Abstract: Method for assessing hydrocarbon source rock potential of a subsurface region without well log information. The method uses surface electromagnetic (121) and seismic (122) survey data to obtain vertical profiles of resistivity and velocity (123), which are then analyzed in the same way as well log data are analyzed by the well known Delta Log R method (124).

Abstract: A stabilized field sensor apparatus collects field data, in particular magnetic field data, with reduced motion noise. The apparatus includes a tear drop shaped housing, a tow frame in the housing, a plurality of vibration isolating dampers spaced around the frame, a base assembly mounted to the dampers, a support pedestal having a bottom end fixed to the base assembly and an upper free end, a single spherical air bearing connected to the upper free end of the pedestal, an instrument platform with a lower hollow funnel having an upper inside apex supported on the air bearing for a one point support, principal and secondary gyro stabilizers for maintaining pivotal and rotational stability, and at least one field sensor mounted to the instrument platform for collecting the field data while being stabilized against motion noise including vibration, pivoting and rotation from the base assembly, from the tow frame and from the housing.

Abstract: Cost-effective compact magnetometers can be deployed across large ocean areas to record magnetic field strengths. Each magnetometer has a canister containing a magnetometer sensor at its upper end to detect magnetic field strengths of magnetic influence sweep systems and provide representative data signals. Each magnetometer also has sensors to collect data representative of the orientation of the magnetometer as well as temperature and depth to aid in post operational evaluation of the gathered magnetic strength data. A computer processor connected to the sensors controls receipt of the data signals and stores them in a memory device. Batteries at the canister's lower end supply power for the sensors, processor, and memory. An anchor release mechanism causes an anchor to separate from the canister, allowing it to float to the surface for recovery or to transmit data via a UHF transceiver.

Abstract: Disclosed herein is a three dimensional small bin electromagnetic consecutive array data acquisition method used in oil exploration comprising the steps of recording data using small bin lattices on execution of arrangement electrodes, each acquisition station (Ex, Ey) records the natural electromagnetic field time series data with the same acquisition parameter simultaneously; interference is removed first, for recorded data processing to get interference-free data; for border points and center point, the recording point serves as center point, and the same component from adjacent two points to total points area added together to obtain the average value of electric field data at time domain for each observation point; for corner points, the average values of the same electric field component from adjacent two points to total survey points toward bin direction is calculated; the electric field components acquired in maximum space serve as the new electric field values respectively; new time series data acqui

Abstract: A geophysical measurement device is provided for exploration of natural resources of the floor in an aquatic domain. The device includes a frame designed to rest in a stable manner on the floor of the aquatic environment, at least one buoyancy caisson resting on the frame, measurement instruments. The measurement instruments including: at least one sensor making it possible to measure locally at least one of the components of the magnetic field; an analogue and/or digital data acquisition and processing device; and a source of energy supply of the sensor and of the data acquisition and processing device. The at least one sensor is contained inside the buoyancy caisson.

Abstract: A marine electromagnetic acquisition apparatus includes a sensor module having at least one sensor associated therewith. A sensor arm assembly is coupled to the sensor module. The sensor arm assembly has at least one sensor arm and at least one sensor disposed along the at least one sensor arm. An actuator is coupled to the sensor arm assembly for moving the sensor arm assembly between a folded position and an unfolded position.

Abstract: A method for determining a component of electric field response of the Earth's subsurface to a time-varying electromagnetic field induced in the Earth's subsurface is provided. The method includes measuring electric field response along a nonlinear pattern on at least one of the Earth's surface and the bottom of a body of water. The method includes measuring magnetic field response in three directions along the nonlinear pattern on at least one of the Earth's surface and the bottom of the body of water. The method further includes determining an electric field response in a direction normal to the measured electric field response using the electric field response and magnetic field response measurements.

Abstract: Disclosed herein is a three dimensional small bin electromagnetic consecutive array data acquisition method used in oil exploration comprising the steps of recording data using small bin lattices on execution of arrangement electrodes, each acquisition station (Ex, Ey) records the natural electromagnetic field time series data with the same acquisition parameter simultaneously; interference is removed first, for recorded data processing to get interference-free data; for border points and center point, the recording point serves as center point, and the same component from adjacent two points to total points area added together to obtain the average value of electric field data at time domain for each observation point; for corner points, the average values of the same electric field component from adjacent two points to total survey points toward bin direction is calculated; the electric field components acquired in maximum space serve as the new electric field values respectively; new time series data acqui

Abstract: A method for removing a static frequency domain, by (1) installing multiple sampling devices; (2) performing conventional processing whereby obtaining conventional observation curves; (3) adding electric-filed components of two adjacent observation points, those of three adjacent observation points, and adding electric field components of up to n adjacent observation points whereby obtaining electric fields of different polar distance, i.e. a sequence of electric field components of topology points; (4) using the sequence of the electric field components and the encrypted topology points sequence as an observation field value to perform power spectrum analysis and impedance tensor estimate; (5) drawing a resistance-variation of polar distance curve of a single frequency point of the record point; observing and analyzing a variation rule of resistivity, editing and deleting static interference resistance; and (6) repeating the previous step to complete edition of all frequency points of all record points.

Abstract: Method for removing air wave noise from shallow water controlled source electromagnetic survey data, using only the measured data and conductivity values for sea water (140) and air. The method is a calculation performed numerically on CSEM data and resulting in an estimate of those data that would have been acquired had the water layer extended infinitely upward from the seafloor. No properties of the sub-sea sediments are used. Synthetic electromagnetic field data are generated for (a) an all water model (141) and (b) an air-water model (146-147) of the survey region. These simulated results are then used to calculate (148-150) electromagnetic field values corresponding to a water-sediment model with water replacing the air half space, which represent measured data adjusted to remove air wave noise.

Abstract: A method of determining an orientation of a data acquisition system deployed on a seafloor includes measuring horizontal magnetic fields using detectors on the data acquisition system while the data acquisition rotates and descends to the seafloor or rises from the seafloor. Resting horizontal magnetic fields are measured after the data acquisition system is on the seafloor. A heading of the data acquisition system on the seafloor may be determined based on maximum and minimum horizontal magnetic fields measured during the descent and the resting horizontal magnetic fields.

Abstract: A detector for a marine electromagnetic survey system includes a housing arranged to minimize turbulence when the housing is towed through a body of water, and to minimize motion of the housing in any direction other than the tow direction. The housing includes at least one of an electric field and a magnetic field sensing element associated therewith.

Abstract: A high resolution magnetotelluric method for removing a static frequency domain, comprising steps of: (1) installing multiple sampling devices at the surface of the area to be explored in a form of an array, adjacent observation points sharing a same electrode; (2) performing conventional processing on all observation results whereby obtaining conventional observation curves of the observation points; (3) for the time series data of the original observation value of the electric field along the same direction of all observation points, adding electric-filed components of two adjacent observation points, those of three adjacent observation points, . . . , and those of all points whereby obtaining electric fields of different polar distance, i.e.

Abstract: A sensor cable for surveying. The sensor cable may comprise at least one pair of current sensor electrodes and an amplifier. The current sensor electrodes may be disposed along opposite sides of the sensor cable. The current sensor electrodes may be configured to detect current in an electromagnetic field transverse to an inline direction of the sensor cable. The amplifier may be configured to amplify the current in the electromagnetic field for detection by the electrode pair.

Abstract: The invention a method and device for carrying out geoelectric mappings. The device includes electrodes, a power supply and a voltage measuring device, for carrying out the method. The electrodes are disposed apart from one another on ground to be investigated and electric current is supplied by certain electrodes to the ground and electric potential is measured by remaining electrodes using the voltage measuring device. The device and method carry out a continuous, geoelectric mapping, with which accurate measurements are achieved. The device is embodied such that the electrodes are disposed at a outside of an endless loop and that this endless loop revolves continuously at the same rate at which the device is moving over the ground to be investigated during the mapping, so that at least two supplying and two potential electrodes rest on the ground investigated during each individual measurement.

Abstract: A method of electromagnetic surveying of an area of seafloor that is thought or known to contain a subterranean hydrocarbon reservoir is described. The method includes broadcasting an EM signal from a horizontal electric dipole (HED) transmitter and obtaining vertical electric dipole (VED) response data at a remote receiver in response thereto. Survey data are analysed by comparing the VED response data with background data which are not sensitive to the postulated hydrocarbon reservoir. Accordingly, differences between the VED response data and the background data allow for the identification of buried hydrocarbon reservoirs. The background data may be provided by magneto-telluric surveying, controlled source electromagnetic surveying or from direct geophysical measurement.

Abstract: A method for measuring magnetotelluric response of the Earth includes measuring transient controlled source electromagnetic response of the subsurface below a body of water over a plurality of actuations of an electromagnetic transmitter. The transient response measurements are stacked. The stacked transient responses are subtracted from measurements of total electromagnetic Earth response over a time period including the plurality of transient response measurements to generate the magnetotelluric response. A method for determining a component of electric field response to a time varying electromagnetic field induced in the Earth's subsurface, includes measuring magnetic field gradient in at least two orthogonal directions in response to the induced electromagnetic field and determining an electric field response in a direction normal to the magnetic field gradient measurements.

Abstract: The invention covers the mining industry. Detection method of the occurrence zone of the mantle substance diapir finger consisting in that that geological observation are carried out in riftzones banked up by diapirs of the abnormal mantle and places of location of the mantle substance finger in the earth's crust are determined. Based on the presence of influence traces of silanes streams upon the rock and/or presence of separate ridges having the length up to 200 km by the width of up to 20 km and/or based on the blast traces of silanes and/or presence of scoria cones, differing by that that the correspondence of 3He/4He from gases sampled from sources of 10?5 order is additionally determined for determination of silicides at the small depth as physical parameters for the location of the mantle substance diapir finger in the earth's crust. The presence of high semiconductor conductivity, locality and activity of magnetic anomalies and anomalies of gravity field is additionally established.

Abstract: A method for interpreting transient electromagnetic survey data includes measuring transient response of a medium over a plurality of switching events. The measured transient response to a first one of the current switching events is modeled. Transient response to the model for at least one current switching event prior in time to the at least a first current switching event is calculated. The calculated transient response is summed with the first event measured response and the sum is compared to the electromagnetic survey measurements. The model is adjusted and the calculating summed transient responses is repeated until a difference between the summed calculated responses and the survey measurements falls below a selected threshold.

Abstract: A marine measurement system is disclosed for obtaining measurements in an underwater operating environment. The system includes a base structure having a top surface, a bottom surface, and cavities provided therebetween. The cavities are also open to the top surface. The measurement system further includes measurement equipment (e.g., electronic equipment and/or instrumentation), which are retained in the cavities. A diaphragm membrane is applied adjacent the top surface and seals the cavities. The diaphragm member is positioned in pressure communication with the operating environment. Furthermore, the cavities are defined by the membrane and the base structure and filled with a pressure compensating fluid that is in pressure communication with the operating environment through the diaphragm membrane.

Abstract: An airborne exploration system used with an aircraft for exploration of oil and gas, mineral deposits and aquifers. The survey system uses natural electromagnetic EM fields as an energy source. The exploration system includes at least one non-directional, wideband, total electric field, water sensor. The water sensor includes a water container filled with water and adapted for mounting on an aircraft. The water container has a pair of conductive metal bands mounted around opposite ends thereof. The metal bands provide a capacitive electrical coupling with the water in the container. The water container has a diameter substantial the size as a length of the container. Therefore, the sensor responds to an electric field substantially independent of the relative orientation of the electric field. The bands are electrically connected to a computer data processing unit mounted on the aircraft.

Abstract: A system for mapping electrical conductivity of the seafloor incorporates a plurality of data logging units, where each unit (100) is an assembly adapted for deployment at a location on the seafloor for measurement of horizontal electric and magnetic fields. Rigidly attached and extending vertically from the unit assembly is a vertically-oriented substantially rigid arm (158) having a pair of vertically-displaced electrodes (160, 162) disposed on the arm to create a vertically-oriented dipole antenna. The electrodes are in electrical communication with an amplifier located within the assembly which generates an amplified signal which is then provided to a data logging processor also located within the assembly. The processor collects time series of amplified electric field and magnetic signals over a predetermined period of time.

Abstract: An airborne exploration system used with an aircraft for shallow and deep exploration for oil and gas, mineral deposits and aquifers. The survey system uses natural electromagnetic EM fields as an energy source. The exploration system includes a pair of aerodynamic housing pods adapted for mounting on wing tips of the aircraft. The housing pods include electric field sensors with three orthogonal electric dipoles oriented along an X, Y and Z axis. An optional third set of orthogonal electric dipoles can be mounted in the tail of the aircraft. The field sensors are electrically attached to angular motion detectors mounted inside housing pods. The motion detectors are used for compensating for errors caused by angular motion of the aircraft when in the presence of strong electric field gradients. The system also includes a total field magnetometer mounted in the aircraft.

Abstract: A marine oil and gas exploration system used for measuring telluric currents as a natural electromagnetic energy source flowing beneath an ocean and sea floor. The marine exploration system includes a towed fish adapted for being pulled behind a survey ship. The coordinates of the ship and the towed fish can be controlled by a GPS satellite and time data. The fish includes an aerodynamic waterproof housing, somewhat reminiscent of a shape of a fish. Metal sheet electrodes of electric field sensors are mounted on the surface of a tubular nose extending outwardly from the housing. The electric field sensors are in direct contact with the sea water. Typically, the fish is towed approximately 100 feet above the sea floor. The positioning of the fish is maintained by the use of an acoustic “pinger” on the fish. The pinger transmits signals up a towed cable to the survey ship. The fish can also include a cesium magnetometer connected to the electric field sensors.

Abstract: A method for processing magnetotelluric signals to identify subterranean deposits is provided for. The methods comprise obtaining magnetotelluric data from an area of interest. The magnetotelluric data comprises the amplitude of magnetotelluric signals recorded over time at one or more defined locations in the area of interest. The data for each location then is filtered through a set of frequency filters. The frequency filters correspond to subterranean depths over a range of interest. Amplitude peaks in the filtered data then are identified and analyzed to determine a value correlated to the resistance of the earth at each frequency and location. The resistance values are indicative of the presence or absence of deposits at the corresponding subterranean depth.

Abstract: A sea-floor electromagnetic measurement device for obtaining underwater measurements of earth formations including a central structure and arms attached to the central structure so that they can pivot relative to the central structure. An electrode is attached to the end of each of the arms or to the central structure, and/or magnetometers are attached to the arms. A method for undertaking sea-floor electromagnetic measurements of earth formations including measuring electric fields at a selected distance from a central structure of an electromagnetic measurement system. Magnetic fields are then measured at the same location.

Abstract: A geophysical prospecting method and apparatus that utilizes the harmonics and sub harmonics waves induced into the earth by the electric power grid. The amplitude-frequency data are converted to amplitude-depth data and further converted to a differential curve. The resulting differential curve is compared to known patterns of the interested geological features, such as hydrocarbon reservoir, to identify whether the features exist and at what depth such geological features lie.

Abstract: A subsurface imaging cable includes a plurality of sensor modules, wherein the plurality of the sensor modules are flexible and each of the plurality of the sensor modules is spaced apart on the subsurface imaging cable at a selected distance; and a flexible medium connecting the plurality of the sensor modules, wherein the subsurface imaging cable is flexible and adapted to be wound on a reel. A method for subsurface images includes acquiring direct-current measurements at a plurality of sites in a survey area; acquiring a first set of electric and magnetic measurements from natural electromagnetic fields at the plurality of sites; acquiring a second set of electric and magnetic measurements using controlled electric and magnetic sources at the plurality of sites; and determining a subsurface conductivity distribution from the direct-current measurements and the first set and the second set of electric and magnetic measurements.

Abstract: A sea-floor electromagnetic measurement device for obtaining underwater measurements of earth formations including a central structure and arms attached to the central structure so that they can pivot relative to the central structure. An electrode is attached to the end of each of the arms or to the central structure, and/or magnetometers are attached to the arms.

Abstract: Apparatus for determining the distribution of the electric polarization of a vessel in water, comprises an array of electric potential sensors (22) which are located in the water beneath the vessel and each generates a signal corresponding to the amplitude of the electric potential at the sensor as effected by the vessel, a signal processor unit (48) and cables (42) connecting each sensor (22) to the signal processor unit (48) to deliver each respective signal to the signal processor unit, the signal processor unit (48) serving to determine the potential difference between respective pairs of sensors (22) from the respective signals they generate, and the signal processor unit (48) processing multiple such potential difference determinations to deduce the distribution of electric polarization in the vessel which would cause said electric potential at each of the electric potential sensors (22).

Abstract: A method and apparatus for imaging deep underground objects utilizes the application of a spectral regularization routine that is applied to measured data prior to the application of an inversion routine to measured surface impedances to map subsurface conductivity. Specifically, measurement of magnetic and electric fields is conducted utilizing sensors at a measurement site, a processor is used to generate either resistivity or conductivity data based on the measurements of the sensors, the processor then performs the spectral regularization routine and an inversion routine to generate data that can be utilized to identify underground targets. The spectral regularization routine and inversion routine can either be processed separately (decoupled) or together. The measurements of the magnetic and electric fields are preferably made under the assumption of the presence of a plane wave at the measurement site.

Abstract: A method of data acquisition for geophysical surveys. The method includes the steps of providing a least one magnetic sensor unit for sensing and recording magnetic field fluctuations; providing a plurality of electric sensor units for sensing and recording electric currents induced by the magnetic field fluctuations; receiving a satellite based timing signal and responsively synchronizing and bag a position for each of the plurality of electric sensor units and the at least one magnetic sensor unit; and synchronously recording the positions, the magnetic field fluctuations and the electric currents for downloading for processing.

Abstract: The magnetotelluric system for seafloor petroleum exploration comprises a first waterproof pressure case containing a processor, AC-coupled magnetic field post-amplifiers and electric field amplifiers (the "logger unit"), a second waterproof pressure case containing an acoustic navigation/release system, four silver-silver chloride (Ag-AgCl) electrodes mounted on booms and at least two magnetic induction coil sensors. These elements are mounted together on a plastic and aluminum frame along with flotation devices and an anchor for deployment to the seafloor. The acoustic navigation/release system serves to locate the system by responding to acoustic pings generated by a ship-board unit and receives a release command which initiates detachment from the anchor so that the buoyant package floats to the surface for recovery. The electrodes used to detect the electric field are configured as grounded dipole antennas.

Abstract: A portable underwater mount for magnetic and electric field sensors. The mount has a gimbal-like structure comprising a ball member having a mounting flange for coupling to the magnetometer. The ball member is cast from polyurethane with Teflon powder suspended therein, providing a substantially frictionless surface. The ball member is disposed over a support platform having a spherical contour so that the weighted sensor positions itself to true vertical. A collar having a lock terminating in the support platform rests over the ball member so that once the sensor is oriented to magnetic north, the lock is engaged to secure the sensor in true vertical and magnetic north positions.

Abstract: A method and apparatus for determining the location in space of a formation sample is set forth. This involves a tool which is typically run on a wireline in a well, the tool supporting orthogonal positioned magnetometers and similar orthogonal accelerometers for measuring movement of the tool along the well borehole whereby components of the magnetic field and gravity field are measured to enable determination of the well in space. The tool supports an axially positioned centrally located core cutting apparatus and a laterally projecting plug cutting apparatus; scribe marks formed on the formation samples orient the samples relative to the tool and the tool is located in space from the above mentioned measurements so that the position of formation samples can be determined.

Abstract: A method of magnetotelluric exploration is described, wherein measures of the earth's electric and magnetic fields are obtained at a plurality of sensing locations comprising an areal array of sensing locations. Specifically, first and second components of the earth's electric field are measured at a plurality of sensing locations within an areal array of sensing locations whereby each of the components of the earth's electric field are spatially continuous. Additionally, first and second components of the earth's magnetic field are measured simultaneously at at least one location within the area covered by the areal arrays of sensing locations.

Abstract: A low noise, magnetotelluric, intercalation electrode is disclosed that is both electronically and ionically conductive and stable in ground water brine. The electrode is comprised of an inert or chemically inactive long cylindrical and porous support shield, a semipermeable membrane located in the shield selectively permeable to the positive ions of a metal selected from the group consisting of Na+ and K+ ions, a brine electrolyte, and an electrode axis located in the brine electrolyte that includes an electronically conductive, metallic material ionically reversible to Na+ and K+. The electrode is sealed and attachable at its electrode axis to a wire leading to another electrode spaced apart therefrom. The wire connection permits voltage measurements and thereby provides a method for determining the resistivity between the electrodes at frequencies that are quite low without being interferred with by noise attendant prior art magnetotelluric electrodes.

Abstract: A method for detecting a geologic deposit of a selected substance. In the method, the intensity of the local geomagnetic field at one or more measurement sites at a first location is resolved. During selected time intervals, at each site, the amplitude of the indigenous terrestial alternating electromagnetic radiation is measured at a frequency in the vicinity of a frequency equal to the local geomagnetic field intensity multiplied by the Larmor constant of the selected substance and divided by 2.pi.. Each measurement is intergated over a respective time interval to yield one or more integrated values. The integrated values are analyzed.

Abstract: The present invention relates generally to a method of geophysical exploration and more particularly to a novel method of magnetotelluric exploration. In one embodiment, a series of continuous electric dipole measurements of one component of the earth's electric field are obtained along a zigzag line. Concurrently, at least two components of the earth's magnetic field are obtained in the vicinity of the zigzag line. Each electric dipole measurement of the earth's electric field comprise measures of the potential difference between electrodes spaced along the zigzag line in which the included angle between adjacent pairs of electric dipole measurements is less than 180.degree.. Preferably, the included angle between adjacent electric dipole measurements is between 90.degree. and 150.degree..

Abstract: An electromagnetic survey method for geophysical exploration, in which the variations in the earth's magnetic field are measured in two, non-parallel directions at one point in the survey area. Simultaneously, the variations in the earth's electrical field parallel to the survey line are measured at a number of points along the survey line. These measured variations are transformed to the frequency domain, and then the horizontal component of the magnetic field orthogonal to the direction of the measured electrical field is calculated. The impedance at each measurement point on the survey line is calculated as a function of frequency, and weighted averages of the impedance for predetermined frequencies using a zero phase length weight function corresponding to a low pass filter applied to the electric field are used to calculate the subsurface conductivity distribution.

Abstract: An electromagnetic survey system for geophysical exploration comprising a reconnaissance method and a survey line method. The reconnaissance method finds the principal electrical axis of conductivity of anomalous zones of conductivity by measurement of variations in the earth's magnetic field in a grid pattern over the survey area, calculation of Tipper vectors for each site, and plotting the vectors to scale. The conductivity axis is determined as being perpendicular to a set of the largest parallel vectors, one or more of which reverse direction, and passing between the direction reversal. A Survey line may be collimated with the set of enlarged vectors to make three dimensional effects more symmetrical so they are attenuated.

Abstract: An electromagnetic survey method for geophysical exploration, in which the variations in the earth's magnetic field are measured in two, non-parallel directions at one point in the survey area. Simultaneously, the variations in the earth's electrical field parallel to the survey line are measured at a number of points along the survey line. These measured variations are transformed to the frequency domain, and then the horizontal component of the magnetic field orthogonal to the direction of the measured electrical field is calculated. The impedance at each measurement point on the survey line is calculated as a function of frequency, and weighted averages of the impedances for predetermined frequencies using a zero phase length weight function corresponding to a low pass filter applied to the electric field are used to calculate the subsurface conductivity distribution.

Abstract: In magnetotelluric exploration, plots of equal values of the phase of the determinant of the impedance tensor relating the earth's electric fields to the earth's magnetic fields are plotted to distinguish the structure of resistivity anomalies and separate near surface and deep anomalies. Plots of equal values of the absolute value of the determinant of the impedance tensor are also made. These plots display the dimensionality of the anomaly and produce a better display of anomalies which are at depth.

Abstract: The depth to and size of an underground object may be determined by sweeping a CSAMT signal and locating a peak response when the receiver spans the edge of the object. The depth of the object is one quarter wavelength in the subsurface media of the frequency of the peak.

Abstract: An electromagnetic survey method for geophysical exploration, in which the variations in the earth's magnetic field are measured in two, non-parallel directions at one point in the survey area. Simultaneously, the variations in the earth's electrical field parallel to the survey line are measured at a number of points along the survey line. These measured variations are transformed to the frequency domain, and then the horizontal component of the magnetic field orthogonal to the direction of the measured electrical field is calculated. The impedance at each measurement point on the survey line is calculated as a function of frequency, and weighted averages of the impedances for predetermined frequencies using a zero phase length weight function corresponding to a low pass filter are used to calculate the subsurface conductivity distribution.

Abstract: A method for removing the effects of said subsurface anomalies in magnetotelluric survey data is disclosed wherein resistivity curves are compared and a correction factor is determined. Resistivity curves having a deviation from a mean are multiplied by this correction factor.

Abstract: In magnetotelluric exploration, channels of measurements of the earth's electric and magnetic field are digitized. Blocks of digital samples are transformed into frequency domain representations of the power spectra, including amplitude and phase, at selected harmonics. The coherence between the power spectra from different channels is determined and compared to a threshold. The power spectra which have a coherence exceeding the threshold are stacked and recorded on magnetic tape.

Abstract: A method for removing the effects of said subsurface anomalies in magnetotelluric survey data is disclosed wherein resistivity curves are compared and a correction factor is determined. Resistivity curves having a deviation from a mean are multiplied by this correction factor.

Abstract: A subsurface electromagnetic sensing device for use in boreholes and mine shafts which includes a sonde for lowering underground which is supercooled and contains Squid gradiometers for measuring vector current density, and magnetometers for measuring vector magnetic fields. When these measured values are combined, information is obtained to construct a conductivity model of the underlying and surrounding subsurface region.

Abstract: A method of magnetotelluric exploration is disclosed which features multiple measurements of the earth's electric field at a plurality of locations and one or more magnetic field measurements being made simultaneously. A specific form of cable for connecting measurement apparatus with recording apparatus is also disclosed.