Ground Penetrating Radar
The ground penetrating radar (GPR) methodology is a very high frequency EM technique used to produce high-resolution images from the subsurface. GPR is used for both target detection (i.e. detecting abandoned USTs, utilities, drums, burials, archaeological artifacts, etc.) and for characterizing the subsurface stratigraphy, water-table, or geology in hydrogeological, environmental, and geotechnical site characterizations. When used at the same site, but surveyed over time, GPR is also an effective methodology for monitoring contamination migration from the surface.
Time Domain Electromagnetic
Manufactured by Geonics, Inc., the EM-61 is a time-domain electromagnetic instrument was developed to find unexploded ordinances (UXOs) and other buried metal targets in environments where there may be a lot of interference from surface metal and overhead power lines. According to tests conducted by the manufacturer and reports from other geophysical service providers, the EM-61 has a sensitivity sufficient to detect metal objects as small as a few centimeters.
Magnetometry- Total Field and Gradiometer
The magnetic method works on the principle that ferrous (iron-steel) objects cause localized disturbances (anomalies) in the earth's total magnetic field that are measurable with the magnetometer. Iron or steel objects may be detected to depths as great as 25 feet, depending upon the magnetic mass of the target(s) in question and the type of magnetometer used.
RSI uses a simple EM induction hand-held pipe and cable locator to detect buried utilities and cables. This tool operates in 4 different modes: (1)Receiver only mode for detecting 60 cycle power (or harmonics thereof), (2)Direct Contact/Clamp mode where the transmitter hooks the positive lead directly to the pipe or cable while the negative lead is grounded, (3)Direct-coupled/ Inductive mode where an inductive clamp is place around the pipe or cable, or the transmitter placed over the inferred trend of the buried utility, and (4) Remote beacon mode where a transponder beacon is placed on a fiberglass rod and threaded through non-conductive pipes and traced from the surface using a receiving antenna.
The electrical resistivity method is one of the oldest geophysical methods, originally designed in the 1920s for mineralogical prospecting by the Schlumberger Company in France. Since then, electrical resistivity methods have been used to find potable groundwater supplies, trace contamination as it migrates through the saturated zone, determine soil resistivity for purposes of designing electrical substations, grounding arrays, and estimating pipeline corrosion. Electrical resistivity is used to detect shallow structures and subtle changes in soil apparent resistivity, which also makes it conducive for using it in archaeology surveys.
Refraction is a geophysical method frequently used for surveying depth to bedrock and investigating groundwater and/or a bedrock water supply queries. Seismic refraction provides density information of subsurface layers. Seismic refraction is based on the principle that the rate at which acoustic energy (i.e. sound waves) travel through the ground, refract along the surface of higher velocity layers, and first arrive at a geophone (i.e. a sensitive device that measures three-component movement of the ground as the acoustic energy moves past it), is proportional to the density of the velocity layers.
Electromagnetic Terrain Conductivity- EM-31
The terrain conductivity survey is typically conducted using a Geonics Model EM31-DL Terrain Conductivity Meter, such as the one shown below. This induction-type instrument measures terrain conductivity without electrodes or direct soil contact. This technique operates on the principle that secondary electric and magnetic currents can be induced in metal objects and conductive bodies, such as USTs, sludge, and leachate when an electric field is applied.
Gravity is a methodology used to differentiate between materials of differing densities. In Environmental and Geotechnical projects, gravity can by used to differentiate between bedrock and unconsolidated materials, find caverns, septic tanks, and other voids, as well as delineate landfill from native soil. In Archaeological applications, Gravity can be used to detect small void spaces, such as from tombs. In many instances, gravity may be preferable to, and obtain more accurate data than, Seismic Refraction, especially in urban environments where type of source used, noise from vehicular traffic, presence of pavement, and lack of space may be issues.
Very Low Frequency Surveying
The VLF method uses very low frequency EM signals propagating from external sources, such as from military radio stations located throughout the United States and internationally, to locate water bearing fractures and/or otherwise conductive mineralization or contaminant zones in bedrock. The government-sponsored stations propagate signal, up to 1000 Watts at frequencies varying between 15 to 30 KHz. These signals propagate well along water-filled or mineralized fractures that are oriented in strike with these transmitting base stations.