Electromagnetic (EM) Surveys

Electromagnetic induction (EM), as the name implies, uses the principle of induction to measure the electrical conductivity of the subsurface. A primary alternating electric current of known frequency and magnitude is passed through a sending coil creating a primary magnetic field in the space surrounding the coil, including underground. The eddy currents generated in the ground in turn induce a secondary current in underground conductors which results in a alternating secondary magnetic field, that is sensed by the receiving coil. The secondary field is distinguished from the primary field by a phase lag. The ratio of the magnitudes of the primary and secondary currents is proportional to the terrain conductivity. The depth of penetration is governed by the coil separation and orientation.

Unlike conventional resistivity techniques, no ground contact is required. This eliminates direct electrical coupling problems and allows much more rapid data acquisition. For shallow profiling (up to 20 feet), a Geonics, Inc. EM-31 Terrain Conductivity meter is used. One person can collect as many as 10,000 data points per day with this instrument. An EM-34 is used for depths of investigation between 30 and 180 feet. This instrument requires two people to operate, and up to 500 data points per day can be collected under good conditions. These tools are extremely sensitive and accurate, capable of detecting variations in conductivity of as little as 3%. Data are automatically stored in an electronic data logger for later transfer to a computer.

The EM-61 instrument is a high resolution, time-domain device for detecting buried conductive objects. It consists of a powerful transmitter that generates a pulsed primary magnetic field when its coils are energized, which induces eddy currents in nearby conductive objects. The decay of the eddy currents, following the input pulse, is measured by the coils, which in turn serve as receiver coils. The decay rate is measured for two coils, mounted concentrically, one above the other. By making the measurements at a relatively long time interval (measured in milliseconds) after termination of the primary pulse, the response is nearly independent of the electrical conductivity of the ground. Thus, the instrument is a super-sensitive metal detector. Due to its unique coil arrangement, the response curve is a single well defined positive peak directly over a buried conductive object. This facilitates quick and accurate location of targets. Conductive objects, to a depth of approximately 10 feet can be detected.

The EM 31 uses an alternating electromagnetic field, which fills the space, below and above ground, surrounding the transmitting coil. When the electromagnetic field couples with a conductor, for example a steel pipe under the ground, AC eddy currents are induced to flow in the pipe. This generates a secondary magnetic field, which is sensed by the co–planar (12’ offset) receiver coil. Due to phase lag the computer on board can discriminate between the primary and secondary fields and outputs the measurements of the secondary field (thus, a conductive zone is
sensed by the induced secondary magnetic field).

Gravity Surveys

State–of–the–art gravity meters can sense differences in the acceleration (pull) of gravity to one part in one billion. Measurements taken at the Earth’s surface express the acceleration of gravity of the total mass of the Earth but because of their high sensitivity the instruments can detect mass variations in the crustal geology. For example a high angle, basin and range type fault will have older consolidated rocks on one side and relatively unconsolidated valley fill sediments on the other side of the fault. Mass is volume x density, and there is a density contrast in the order of 0.5 gm/cc across the basin and range fault, therefore the gravity field will express the position of the fault, in the high gradient zone, between the mountain and the valley. The amplitude of the variation from the high to the low of the gravity gradient zone is a function of the displacement on the fault. In addition to providing insights to fault problems, gravity methodology applies to any geologic problem involving mass variations

Ground Penetrating Radar (GPR) Surveys

Ground Penetrating Radar (GPR) is used to pinpoint the location of buried objects. Unlike conventional metal detectors, radar can locate both metal and nonmetal objects. It can also locate void spaces. A radar record provides a permanent detailed picture of the size, location and depth of an object.The GPR instrument beams energy into the ground from its transducer/antenna in the form of electromagnetic waves. A portion of this energy is reflected back to the antenna at any boundary in the subsurface, across which there exists an electrical contrast. The recorder continuously records an image of the reflected energy as the antenna is traversed across the ground surface. The EM wave travels at a velocity unique to the material properties of the ground being investigated and when these velocities are known, or closely estimated from ground conductivity values and other information, two–way travel times can be converted to depth measurements. Penetration into the ground and resolution of the image produced are a function of ground electric conductivity and dielectric constant. Images tend to be graphic, even at considerable depth, in dry sandy soils, but penetration and resolution is limited in drastically more conductive, moist clayey soil.

Seismic Downhole and Crosshole Surveys This kind of survey measures geological boundaries and rock velocities in the vicinity of boreholes. Compression and shear waves are transmitted from a source on the surface to a receiver in the adjacent borehole (Downhole Survey) or from a source located in another borehole (Crosshole Survey).

Downhole Survey - A sturdy hardwood board is laid beside the wellhead, and a downhole seismic, three – component, clamping geophone is lowered into the hole. It is then clamped at intervals, commonly five feet, and at each interval the board is hammered. First it is hammered on one of its ends and a record is made. Then the procedure is repeated at the other end. This is necessary to identify the shear wave arrival by virtue of the two arrivals having opposite polarity. The geophone is then lowered, incrementally, down the borehole and more measurements are done. By this process the compression and the shear waves’ velocities are determined at intervals as a function of depth in the vicinity of the borehole. Crosshole Survey – The difference between this survey and the Downhole one is that in the Crosshole approach there are two boreholes. A downhole seismometer is in one of the holes and the shear wave source in the other. This approach is a little more definitive for acquiring information on discrete layers. The information applies to a little larger volume of sediment/rock between the holes as opposed to the near vicinity of one borehole.

Very Low Frequency (VLF) Surveys

Governments with naval forces have established a grid of tall, high – powered transmitters, up to 1000 watts, that broadcast a signal field in the 15 to 28 kHz frequency range. These broadcast fields propagate thousands of miles over the Earth’s surface and are essentially uniform in the
atmosphere. Due to their power the signals penetrate into the ground to depths of several hundred feet. Because of the high material properties contrast at the ground/air interface, the signals are refracted down into the ground at steep angles. Since the Earth is relatively less homogeneous then the atmosphere, the EM flux crowds into zones of higher conductivity and rarefies in zones of higher resistivity. A VLF receiver tuned to the frequency of a VLF transmitter, traversed across the Earth’s surface, will exhibit high signals over conductive water–
bearing fracture zone, for example, and a low strength signals over the resistive portions of the crystalline rock mass. Any linear conductive body, in addition to water– bearing fractures, can also be detected by VLF methodology