Geophysics

ERT started in 1996 primarily as a geophysics services company. Our expertise in the field and our long running experience in the mid-atlantic regions have made us the “go to” organization for geophysical services.

Ground Penetrating Radar

Ground Penetrating Radar (GPR) is a device consisting of a transmitter and receiver antenna that are pointed into the earth in order to detect materials of differing electrical properties than the surrounding matrix (usually soil or concrete). Various frequencies of electromagnetic signal ranging from 50 MHz to 1500 MHz are used. The lower the frequency, the greater the penetration but the lower the resolution. Frequencies in the range of 200 to 900 MHz are useful for detecting Underground Storage Tanks (UST’s), pipes, utilities, buried drums, graves, and under certain circumstances contaminated soil. Higher frequencies are useful for mapping rebar and utilities within concrete, and for performing pavement evaluations. One advantage of GPR over other techniques is that the data can be displayed and interpreted almost instantly if necessary – other techniques require various degrees of processing.

Seismic Refraction

Seismic refraction is a commonly used technique for determining the depth to bedrock and the "rock velocity" (speed at which compressional waves, or P-waves, travel through the rock). The equipment consists of a seismograph, vibration sensors called geophones, a seismic source such as a sledgehammer, and cables connecting these components. For environmental applications, typically 12 to 48 geophones are laid out in a straight line called a spread or array, and the source is used to produce seismic energy at several points along the spread. The seismograph records the signals from the geophones, and this information can be processed to produce profiles of the shallow subsurface in the form of layer models (showing the depth to the top of the weathered bedrock and/or the deeper, competent bedrock) or contoured P-wave velocity models. This information in turn can be used to predict the rippability of the various layers for excavation cost estimation.

Multichannel Analysis of Surface Waves

Multichannel Analysis of Surface Waves (MASW) is another seismic technique that was developed by the Kansas Geologic Survey in the late 1990's. The equipment is essentially the same as that used for Seismic Refraction (above), but the data acquisition is more time- consuming and thus more costly. The results are presented in the form of contoured shear-wave (S-wave, as opposed to P-wave) velocity models. The penetration is somewhat greater than that gained by seismic refraction, but resolution decreases with depth. Because the technique relies on lower frequencies of seismic waves, it is less sensitive to higher frequencies of noise, which can be a serious problem when attempting to acquire seismic refraction data. This is the main advantage of MASW over seismic refraction.

Electrical Resistivity

Resistivity is a technique that involves injection of electrical current into the ground via electrodes at one or more points and measuring the voltage at one or more points to calculate the resistance of the earth. Resistivity data can be acquired in the orm of one-dimensional "soundings" using two to four electrodes set up at various geometries, or two-dimensional tomographic profiles using 20 or more electrodes and automated controls. Soundings are relatively fast and cheap, and can be used to document the earth’s electrical properties for purposes of construction of radio or cell towers, or to estimate the depth to ground water. Tomographic profiles show a contoured model of the resistivity of the subsurface, usually in Ohm-meters or Ohm-feet. The profile can show bedrock lows and highs as well as air-filled or clay- filled voids, particularly in karst areas. Tomography can rarely be used in urban areas due to the presence of electrical utilities which greatly interfere with the signals produced by the equipment.

Microgravity Surveys

This technique employs a device called a gravimeter to passively measure the gravitational field strength in the units of milliGals at points along a transect or on a grid. These devices are extremely sensitive, delicate, and expensive. The results are displayed in the form of simple graphs or as contour maps. This technique can be used to search for voids, faults, bedrock highs and lows, changes in lithology, to map buried valleys in glacial areas, and to estimate the density of materials in the shallow subsurface. The technique is somewhat time-consuming because a single reading takes several minutes, and each station must be precisely surveyed for elevation. For this reason, it is usually cost- effective to combine it with some other, faster technique such as magnetics, electromagnetics, or GPR.

Magnetics

This technique passively measures the existing magnetic field, usually in units of nannoTeslas (nT) at points along a transect or on a grid. The data are acquired with a magnetometer (which simply records the field at the point of the instrument) or a gradiometer (consisting of two sensors separated by a fixed vertical distance, and the two signals are subtracted to obtain the magnetic gradient). This technique is useful for searching for objects of ferrous metal, or utilities. Magnetic data are most frequently presented in the form of a contour map, showing magnetic anomalies of various magnitudes. Anomalies may correspond with Underground Storage Tanks (USTs), buried metal, Unexploded Ordnance (UXO), or even ore bodies. An advantage of magnetics over other techniques is that it can be used to cover a large area relatively quickly.

Electromagnetics

This technique employs a wide variety of devices that actively produce electromagnetic signals and then measure the response of the earth to those signals. These devices can be used for such diverse applications as utility location, ground water studies, contaminant mapping, mineral exploration, geologic mapping, landfill delineation, and karst feature mapping. Many devices can be interfaced with Global Positioning System (GPS) in order to perform high-resolution data acquisition. An advantage of electromagnetics over other techniques is that some devices can be used to cover a large area relatively quickly.

Spontaneous Potential

Spontaneous Potential, also known as Self-potential and Natural Potential and abbreviated SP or NP, is a measure of naturally occurring electrical voltage between two points on the surface of the earth. Natural currents in the earth are caused by the flow of heat or ions within the earth. In the shallow subsurface the current is usually caused by ground water movement. The equipment is inexpensive, consisting of a digital mulimeter capable of averaging voltage readings over several minutes, wire, non-polarizing liquid-junction electrodes, and a clock. Measurements along a transect or on a grid can be presented as graphs or contour maps, with anomalies indicating areas of ground water movement. Overhead power lines and utilities are a source of considerable interference. The technique is somewhat time-consuming because a single reading takes several minutes. For this reason, it is usually cost-effective to combine it with some other, faster technique such as magnetics, electromagnetics, or GPR.

Borehole Logging

Various geophysical tools have been developed for extracting information from boreholes, in the form of some parameter versus depth. Many are actually variants of the techniques discussed above, such as SP and resistivity, where electrodes are in contact with the walls of the borehole. Induction tools are similar to electromagnetic devices mentioned above. Other devices include gamma ray and neutron density tools for stratigraphic correlation and porosity determination. Sonic tools measure the P-wave velocity. Data such as this is useful in its own right, but it can also be useful for calibrating other geophysical data acquired at the surface near the borehole.

Crosshole Seismic Techniques

Geophysical tools may be employed to gather data between boreholes. Seismic data can be collected by placing geophones at intervals down one or more boreholes, and setting off explosive charges at various depths in another borehole nearby. Similar to seismic refraction (see above), the arrival of seismic waves at the geophones at different times allows one to determine the rock velocity between the boreholes, and possibly detect the presence of voids. The equipment is somewhat similar to that of seismic refraction, but is more specialized and expensive. The technique is obviously most cost-effective in areas where numerous boreholes already exist.