Abstract: A power detection regulation device including a power detection signal generator, a power state detector and a regulated output unit is disclosed. The power detection signal generator receives the input power from an external power supply and generates a power detection signal. The power state detector generates a power state signal based on the power state derived from the power detection signal. The regulated output unit receives the power state signal and generates a driving signal to an external electrical device in accordance with the feedback signal from the external electrical device. The power state signal is provided for the external electrical element to perform relevant processes, and the regulated output device can output the predetermined driving signal on receiving the power state signal indicating some abnormal situation in the input power so as to maintain the normal operation performed by the actuating element in the external electrical device.

Abstract: The invention relates to instruments designated for geophysical survey, in particular for soil mass exploration using electromagnetic waves. Radio frequency assisted geostructure analyzer comprising transmitting antenna and radio transmitter (2) installed on the first pillar (3), with transmitting antenna consisting of transmitting loop (1) and antenna rod (10), and also receiving loop (4) and radio receiver (7) installed on the second pillar (9), and receiving ferrite antenna (5).

Abstract: A method for removing the effects of an airwave from marine electromagnetic data comprising providing an electromagnetic source and at least one receiver in the water; measuring the electromagnetic response at a first source-receiver separation; determining a scaled version of the airwave response at a source-receiver separation where the earth response is negligible and using the scaled airwave response to determine the earth response measured at the first separation. Using this method, an improved estimate of the earth's response can be achieved.

Abstract: A spot indicating metal detector apparatus rides on a motorized platform having a rotary framework having a plurality of metal detectors mounted thereon with a plurality of metal detector coils positioned adjacent the ground below. The metal detector apparatus has a plurality of spray heads each associated with one metal detector detector coil for spraying a fluid onto the earth at the position the metal detector detects the presence of a metallic object.

Abstract: A method of monitoring a behavior of carbon dioxide in a stratum by using a marine Controlled-Sources Electromagnetic survey, includes: (S10) collecting a sample of sandstone at a stratum for charging carbon dioxide; (S20) calculating an effective porosity of the collected sample; (S30) measuring an electrical resistivity by saturating the collected rock sample with a pore fluid having a different concentration; (S40) forecasting a change of an electrical resistivity of stratum by charging carbon dioxide; (S50) carrying out a Marine CSEM exploration before and after charging the carbon dioxide separately in each step; and (S60) monitoring a charging behavior of carbon dioxide in stratum by a Marine CSEM exploration.

Abstract: A method for determining a formation type within a wellbore includes forming the wellbore such that the wellbore intersects fractures within a formation. Hydrocarbons are produced from the formation. An induction tool is disposed into the wellbore after producing the hydrocarbons. A resistivity of a portion of the formation is measured with the induction tool. The resistivity of the portion of the formation is compared with a known formation type. The formation type of the portion of the formation is determined based on the resistivity. A characteristic of the formation type is output.

Abstract: A method of surveying the condition of an underground enclosure including the steps of (a) positioning at least one transmitter/receiver unit (including an antenna) within an underground, substantially nonconductive enclosure, such that a substantial air gap exists between the antenna and the inner wall of the enclosure; (b) transmitting an ultra wideband (UWB) signal toward at least a portion of the inner wall; and (c) processing the return signal in order to identify the interface between the soil and a region of conductivity different from the soil.

Abstract: A system for monitoring movement in a subsurface environment, which may be used to determine flow dynamics within a fluid mass such as an ore body, or track subsurface persons or moving assets. A plurality of underground positioning system (UPS) elements in the subsurface environment transmit characteristic signals to a plurality of antennas, which transmit the signals to a data processing apparatus. The system thus determines changes in the positions of the UPS elements to derive an indication of the motion of the fluid mass or the locations and movement of subsurface persons or assets.

Abstract: Methods and apparatus for detecting nearby conductors such as pipes, well casing, etc., from within a borehole. A nearby casing string can be detected by transmitting an electromagnetic signal from a transmit antenna on a downhole logging tool and measuring a parallel response signal with a first receive antenna parallel to the transmit antenna and a perpendicular response signal with a second receive antenna perpendicular to the transmit antenna, both receive antennas on the downhole logging tool. As the tool rotates, the transmitting and measuring are repeated to determine the azimuthal dependence of the parallel and perpendicular response signals. The azimuthal dependence is analyzed to determine a diagonal component of said azimuthal dependence for each response signal. Distance to a casing string can be estimated using the diagonal component of each response signal. At least one of the antennas is preferably tilted.

Abstract: A transient electromagnetic (TEM) signal is recorded in an earth formation during vibration of a logging tool. Concurrently, accelerometer measurements are made. The accelerometer measurements are used to correct the TEM signal for the vibration using the accelerometer measurement and a transfer function relating the TEM signal and the accelerometer. The transfer function may be determined using measurements in a water tank or by using the tail end of the TEM measurements.

Abstract: A method for generating electromagnetic waves in the ELF/ULF comprising the steps of using a ground-based Horizontal Electric Dipole (HED) antenna to send electromagnetic pulses upwardly in the E-region of the ionosphere to form an oscillatory or pulsed electric field; allowing said pulsed electric field to interact with magnetized plasma of the lower ionosphere to generate a pulsed horizontal and vertical current which have associated Horizontal and Vertical Electric Dipole moment; and allowing them to radiate.

Abstract: A method for electromagnetic geophysical surveying of rock formations (1) under a sea-floor (3) comprising the following steps: *—towing first and second alternating field (E1, E2) emitting sources (s1, S2) in first and second depths below the sea surface, said first field (E1) having a first phase (?1);—said second alternating field (E2) given a second phase (?2) different from said first phase (?1), said sources (S1, S2) constituting a phased array emitter antenna with directivity for transmitting a major proportion of the combined electromagnetic energy downwards;—said first and second fields (E1, E2) for propagating partly down through the sea-floor (3) and being reflected and/or refracted through said rock formations (1) and partly propagating back through the seafloor (3);—said first and said second fields (E1, E2) for merging to a total field and being measured by electromagnetic receivers (r1, r2, . . . , rn) recording corresponding field registrations (Er1(t), Er2(t), Er3(t), . . . , Ern(t)).

Abstract: A retrievable tool for steering through an earth formation includes a first tool assembly and a tilted antenna attached to the first tool assembly. The tool also includes a second tool assembly attached to the first tool assembly and a tilted antenna attached to the second tool assembly. The first tool assembly attaches to the second tool assembly so that the antennas are tilted in predetermined directions. The tilted antennas are transmitter antennas or receiver antennas. Each tool assembly is a tubular cylinder with a longitudinal axis running the length of the cylinder, wherein the tubular cylinder has two ends, each end including a rotational attachment mechanism. The tool assemblies attach to each other through their rotational attachment mechanisms. The rotational attachment mechanism may be a screw-on mechanism, press-fit mechanism, or welded mechanism.

Abstract: An apparatus for detecting a position of a component in an earth formation is disclosed. The apparatus includes: a transmitter configured to emit a first magnetic field into the earth formation and induce an electric current in the component, the transmitter having a first magnetic dipole extending in a first direction; and a receiver for detecting a second magnetic field generated by the component in response to the first magnetic field, the receiver having a second magnetic dipole extending in a second direction orthogonal to the first direction. A method and computer program product for detecting a position of a component in an earth formation is also disclosed.

Abstract: A method for measuring resistivity variations in the earth comprising passing a transient current between two source electrodes; measuring the transient current at the source electrodes; measuring the resultant transient voltage between at least one pair of receiver electrodes; estimating one or more processing functions for applying to the measured input current to provide a step current profile, and applying the same one or more processing functions to the measured voltage to provide an estimate of the step response voltage between the receivers, using both the step current and the estimated step response voltage to determine the resulting apparent earth resistance, and using the apparent earth resistance to determine the resistivity of the earth.

Abstract: A method for estimating the effects of an airwave in marine electromagnetic data measured using a source and at least one receiver. The method involves measuring the electromagnetic response at two different under water source-receiver separations and using measurements at the larger separation to estimate the airwave response at that separation. The airwave effect in the response measured at the shorter separation can then be determined using the estimated airwave response at the larger separation.

Abstract: An apparatus for evaluating an earth formation, the apparatus including: a logging tool configured to be conveyed through a borehole penetrating the earth formation; a coil inductively coupled to the earth formation, the coil being disposed at the logging instrument; and a circuit coupled to the coil wherein the circuit and the coil form an oscillator circuit, the oscillator circuit being configured to oscillate when a circuit parameter of the oscillator circuit satisfies an oscillation criterion, the circuit parameter being related to a property of the earth formation.

Abstract: A retrievable tool for steering through an earth formation includes a first tool assembly and a tilted antenna attached to the first tool assembly. The tool also includes a second tool assembly attached to the first tool assembly and a tilted antenna attached to the second tool assembly. The first tool assembly attaches to the second tool assembly so that the antennas are tilted in predetermined directions. The tilted antennas are transmitter antennas or receiver antennas. Each tool assembly is a tubular cylinder with a longitudinal axis running the length of the cylinder, wherein the tubular cylinder has two ends, each end including a rotational attachment mechanism. The tool assemblies attach to each other through their rotational attachment mechanisms. The rotational attachment mechanism may be a screw-on mechanism, press-fit mechanism, or welded mechanism.

Abstract: To survey a subterranean structure, for acquiring an electromagnetic (EM) measurement signal across EM receivers having a target axial spacing, a group of more than two EM receivers in an interval defined by the target axial spacing along a sensor cable is provided. The spacing between successive ones of at least some EM receivers in the group is less than the target axial spacing. EM measurements are acquired using the EM receivers in the group.

Abstract: To perform marine electromagnetic (EM) surveying of a subterranean structure, a marine cable system is provided including a tow cable, a plurality of electromagnetic (EM) sources coupled to the tow cable, and a plurality of EM receivers coupled to the tow cable. The system is configured for deployment in a body of water to perform marine EM surveying of a subterranean structure.

Abstract: A method for designing a controlled-source electromagnetic survey that will discriminate between a defined deep marginal-interest reservoir (2) and specified false positive resistivity structures of concern (3, 4, 5). A reservoir model and a false positive model are constructed for each false positive scenario. The resistivity of the false positive model may be tuned to give electromagnetic data similar enough to the reservoir model when forward modeled that any differences fall in the model null space. A null-space discriminating ratio (“NSDR”) is defined, for example as the peak normalized difference of the two related modeled electromagnetic field data sets. An area coverage display of NSDR values (6) allows determination of such additional data as may be needed to distinguish the false positive body, and a survey design is developed accordingly (7). Reduction of the number of variables affecting the area coverage displays is a key feature of the method.

Abstract: An eddy current system and method enables detection of sub-surface damage in a cylindrical object. The invention incorporates a dual frequency, orthogonally wound eddy current probe mounted on a stepper motor-controlled scanning system. The system is designed to inspect for outer surface damage from the interior of the cylindrical object.

Abstract: A method for electromagnetic surveying subsurface formations includes inducing an electromagnetic field in the subsurface formations by passing electric current through a transmitter. Response of the subsurface formations to the induced electromagnetic field is detected at a first plurality of spaced apart positions disposed longitudinally within a bipole length of the transmitter. A direct induction response is removed from the detected response.

Abstract: A system for offshore hydrocarbon electromagnetic prospecting is described. The system includes a transmitter generating electromagnetic energy and injecting an electrical current into a flooded vertical cable. The circulated induced vertical current time derivative's response generated by this current in the medium is measured by a circular chain of magnetometers. The measured response which is sensitive to the resistivity of targets is proposed to be used to search for and identify hydrocarbons reservoirs.

Abstract: To perform marine electromagnetic (EM) surveying of a subterranean structure, a marine cable system is provided including a tow cable, a plurality of electromagnetic (EM) sources coupled to the tow cable, and a plurality of EM receivers coupled to the tow cable. The system is configured for deployment in a body of water to perform marine EM surveying of a subterranean structure.

Abstract: The instrument utilizes at least one electromagnetic signal transmitter and at least one electromagnetic signal receiver with the receiver detecting a receiver signal responsive to the transmitter signal and indicative of subterranean details adjacent to the instrument. The instrument is attachable to a tow vehicle for transport and includes a GPS antenna and tags data gathered by the instrument with GPS position information. The instrument is formed in modular sections which can be removably attached for collapsing into a smaller space and to allow for flexible configuration of the instrument in various different ways. A wheeled carriage is provided to carry the instrument over the ground. Antennas for the transmitter and receiver, as well as circuitry and cooling systems are all provided to supply the instrument with high resolution and a clear signal indicative of the position and characteristics of subterranean details of interest.

Abstract: This invention is directed to a downhole method and apparatus for simultaneously determining the horizontal resistivity, vertical resistivity, and relative dip angle for anisotropic earth formations. The present invention accomplishes this objective by using an antenna configuration in which a transmitter antenna and a receiver antenna are oriented in non-parallel planes such that the vertical resistivity and the relative dip angle are decoupled. Preferably, either the transmitter or the receiver is mounted in a conventional orientation in a first plane that is normal to the tool axis, and the other antenna is mounted in a second plane that is not parallel to the first plane. This invention also relates to a method and apparatus for steering a downhole tool during a drilling operation in order to maintain the borehole within a desired earth formation.

Abstract: To provide a vehicle which can move independently as an own independent vehicle and can run integrally in linkage with another vehicle. A plurality of single-seat vehicles which can move independently are combined and they move integrally while a predetermined formation is maintained through linkage among respective vehicles. One of all the vehicles moving in linkage serves as a host vehicle and an occupant in the host vehicle becomes a driver in the linkage moving. A host vehicle 1 runs with speed/direction according to running operation conducted by an occupant. Simultaneously therewith, the host vehicle 1 instructs following vehicles 2 to 4 to synchronize (follow) the host vehicle. The host vehicle 1 transmits a speed, a direction, and a relative position to the host vehicle to the following vehicles 2 to 4 as moving information in order to synchronize the following vehicles with the host vehicle (maintain linkage relationship).

Abstract: To perform an electromagnetic (EM) surveying of a subterranean structure, a signal generator produces a signal having an analog continuous waveform without steps in the waveform. Producing the signal is in response to an indication specifying a characteristic of the signal. An antenna is responsive to the signal having the analog continuous waveform to emit an EM field to produce the EM surveying of the subterranean structure.

Abstract: A method for electromagnetic surveying below the bottom of a body of water includes deploying a plurality of nodal recording devices in a selected pattern on the water bottom. An electromagnetic transmitter is towed in the water. At least one electromagnetic sensor streamer is concurrently towed in the water. The electromagnetic transmitter is actuated at selected times and signals detected by sensors in the nodal recording devices and in the at least one streamer are recorded.

Abstract: A method for electromagnetic exploration includes imparting a first electromagnetic signal into subsurface formations from a first location and imparting a second electromagnetic signal into the formations from a second location substantially contemporaneously with imparting the first electromagnetic signal. The first and second electromagnetic signals are substantially uncorrelated with each other. A combined electromagnetic response of the formations to the first and second imparted electromagnetic signals is detected at a third location. A response of the formations to each of the first and the second imparted signals is determined from the detected response.

Abstract: A system, method and device for interrogating a downhole environment in a borehole beneath a surface includes a source of electromagnetic energy operable to transmit an electromagnetic signal in the borehole, a sensor module, including a passive resonating circuit including a crystal oscillator having a resonant frequency that varies with changes in the condition in the downhole environment in response to a condition in the downhole environment in the borehole and a detector positionable to receive the reflected modulated electromagnetic signal. In an embodiment, a solids-free dielectric medium is provided within an annular volume in the borehole defined by the casing through which the electromagnetic signal is transmitted.

Abstract: A method for analyzing acquired electromagnetic measurements (R) made at or in a sea (4) over a seafloor (1) with rock formations (3) having relatively low resistivity (?3) for detecting a possibly underlying petroleum bearing reservoir formation (2) having relatively high resistivity (?2), wherein a low frequency electromagnetic transmitter (5) arranged in the sea (4) emits an electromagnetic field (P) propagating in the sea (4), in the rocks (3, 2) and in the air (0) above the sea; wherein electromagnetic sensors (6) are arranged with desired offsets (x) in the sea (4) for measuring the electromagnetic field (P(x)) while the field propagates, characterized in that one or more component of the electromagnetic field (P) is measured at least one large offset (xL) from the transmitter (5) where the field (P) essentially only has its origin from the field propagating as a field (P0) through the air (0); that the one or more components of the electromagnetic field (P) measured at the large offset (xL) is calculat

Abstract: A system, method and device for interrogating a downhole environment in a borehole beneath a surface includes a source of electromagnetic energy, operable to transmit an electromagnetic signal in the borehole, a sensor module, including a passive resonating circuit including a crystal oscillator having a resonant frequency that varies with changes in the condition in the downhole environment to reflect the electromagnetic signal and to modulate the electromagnetic signal in response to a condition in the downhole environment in the borehole and a detector positionable to receive the reflected modulated electromagnetic signal.

Abstract: This invention is directed to a downhole method and apparatus for simultaneously determining the horizontal resistivity, vertical resistivity, and relative dip angle for anisotropic earth formations. The present invention accomplishes this objective by using an antenna configuration in which a transmitter antenna and a receiver antenna are oriented in non-parallel planes such that the vertical resistivity and the relative dip angle are decoupled. Preferably, either the transmitter or the receiver is mounted in a conventional orientation in a first plane that is normal to the tool axis, and the other antenna is mounted in a second plane that is not parallel to the first plane. Although this invention is primarily intended for MWD or LWD applications, this invention is also applicable to wireline and possibly other applications.

Abstract: An electromagnetic survey sensing device includes at least two electrodes disposed at spaced apart locations. An electrical to optical converter is electrically coupled to the at least two electrodes. The converter is configured to change a property of light from a source in response to voltage imparted across the at least two electrodes. The device includes an optical fiber optically coupled to an output of the electrical to optical converter, the optical fiber in optical communication with a detector.

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 analyzed 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 phase error in measurements made by a resistivity logging tool is estimated by positioning the tool above a conducting surface. Deviation of the measured phase difference between the transmitter and the receiver from 90° gives the phase error in the electronics. It is emphasized that this abstract is provided to comply with the rules requiring an abstract which will allow a searcher or other reader to quickly ascertain the subject matter of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims.

Abstract: A method for determining resistivity anisotropy of subsurface rock formations includes imparting a transient electromagnetic field into the subsurface rock formations. Electromagnetic response of the formations is measured at a plurality of offsets from a position of the imparting. For each offset, an arrival time from the imparting is determined of a peak of an impulse response such that the response is related to subsurface horizontal and vertical resistivities. For each offset, a step response of the formations is determined at a time from the imparting selected such that the step response is related substantially only to mean resistivity. The arrival time of the peak of the impulse response and the late time value of the step response are used to determine the resistivity anisotropy.

Abstract: A method for determining reservoir formation properties that consists of exciting the reservoir formation with an electromagnetic exciting field, measuring an electromagnetic signal produced by the electromagnetic exciting field in the reservoir formation, extracting from the measured electromagnetic signal a spectral complex resistivity as a function of frequency, fitting the spectral complex resistivity with an induced polarization model and deducing the reservoir formation properties from the fitting with the induced polarization model.

Abstract: A method of surveying the condition of an underground enclosure including the steps of (a) positioning at least one transmitter/receiver unit (including an antenna) within an underground, substantially nonconductive enclosure, such that a substantial air gap exists between the antenna and the inner wall of the enclosure; (b) transmitting an ultra wideband (UWB) signal toward at least a portion of the inner wall; and (c) processing the return signal in order to identify the interface between the soil and a region of conductivity different from the soil.

Abstract: To determine effect on a magnetic field caused by a lining structure in a wellbore, an array may be deployed into the wellbore lined with the lining structure. The array comprises a plurality of sensors including sensor A configured to operate as a transmitter, sensor B configured to operate as either a transmitter or a receiver, and sensor C configured to operate as a receiver. The array measures magnetic fields using sensor B as a receiver and sensor C in response to activation of sensor B as a transmitter and sensor A. A plurality of lining structure correction factors can be calculated based on the measured magnetic fields, based on the reciprocity of the sensors.

Abstract: A method and apparatus for sensing conditions in a subsurface geologic formation heated for the extraction of hydrocarbons is disclosed. A time domain reflectometer in conjunction with an open wire transmission line is employed in real time to determine impedance discontinuities in the geologic formation. These impedance discontinuities correspond to physical conditions in the geologic formation. The open wire transmission line can include pipes running into the subsurface geologic formation or other conductors, including a split tube well casing. The method may operate in the low frequency window for subsurface electromagnetic propagation.

Abstract: A ground conductivity meter that includes a transmitter coil and a receiver coil that are horizontally spaced apart from each other, and a conductivity meter controller connected to the transmitter and receiver coils and including an electronic storage element and at least one processor, the conductivity meter controller being operative to: determine a first conductivity reading in dependence on signals from the receiver coil when the transmitter coil and receiver coil are positioned a predetermined distance above a ground surface in one of a vertical dipole orientation or a horizontal dipole orientation; determine a second conductivity reading in dependence on signals from the receiver coil when the transmitter coil and receiver coil are positioned the predetermined distance above the ground surface in the other of the vertical dipole orientation or horizontal dipole orientation; calculate a correction factor in dependence on the first and second conductivity readings and store the correction factor in the s

Abstract: A method for marine geophysical surveying according to one aspect of the invention includes towing at least one geophysical sensor streamer in a body of water. The streamer includes a plurality of spaced apart electromagnetic field receivers disposed at spaced apart locations along the streamer. The streamer also includes a plurality of seismic sensors disposed at spaced apart locations. The seismic sensors each include at least one pressure responsive receiver and at least one particle motion responsive receiver. At selected times, a seismic energy source is actuated in the water. Particle motion and pressure seismic signals, and electromagnetic field signals are detected at the respective receivers.

Abstract: A method is provided for processing marine controlled source electromagnetic data. Inline and broadside marine controlled source electromagnetic data are provided, for example by means of one or more horizontal electric dipoles and one or more receivers disposed in a water column above a subsurface to be surveyed. A linear combination of the inline and broadside data is formed so as to reduce the airwave content.

Abstract: Systems and methods for performing bed boundary detection and azimuthal resistivity logging with a single tool are disclosed. Some method embodiments include logging a borehole with an azimuthally-sensitive resistivity logging tool; deriving both a resistivity log and a boundary detection signal from measurements provided by said tool; and displaying at least one of the boundary detection signal and the resistivity log. The resistivity log measurements may be compensated logs, i.e., logs derived from measurements by one or more symmetric transmitter-receiver arrangements. Though symmetric arrangements can also serve as the basis for the boundary detection signal, a greater depth of investigation can be obtained with an asymmetric arrangement. Hence the boundary detection signal may be uncompensated.

Abstract: A system for marine electromagnetic surveying of hydrocarbon reservoirs is proposed. The system proposed is characterized by high sensitivity to targets containing hydrocarbons and an ability to work in shallow and deep waters. The system includes a transmitter setting up current pulses in water (2) through a submerged, vertical or horizontal transmitter cable (7a, 7b, 8) and a registration subsystem (9) connected to electrodes (11) on vertical or horizontal receiver cables (10a, 10b). The transmitter generates a special sequence of sharply terminated pulses of the electric current, the electric field being measured in the water in the pauses between these pulses. The straight line through the receiver electrodes lies in the same vertical plane as the terminations of the transmitter cable (7a, 7b).

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 analyzed 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 system for detection of linear underground anomalies passing under surface roads comprises an electromagnetic (EM) gradiometer mounted on a vehicle trailer. A transmitter is mounted to the front bumper of a car or vehicle towing the trailer and provides carrier synchronization information to the EM-gradiometer. An opportunistic radio station can be used as an illuminator. The transmitter or ground wave from an opportunistic radio station directs radio waves down into the ground where objects like linear underground anomalies and their equipment will produce reflections and scattered waves. These reflections will have phase angles and magnitudes that can be interpreted for characterizing information about the linear underground anomalies. Each EM-gradiometer measurement is tagged with GPS location information and then stored in a database. Subsequent passes over the same roadways and tracks are compared (change detection) to the earlier stored data.