Ground Penetrating Radar
- Direct-Push Technologies
- Fiber Optic Chemical Sensors
- Gas Chromatography
- High-Resolution Site Characterization (HRSC)
- Infrared Spectroscopy
- Laser-Induced Fluorescence
- Mass Flux
- Mass Spectrometry
- Open Path Technologies
- Passive (no purge) Samplers
- Test Kits
- X-Ray Fluorescence
Ground penetrating radar (GPR) is a geophysical method that has been developed for shallow, high-resolution, subsurface investigations of the earth. GPR uses high frequency pulsed electromagnetic waves (generally 10 MHz to 1,000 MHz) to acquire subsurface information. Energy is propagated downward into the ground and is reflected back to the surface from boundaries at which there are electrical property contrasts (click to see a schematic diagram of the process). GPR is commonly used for environmental, engineering, archeological, and other shallow investigations. As with most geophysical techniques, the results are non-unique and should be compared with direct physical evidence.
GPR is used to map geologic conditions that include depth to bedrock, depth to the water table, depth and thickness of soil and sediment strata on land and under fresh water bodies, and the location of subsurface cavities and fractures in bedrock. Other applications include the location of objects such as pipes, drums, tanks, cables, and boulders, mapping landfill and trench boundaries, mapping contaminants, and conducting archeological investigations.
Integration of GPR data with other surface geophysical methods, such as seismic, resistivity, or electromagnetic methods, reduces uncertainty in site characterization.
GPR is now a widely accepted field screening technology for characterizing and imaging subsurface conditions. The American Society for Testing and Materials (ASTM) has an approved Standard Guide for Using the Surface Ground Penetrating Radar Method for Subsurface Investigation.
Useful EPA and USACE resources for geophysical methods include:
- EPA/625/R-92/007 Use of Airborne, Surface, and Borehole Geophysical Techniques at Contaminated Sites
- EPA/600/R-92/089 A Study to Determine the Feasibility of Using a Ground Penetrating Radar for More Effective Remediation of Subsurface Contamination
- EPA/625/R-93/003a Subsurface Characterization and Monitoring Techniques A Desk Reference Guide Volume I: Solids and Ground Water Appendices A and B.
- USACE, EM1110-1-1802, Geophysical Exploration for Engineering and Environmental Investigation
GPR uses high frequency pulsed electromagnetic waves (typically from 10 MHz to 1,000 MHz) to acquire subsurface information. The wave spreads out and travels downward, until it hits a buried object or boundary with different electrical properties. Then part of the wave energy is reflected or scattered back to the surface, while part of the energy continues to travel downward. The wave that is reflected back to the surface is captured by a receiving antenna and recorded on a digital storage device for later interpretation. The most common display of GPR data is one showing signal versus amplitude and is referred to as a trace. A single GPR trace consists of the transmitted energy pulse followed by pulses that are received from reflecting objects or layers. Several traces from the same location are typically stacked and averaged to provide better resolution of weaker reflections. A scan is a trace where a color or gray scale has been applied to the amplitude values. As the antenna(s) are moved along a survey line, a series of traces or scans are collected at discrete points along the line. These scans are positioned side by side to form a display profile of the subsurface (Daniels 2000).
Electromagnetic waves travel at a specific velocity that is determined primarily by the electrical permittivity of the material. The velocity is different between materials with different electrical properties, and a signal passed through two materials with different permittivities over the same distance will arrive at different times. The interval of time that it takes for the wave to travel from the transmitting antenna to the receiving antenna is simply called the transit time. The basic unit of electromagnetic wave travel time is the nanosecond (ns), where 1 ns=10-9 s. Since the velocity of an electromagnetic wave in air is 0.3 m/ns, the travel time for an electromagnetic wave in air is approximately 3.3333 ns/m traveled. The velocity is proportional to the inverse square root of the permittivity of the material, and since the permittivity of earth materials is always greater than the permittivity of the air, the travel time of a wave in a material other than air is always greater than 3.3333 ns/m. Click to see a table that shows permittivities and velocities for various earth materials.
GPR equipment utilized for the measurement of subsurface conditions normally consists of a radar control unit, transmitting and receiving antennas, and suitable data storage and/or display devices. In order to generate a sampled waveform of the reflected radar pulses, the radar control unit generates synchronized trigger pulses to the transmitter and receiver electronics in the antennas.
Antennas are transducers that convert electrical currents on the metallic antenna elements (usually simple bowtie dipole antennas) to transmit electromagnetic waves that propagate into a material. Antennas radiate electromagnetic energy when there is a change in the acceleration of the current on the antenna. Radiation occurs along a curved path, and radiation occurs anytime the current changes direction (e.g. at the end of the antenna element). Controlling and directing the electromagnetic energy from an antenna is the purpose of antenna design. Antennas also convert electromagnetic waves to currents on an antenna element, acting as a receiver of the electromagnetic energy by capturing part of the electromagnetic wave.
GPR systems are digitally controlled, and data are usually recorded digitally for post-survey processing and display. The digital control and display part of a GPR system generally consists of a microprocessor, memory, and a mass storage medium to store the field measurements. A small micro-computer and standard operating system is often used to control the measurement process, store the data, and serve as a user interface. Data may be filtered in the field to remove noise, or the raw data may be recorded and the data processed for noise remove at a later time. Field filtering for noise removal may involve electronic filtering and/or digital filtering prior to recording the data on the mass data storage medium. Field filtering should be minimized except in those cases where the data are to be interpreted immediately after recording.
The most common mode of GPR data acquisition is referred to as the reflection profiling method. In the reflection mode of operation, radar waves are transmitted, received, and recorded each time the antenna has been moved a fixed distance across the surface of the ground, in a borehole, or across any other material that is being investigated. In addition to surveys on land and ice, surveys can also be made in lakes and rivers with low conductivity water.
Three-dimensional (3D) GPR involves collecting GPR data on closely spaced (less than 1 meter) lines. Computers are then used to composite these lines into a 3D data volume that can be observed from any angle using any subset of the data.
Transillumination measurements can be used in locations, such as mines and boreholes, where the transmitter and receiver can be put on opposite sides of a medium so as to look through it. Tomographic reconstruction techniques can be used to image the volume between the measurement points.
Cross-borehole surveys are often used for imaging complex subsurfaces, such as fractured rock. They also can increase the effective depth of the GPR instrument. In the constant offset technique both the receiver and transmitter antennas are lowered to equal predetermined depths before a measurement is made. The process is repeated over the depth of interest. In a multiple offset gather, the transmitter is held at a predetermined depth in one borehole while the receiver(s) is lowered in regular steps down the other(s). After the receiver(s) collects data over the depth of interest, the transmitter is lowered to the next interval and the process is repeated until the transmitter reaches the bottom of the depth of interest (Kayen 2000). The gathers are then manipulated to give a detailed 3-D depiction of the subsurface between the boreholes.
The objective of GPR data presentation is to provide a display of the processed data that closely approximates an image of the subsurface including anomalies in their proper spatial positions. Data display is central to data interpretation, and is an integral part of interpretation.
The types of displays of surface GPR data include: (1) one-dimensional trace, (2) two dimensional cross-section, and (3) three-dimensional display. Borehole data can be displayed as a two-dimensional (2-D) cross section, or processed to be displayed as a velocity or attenuation tomogram. A one-dimensional trace does not have very much value until several traces are placed side-by-side to produce a 2-D cross section, or placed in a 3-D block view.
The wiggle trace (or scan) is the building block of all displays. A single trace can be used to detect objects (and determine their depth) below a spot on the surface. By moving the antenna over the surface and recording traces at a fixed spacing, a record section of traces is obtained. The horizontal axis of the record section is surface position, and the vertical axis is the round-trip travel time of the electromagnetic wave. A GPR record section is very similar to the display for an acoustic sonogram, or a fish finder. Wiggle trace displays are a natural connection to other common displays used in engineering (e.g., an oscilloscope display), but it is often impractical to display the numerous traces that are measured along a GPR transect in wiggle-trace form. Therefore, scan displays have become the normal mode of 2-D data presentation for GPR data. A scan display is obtained by simply assigning a color (or a variation of color intensity) to amplitude ranges on the trace.
Three-dimensional displays are fundamentally block views of GPR traces that are recorded at different positions on the surface. Data are usually recorded along profile lines in a continuous recording system, or at discrete points on the surface in fixed-mode recording. In either case, the accurate location of each trace is critical to producing accurate 3-D displays. Normally, 3-D block views are constructed, before they may be viewed in a variety of ways, including as a solid block or as block slices.
Obtaining good 3-D images is very useful for interpreting specific targets. Targets of interest are generally easier to identify and isolate on three dimensional data sets than on conventional 2-D profile lines. Simplifying the image by eliminating the noise and clutter is the most important factor for optimizing the interpretation. Image simplification may be achieved by: 1) carefully assigning the amplitude-color ranges; 2) displaying only one polarity of the GPR signal; 3) using a limited number of colors; 4) decreasing the size of the data set that is displayed as the complexity of the target increases; 5) displaying a limited range (finite-thickness time slice); and 6) carefully selecting the viewing angle. Further image simplification in cases involving very complex (or multiple) targets may also be achieved by displaying only the peak values (maximum and minimum values) for each trace. Finite-thickness (pillow) time slices and cross sections have many advantages over infinitesimal thin slices that are routinely used for interpreting GPR data.
The performance of the GPR method depends upon the site-specific surface and subsurface conditions. Performance specifications include requirements for or information about reflections, depth of investigation, resolution, interferences, calibration, quality control, and precision and accuracy.
Reflections are created by an abrupt change in the electrical and magnetic properties of the material the electromagnetic waves are traveling through. In most situations, magnetic effects are small. Most GPR reflections are due to changes in the relative permittivity of material. The greater the change in properties the more signal is reflected. In addition to having a sufficient electromagnetic property contrast, the boundary between the two materials needs to be sharp.
Areas with subsurface contamination often have very different permittivities than non-contaminated areas. GPR has been used to map highly conductive contaminated groundwater plumes (Porsani et al. 2004 and Pomposiello et al. 2004). Also studies have shown that weathered fuel releases create a "halo" of conductive soil and groundwater around them that are detectable by GPR (Sauck et al. 1998, Atewanda et al. 2002, and Bradford 2003). Nonaqueous phase non-polar organic contaminants, such as fuels and chlorinated solvents, generally have very low permittivities. In theory, these should provide a good reflectance contrast, and studies have shown that GPR can track their movement in the subsurface during a controlled release (Sneddon et al. 2000 and Brewster et al. 1995); however, in practice, differentiating relatively thin layers of free product from other reflectors where the release area is not known has not been particularly successful. Work has been done (Lane et al. 2004, Patterson 1997, and Bradford 2004,) to suggest that GPR can be used as a remediation monitoring aid by tracking changes in the subsurface conditions.
Depth of Penetration
The principal limiting factor in depth of penetration of the GPR method is attenuation of the electromagnetic wave in the earth materials. The attenuation predominantly results from the conversion of electromagnetic energy to thermal energy due to high conductivities of the soil, rock, and fluids. Scattering of electromagnetic energy may become a dominant factor in attenuation if a large number of inhomogeneities exist on a scale equal to the wavelength of the radar wave.
GPR depth of penetration can be more than 30 meters in materials having a conductivity of a few milliSiemens/m. In certain conditions, such as thick polar ice or salt deposits, penetration depth can be as great as 5,000 meters. However, penetration is commonly less than 10 meters in most soil and rock. Penetration in conductive (e.g., smectites) clays and in materials having conductive pore fluids may be limited to less than one meter.
The GPR method is sensitive to unwanted signals (noise) caused by various geologic and cultural factors. Geologic (natural) sources of noise can be caused by boulders, animal burrows, tree roots, and other inhomogeneities that cause unwanted reflections or scattering. Cultural sources of noise can include reflections from nearby vehicles, buildings, fences, power lines, and trees. Shielded antennas can limit these types of reflections. Electromagnetic transmissions from cellular telephones, two-way radios, television, and radio and microwave transmitters may cause noise on GPR records.
GPR provides the highest lateral and vertical resolution of any surface geophysical method. Various frequency antennas (10 to 1,000 MHz) can be selected to optimize the resulting data to the project's needs. Lower frequency provides greater penetration with less resolution. Higher frequencies provide less penetration with higher resolution. Resolution of a layer or anomaly a few centimeters thick can be obtained with high frequency antennas (1 GHz) at shallow depths, while lower frequency antennas (10 MHz) may have a resolution of approximately one meter thickness at greater depths. In general, two pulses, one reflected from the top and the other from the bottom of the strata should be distinguishable from each other when offset by ¼ of a pulse width. For example in a typical aquifer with a 100 MHz GPR system the vertical resolution would be about 15 centimeters (Greenhouse et al. 1998). Horizontal resolution is determined by the distance between station measurements, and/or the sample rate, the towing speed of the antenna, and the frequency of the antenna.
The manufacturer's recommendations should be followed for the calibration and standardization of GPR equipment. An operational check should be conducted before each project and before starting fieldwork each day. A routine check of equipment should be made on a periodic basis and after each problem.
Quality control activities can be appropriately applied to the procedures, processing, and interpretation phases of the survey. Good quality control requires that standard procedures (e.g., those given in ASTM Standard Guide D6432-99) are followed and appropriate documentation made.
Precision and Accuracy
Precision is a measure of the repeatability between measurements. Precision can be affected by the location of the antennas, tow speed, coupling of the antennas to the ground surface, variations in soil conditions, and ability and care involved in picking reflections. Assuming that soil conditions remain the same (e.g., soil moisture), repeatability of radar measurements can be 100%.
Accuracy is defined as a measure of closeness to the true value. The accuracy of a GPR survey is dependent upon picking appropriate travel times, and proper attention to processing, interpretation, and site-specific limitations, such as unknown changes in radar velocities (lateral and vertical) or the presence of steeply dipping layers.
GPR measurements are relatively easy to make and are not intrusive. Antennas may be pulled by hand or with a vehicle from 0.8 to 8 kph, or more. GPR data can often be interpreted right in the field without data processing. Graphic displays of GPR data often resemble geologic cross sections. When GPR data are collected on closely spaced (less than one meter) lines, these data can be used to generate multi-dimensional views that greatly improve the ability to interpret subsurface conditions.
The major limitation of GPR is its site specific performance. Often, the depth of penetration is limited by the presence of conductive clays or high conductivity pore fluid. Interpretation of GPR data requires a highly trained operator.
The cost of GPR systems varies widely depending on the complexity of the systems. Most systems fall in the $15,000 to $50,000 range. GPR systems can be rented for about $1,000 per week and a $300 mobilization charge. GPR surveys can be conducted by contractors with costs ranging from $1,000 to $2,000 per day depending on the amount of interpretation needed and if a report is required.
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