primary and secondary A.C. magnetic fields at many vertical positions within the borehole casing thereby providing indication of the presence of relatively conductive formations in the vicinity of the casing, said magnetic field measuring means providing an indication of characteristic parameters of the geological formation through said borehole casing including the resistivity and changes in the resistivity of the geological formation;
• the frequency of the source of the A.C. current being between 0.001 Hz to 20 Hz thereby allowing said longitudinal A.C. magnetic fields to penetrate the casing and whereby the magnitude of the A.C. current conducted between said current conducting electrodes on the surface of the earth exceeds 100 milliamps peak-to-peak;
• and wherein the casing is spaced horizontally apart from said first and second current conducting electrode.

The method of measurement of geological formation properties penetrated by two conductive and magnetic steel borehole casings which comprises the steps of:

• conducting A.C. current from a first current carrying electrical contact engaged with the interior of a first cased borehole and located at a first depth within said first cased bore to a second current carrying electrical contact engaged with the interior of a second cased borehole located at a second depth within said second cased borehole, said first and second casings being spaced apart in the geological formation, which causes flowing A.C. current to selectively follow geological formations which are relatively less resistive than other adjacent geological formations in the vicinity of said first and second casing, and whereby said flowing A.C. currents give rise to primary A.C. magnetic fields which in turn produce longitudinal components which penetrate to the interior of the first and second borehole casings, and whereby said primary A.C. magnetic fields in addition cause the production of eddy currents in the geological formation which in turn produce secondary A.C. magnetic fields which possess longitudinal components which also subsequently penetrate the walls of the casings to the interior of said first and second casings;
• measuring the longitudinal components of both the primary and secondary A.C. magnetic fields on the interior of said second casing at a predetermined vertical position within said second casing, repetitively performing said measurements at different predetermined vertical positions within the second borehole casing, thereby providing a method of measuring characteristic parameters of the formation through said second borehole casing including the resistivity and changes in the resistivity of the geological formation, whereby the frequency of the A.C. current is chosen to be in the range from 0.001 Hz to 20 Hz to allow said longitudinal components of the A.C. magnetic field to penetrate the conductive and magnetic steel borehole casings and whereby the magnitude of the currents conducted between said current conducting electrodes have a magnitude of at least 100 milliamps peak-to-peak which provides a sufficient magnitude A.C. magnetic field for measurement inside said second casing, and determining from said longitudinal components a formation property.

An apparatus for measurement of the properties of a geological formation penetrated by two conductive and magnetic steel borehole casings comprising:

• a means of generating and conducting A.C. current through a geological formation from a first current conducting electrode in electrical contact with the interior of a first cased borehole located at a first depth within said first cased well to a second current conducting electrode in electrical contact with the interior of a second cased borehole located at a second depth within said second cased well, said cased boreholes being spaced apart within the geological formation, whereby said A.C. currents selectively follow geological formations which are relatively less resistive than those adjacent geological formations which produce longitudinal magnetic field components which penetrate to the interior of the first and second borehole casings, and whereby said primary A.C. magnetic fields in addition cause the production of eddy currents in the geological formation which in turn produces secondary A.C. magnetic fields which possess longitudinal components which also subsequently penetrate the walls of the casings to the interior of said first and second casings;
• one or more magnetic field measurement means sensitive to the longitudinal components of the A.C. magnetic fields on the interior of said second cased well for repetitively measuring the longitudinal components of both the primary and secondary A.C. magnetic fields at many vertical positions within said second borehole casing thereby providing indication of the presence of relatively conductive formations in the vicinity of said second casing, said magnetic field measuring means providing an indication of characteristic parameters of the geological formation through said second borehole casing including the resistivity and changes in the resistivity of the geological formation;
• wherein the frequency of the source of the A.C. current is between 0.001 Hz to 20 Hz thereby allowing said longitudinal A.C. magnetic fields to penetrate the casing and whereby the magnitude of the A.C. current conducted between said current conducting electrodes exceeds 100 milliamps peak-to-peak.

An apparatus for measurement of geological formation properties in a formation penetrated by a conductive and magnetic steel borehole casing comprising at least one primary source of exciting A.C. magnetic fields positioned within the interior of the casing, wherein the frequency and amplitude of the A.C. fields are chosen to allow longitudinal and azimuthal components of said primary A.C. magnetic fields to penetrate the walls of the borehole casing thereby subjecting the adjacent formation to the exciting A.C. magnetic fields resulting in the production of eddy currents in the formation which subsequently produce secondary A.C. magnetic fields characteristic of the formation whereby the longitudinal and azimuthal components of said secondary A.C. magnetic fields penetrate the wall of the casing to the interior of the borehole casing in combination with one or more magnetic field sensing means located within said borehole casing, vertically spaced apart from said primary source, which is responsive to the longitudinal components of said primary and secondary A.C. magnetic fields for providing measurements of characteristic formation properties including the resistivity of the formation, wherein the frequency of the source of the exciting A.C. magnetic fields is between 0.001 Hz and 20 Hz thereby allowing the longitudinal and azimuthal components of said exciting A.C. magnetic fields to penetrate the walls of the conductive and magnetic steel borehole casing to the adjacent geological formation, wherein the amplitude of the exciting A.C. magnetic field exceeds one gauss peak-to-peak in the immediate vicinity of said exciting source within said steel borehole casing which provides sufficient amplitude for measurement of said primary and secondary A.C. magnetic fields within said cased well.

An apparatus for measurement of the properties of a geological formation penetrated by two conductive and magnetic steel cased boreholes comprising:

• means of generating a primary A.C. magnetic field on the interior of a first cased borehole at a first depth within said first cased borehole which generates longitudinal components of said primary fields at the walls of said first casing which therefore subjects the adjacent geological formation to the longitudinal components of said primary A.C. magnetic fields which in addition cause the production of eddy currents in the geological formation which in turn produce secondary A.C. magnetic fields which also posses longitudinal components thereby causing the longitudinal components of said primary and second A.C. magnetic fields to penetrate the walls of a second borehole casing to the interior of the second cased well;
• one or more magnetic field measurement means sensitive to the longitudinal components of the A.C. magnetic fields present which is positioned at a second depth within the interior of said second cased well for repetitively measuring the longitudinal components of both the primary and secondary A.C. magnetic fields at many vertical positions within said second borehole casing thereby providing indication of the presence of relatively conductive formations in the vicinity of said second casing, said magnetic field measuring means providing an indication of characteristic parameters of the geological formation through said second borehole casing including the resistivity and changes in the resistivity of the geological formation;
• wherein the frequency of operation of the source of the A.C. generating means is between 0.001 Hz to 20 Hz thereby allowing said longitudinal components of said A.C. magnetic fields to penetrate the casings and wherein the A.C. generating means in said first cased borehole produces A.C. magnetic fields with a magnitude in excess of one gauss peak-to-peak in the immediate vicinity of said generating means which provides sufficient A.C. magnetic fields for measurement within said second cased well.

A method for geophysical exploration of a geological formation which is penetrated by at least one open borehole comprising the steps of:

• generating and applying an A.C. magnetic field to the geological formation with A.C. magnetic field generation means in the frequency range of 0.001 Hz to 20 Hz;
• sensing the A.C. magnetic field at a predetermined depth within said open hole with a precision A.C. magnetic gradiometer which is capable of measuring A.C. magnetic fields smaller than the natural magnetic noise in the geomagnetic field whereby said A.C. magnetic gradiometer is comprised of a pair of induction coils which are disposed vertically in the borehole and which may be operated differentially to sense the longitudinal components of any A.C. magnetic fields present wherein each induction coil is comprised of insulated wire wound around high permeability magnetic material such as mu-metal which serves as the core of the induction coil, said high permeability core material being at least 10 times longer than any lateral dimension of the core, and wherein each vertical pair of said induction coils is adjusted with an A.C. magnetic balancing means at each frequency such that the individual induction coils comprising the gradiometer have identical responses to identical A.C. magnetic fields, and the differential output of the A.C. magnetic gradiometer is sensitive primarily to only those components of the A.C. magnetic field which have different magnitudes and phases at the different vertical locations of the induction coils whereby said differential output is therefore relatively immune to magnetic noise fluctuations in the geomagnetic field.