Ground Penetrating Radar- Principle & Applications in Vadose Zone study
Ground Penetrating Radar- Principle & Applications in Vadose Zone study
Dr. Sanjay Rana
Chief Geophysicist & Director
PARSAN Overseas Private Limited
Soil erosion, groundwater depletion, salinization, and pollution have been recognized for decades as major threats to ecosystems and human health. The top part of the earth between the surface and the water table is called the vadose zone. The vadose zone mediates many of the processes that govern water resources and quality, such as the partition of precipitation into infiltration and runoff, groundwater recharge, contaminant transport, plant growth, evaporation, and energy exchanges between the earths surface and its atmosphere. It also determines soil organic carbon sequestration and carbon-cycle feedbacks, which could substantially affect climate change.
Information about near surface soil water content is a vital component for vadose zone, agricultural and ecological studies, as well as for climate models that require input about processes at the air-soil interface. This paper focuses on investigating the applicability of a surface geophysical method, ground penetrating radar (GPR), for use as a water content estimation tool.
The paper presents review of methods to measure soil water content with ground penetrating radar (GPR). Four methodologies have been discussed: soil water content determined from reflected wave velocity, soil water content determined from ground wave velocity, soil water content determined from transmitted wave velocity between boreholes, and soil water content determined from the surface reflection coefficient. Basic methodologies, capabilities and limitations of each approach have been discussed in the paper.
Water at the land surface is a vital resource for both human needs and natural ecosystems. Society's fresh water needs for agriculture, sanitation, municipal, and industrial supply are ever increasing. At the same time, natural hazards involving water, such as floods, droughts and landslides are major natural threats to society in many countries. The vadose zone, which may be defined as the transition zone between the atmosphere and groundwater reservoirs, is important for water resource management, because it regulates the water availability for vegetation, including crops, while at the same time provides a protective buffer zone between land surface and groundwater against solutes and pollutants.
Hydrologists, soil scientists, ecologists, meteorologists, and agronomists all study the space and time variability of water in the vadose zone, hereafter referred to as soil water content, over a range of scales and for a variety of reasons. At the regional to continental scale, the exchange of moisture and energy between soil, vegetation, and the atmosphere has an impact on near-surface atmospheric moisture and temperature, which in turn define the regional climate. For example, soil water content determines to a large extent the relative magnitudes of sensible and latent heat fluxes and therefore determines the diurnal evolution of the atmospheric boundary layer. Currently, there is a need to establish and quantify the contribution of soil water content regulated land atmosphere coupling to regional climate anomalies, such as continental droughts and large-scale precipitation events.
The vadose zone mediates many of the processes that govern water resources and quality, such as the partition of precipitation into infiltration and runoff, groundwater recharge, contaminant transport, plant growth, evaporation, and energy exchanges between the earths surface and its atmosphere. It also determines soil organic carbon sequestration and carbon-cycle feedbacks, which could substantially affect climate change.
The vadose zones inherent spatial variability and inaccessibility make direct observation of the important below-ground (termed subsurface) processes difficult. Conventional soil sampling is destructive, laborious, expensive, and may not be representative of the actual variability over space and time. In a societal context where the development of sustainable and optimal environmental management strategies has become a priority, there is a strong prerequisite for the development of noninvasive characterization and monitoring techniques of the vadose zone.
In particular, approaches integrating water movement, geological, and physical principles (called hydro-geophysics) applied at relevant scales are required to appraise dynamic belowground phenomena and to develop optimal sustainability, exploitation, and remediation strategies.
Among existing geophysical techniques, ground-penetrating radar (GPR) technology is of particular interest for providing high-resolution subsurface images and specifically addressing water-related questions. GPR is based on the transmission and reception of electromagnetic waves into the ground, whose propagation velocity and signal strength is determined by the soil electromagnetic properties and spatial distribution. As the electric permittivity of water overwhelms the permittivity of other soil components, the presence of water in the soil principally governs GPR wave propagation. Therefore, GPR-derived dielectric permittivity is usually used as surrogate measure for soil water content.
In the areas of unsaturated zone hydrology and water resources, GPR has been used to identify soil layering, locate water tables, follow wetting front movement, estimate soil water content, assist in subsurface hydraulic parameter identification, assess soil salinity, and support the monitoring of contaminants.
GPR has known a rapid development over the last decade, notes Sbastien Lambot, a well known vadose zone expert. Yet, several challenges must still be overcome before we can benefit from the full potential of GPR. In particular, more exact GPR modeling procedures together with the integration of other sources of information, such as other sensors or process knowledge, are required to maximize quantitative and qualitative information retrieval capabilities of GPR. Once this is achieved, GPR will be established as a powerful tool to support the understanding of the vadose zone hydrological processes and the development of optimal environmental and agricultural management strategies for our soil and water resources.
Clearly there is a need for soil water content measurements across a range of spatial scales. High-frequency electromagnetic techniques are the most promising category of soil water content sensors to fulfill this need because this category contains a range of techniques that measure the same soil water content proxy, namely dielectric permittivity, at different spatial scales. Remote sensing with either passive microwave radiometry or active radar instruments is the most promising technique for measuring soil water content variations over large regions. The passive instruments have low spatial resolution and can either be airborne with pixel sizes of thousands of square meters or satellite-borne with footprints in the order of tens of square kilometers. In the near future, passive satellite remote sensing will provide global coverage of critical hydrological data, including soil water content. Active radar instruments have smaller pixel sizes ranging from 1 to several 1000 m2. Although remote sensing will surely play an important role in many future hydrological studies, currently there is still a need to establish transfer functions between remote sensing and the more familiar in situ soil water content measurements. Additionally, because remote sensing approaches for estimating water content estimate water content in the uppermost 0.05 m of the soil and require that the vegetation cover is minimal, remote sensing methods are not applicable in all types of vadose zone studies.
A well-established in situ electromagnetic technique for soil water content investigations is time domain reflectometry (TDR), which was introduced in vadose zone hydrology in the early 1980s. Time domain reflectometry has developed into a reliable method for soil water content determination that can easily be automated. Furthermore, TDR can simultaneously measure dielectric permittivity and bulk soil conductivity, which allows the study of water and solute transport within the same soil volume. Although TDR is highly suited for monitoring the development of soil water content at one location with a high temporal resolution, the small measurement volume makes it sensitive to small-scale soil water content variation (e.g., macropores, air gaps due to TDR insertion) within this volume. Furthermore, assessment of spatial soil water content variation with TDR is labor intensive because TDR sensors need to be installed at each measurement location.
Clearly, there is a scale gap between remote sensing and TDR measurements of soil water content. At intermediate spatial scales, such as agricultural land and small catchments, reliance on sparse TDR measurements or coarse remote sensing measurements might not provide the accurate soil water content information required at these scales (e.g., crop management, precision farming). Therefore, there is a need for soil water content measurement techniques that can provide dense and accurate measurements at an intermediate scale. Since the early days of electromagnetic measurement techniques, ground penetrating radar (GPR) has been conceived as the natural intermediate-scale counterpart of TDR for soil water content measurements. Although the number of TDR applications has increased immensely in the past 20 yr, the number of GPR applications for measuring soil water content has only recently increased. Probably, the most important reason behind this delay is the more complicated behavior of the unguided waves used in GPR as compared with waves guided by a TDR sensor. Furthermore, recent improvements in GPR technology allow more accurate travel time measurements, which are needed for soil water content determination with GPR. In this review, we focus on investigating the applicability of a surface geophysical method, ground penetrating radar (GPR), for use as a water content estimation tool.
Vadose Zone :
The vadose zone, also termed the unsaturated zone, is the portion of Earth between the land surface and the phreatic zone or zone of saturation ("vadose" is Latin for "shallow"). It extends from the top of the ground surface to the water table. Water in the vadose zone has a pressure head less than atmospheric pressure, and is retained by a combination of adhesion (funiculary groundwater), and capillary action (capillary groundwater). If the vadose zone envelops soil, the water contained therein is termed soil moisture.
Movement of water within the vadose zone is studied within soil physics and hydrology, particularly hydrogeology, and is of importance to agriculture, contaminant transport, and flood control. The Richards equation is often used to mathematically describe the flow of water, which is based partially on Darcy's law. Groundwater recharge, which is an important process that refills aquifers, generally occurs through the vadose zone from precipitation.
In speleology, cave passages formed in the vadose zone tend to be canyon-like in shape, as the water dissolves bedrock on the floor of the passage. Passages created in completely water-filled conditions are called phreatic passages and tend to be circular in cross-section.
This zone also includes the capillary fringe above the water table, the height of which will vary according to the grain size of the sediments. In coarse-grained mediums the fringe may be flat at the top and thin, whereas in finer grained material it will tend to be higher and may be very irregular along the upper surface. The vadose zone varies widely in thickness, from being absent to many hundreds of feet, depending upon several factors. These include the environment and the type of earth material present. Water within this interval, which is moving downward under the influence of gravity, is called vadose water, or gravitational water.
The vadose zone mediates many of the processes that govern water resources and quality, such as the partition of precipitation into infiltration and runoff, groundwater recharge, contaminant transport, plant growth, evaporation, and energy exchanges between the earths surface and its atmosphere. It also determines soil organic carbon sequestration and carbon-cycle feedbacks, which could substantially affect climate change.
Principles of Electromagnetic Methods
Electrical properties come from the interaction between electrical fields and charged particles, particularly the electron. Electrical conduction (transport) is the result of charge motion and results in energy dissipation (energy loss or conversion to heat). Electrical polarization (dielectric permittivity) is the result of charge separation over a distance, storing energy. Magnetic polarization (permeability or susceptibility) is the result of electron spin and motion in atomic orbits, and also results in energy loss and storage. Electrical and magnetic processes are also coupled, so accelerating electrons generate electromagnetic radiation, moving charges (currents) generate a magnetic field, and time varying magnetic fields cause charges to move. The velocity of electromagnetic wave propagation (speed of light) is the reciprocal of the square-root of the product of permittivity times permeability. The velocity in low loss, non-magnetic materials is (to a good approximation) the speed of light in vacuum divided by the square root of the relative dielectric permittivity (relative to that in free space).
v = c
Maxwell's equations (Maxwell, 1864, 1991) describe the propagation of an electromagnetic field. It is a coupled process, propagating as a three-dimensional, polarized, vector wave field. At low frequencies and high losses, the equations reduce to the diffusion equation and are called electromagnetic induction. At the high frequencies of radar, the energy storage in dielectric and magnetic polarization creates wave propagation. In the ideal, lossless case (vacuum), the electric and magnetic fields are in phase, orthogonal polarized vector fields, propagating at the speed of light. In real materials, they are out of phase, not completely polarized, propagating with a velocity lower than the speed of light in vacuum, scattered by changes in electric and magnetic properties, and with all of the preceding varying as functions of frequency.
The polarized vector field propagates in a straight line (neglecting relativistic gravitational effects) until it encounters a change in electrical or magnetic properties. At the change, the wave is scattered (reflected, refracted or diffracted) with amplitudes determined by the Fresnel reflection coefficient, angles determined by Snell's Law, and a polarization change described by the Stokes-Mueller matrices. Amplitudes are also varying with direction from the antenna (controlled by the antenna pattern), and with distance from the antenna (geometric spreading losses and material property dissipation losses) as described by the radar equation. Part of what determines whether or not significant scattering occurs is the spatial scale over which the change in properties occurs. This is both a detectability and a resolution issue. Resolution is determined by the spatial geometry of change versus the size of the wavelength of the propagating field.
The real part of the permittivity of water within the megahertz to gigahertz bandwidth is approximately 80, whereas the permittivity of air is 1 and of most other common soil constituents is about 3 to 10. This large contrast in permittivity explains the success of soil water content measurements with electromagnetic techniques working within this frequency bandwidth.
The most commonly used relationship between apparent permittivity, , and volumetric soil water content, (m3 m-3), was proposed by Topp et al. (1980):
This equation was determined empirically for mineral soils having various textures. It has an accuracy of 0.022 m3 m-3 determined in an independent validation on mineral soils. The term apparent is used because the permittivity used in this equation is determined from the measured electromagnetic propagation velocity in the soil.
It is important to realize that most available calibration equations between permittivity and water content were derived using TDR, which mainly operates in the frequency range from 500 to 1000 MHz. However, it has long been recognized that high clay content soils exhibit significant permittivity dispersion at low frequencies. Recently, West et al. (2003) presented frequency-dependent permittivity measurements of fine-grained sandstone samples containing up to 5% clay and soil samples containing Ottawa sand and varying amounts of montmorillonite clay. Their result showed that both the sandstone and soil samples showed significant frequency dispersion below 350 MHz. This implies that site-specific calibration may be required for those applications that require accurate water content measurements with lower antenna frequencies, such as the commonly used 100-MHz antenna. However, even when using published petrophysical relationships derived with TDR (such as above equation) with permittivity values obtained from GPR data, reasonable information about water content variation and spatial patterns can be obtained.
Principles of Ground Penetrating Radar :
Ground Penetrating Radar, also known as GPR, Georadar, Subsurface Interface Radar, Geoprobing Radar, is a totally non-destructive technique to produce a cross section profile of subsurface without any drilling, trenching or ground disturbances. Ground penetrating radar (GPR) profiles are used for evaluating the location and depth of buried objects and to investigate the presence and continuity of natural subsurface conditions and features, including determination of soil water content.
The GPR operates by transmitting electromagnetic impulses into the ground through transmitter antenna. The transmitted energy is reflected from various buried objects or distinct contacts between different earth materials, across which there is a contrast in dielectric constant. The antenna then receives the reflected waves and displays them in real time on screen. Data is also saved in appropriate memory for later processing and interpretation.
Ground penetrating radar waves can reach depths upto 60 meters in low conductivity materials such as dry sand or granite. Clays, shale
and other high conductivity materials may attenuate or absorb GPR signals, greatly decreasing the depth of penetration.
The depth of penetrating in also determined by the GPR antenna used. Antennas with low frequency obtain reflections from deeper depths but have low resolution. These low frequency antennas are used for investigating the geology of a site, such as for locating sinkholes or fractures, and to locate large, deep buried objects.
Antennas with higher frequencies (300 to 2000 MHz) obtain reflections from shallow depths (0 to 10 meters) and have a high resolution. These high frequency antennas are used to investigate surface soils and to locate small or large shallow buried objects, pipes, cables and rebar in concrete.
GPR can detect objects of any material, metallic or non-metallic.
Application Areas :
� Geological and hydro-geological investigations including mapping of bedrock topography, water levels, solution features, glacial structures, soils and aggregates.
� Engineering investigations to evaluate dams, sea walls, tunnels, pavements, roadbeds, railway embankments, piles, bridge decks, river scour, buildings and monuments.
� Location and evaluation of buried structures including utilities, foundations, reinforcing bars, cavities, tombs, archaeological artifacts, and animal burrows.
� Site investigations: location of buried engineering structures and underground storage tanks.
� Subsurface mapping for cables, pipes and other buried structures prior to trench-less operations.
� Rapid ground coverage- Antenna towed either by hand or from a vehicle.
� High-resolution coverage of the survey area, detecting even small objects.
� On-site interpretation possible due to instant graphic display.
� Data acquisition may be slow over difficult terrain.
� Depth of penetration is limited in materials with high electrical conductivities, clays.
� Energy may be reflected and recorded from aboveground features, walls, canopies, unless antennae are well shielded.
� Artifacts in the near surface (reinforcing bars, boulders, components of made ground) may scatter the transmitted energy and complicate the received signal and/or reduce depth of penetration.
Ground Penetrating Radar Theory :
GPR includes a radio transmitter and receiver, connected to an antenna coupled to the ground. The transmitted signal penetrates a short distance into the ground and some of it reflects off any object with different electrical properties than the soil. Since plastic pipe and voids are different than soil, these are some of the objects which reflect the signal. Receiver records the time and amplitude of reflected signal.
Multiple pulses sent into ground, and signal displayed on screen. The radar is moved along the ground and each new echo is plotted alongside the
previous ones. When enough of these signals are plotted side-by-side, the operator can see a pattern which he can interpret as an object.
When the object is ahead of the radar, it takes more time for the echo to bounce back to the antenna. As it passes over, the time grows shorter, and then longer again as it goes past the object. This effect causes the image to take the shape of a curve called a "hyperbola". Experienced users recognize that a hyperbola is actually the image of a smaller object (like a pipe) located at the center. Other patterns are produced by different structures. For example, a buried tank might have a flat image with curves down from either end.
It is not easy to determine the depth of an object without some knowledge about the dielectric constant in the local soil. The GPR can measure time very accurately, but the speed of the radar signal varies considerably with the soil type. In air, radar waves travel at 186,000 miles per second (or about 1 foot in one nanosecond). If you know the dielectric constant (from experience) or if you can look at the image of an object with a known depth, the GPR can be adjusted to read depth accurately. Software today allows construction of synthetic Hyperbola for depth calculations.
Note: Photographs used above: Courtesy Georadar Inc.
The figure as under presents possible propagation paths for surface GPR energy. Principally, all these waves can be used to measure soil water content. In the following section, we focus on soil water content estimation using reflected and ground wave travel time data. In addition, we also discuss the estimation of water content using borehole GPR travel time data and using ground surface reflection amplitude data.
Measuring Soil Water Content with Reflected Waves:
Two classes of methods to estimate soil water content from reflected wave travel time data can be distinguished. The first class contains the methods that use a single antenna separation for soil water content estimation (e.g., soil water content estimation from scattering objects and traditional GPR sections). The second class contains the methods that require multiple measurements with different antenna separations.
Single or common offset reflection methods:
The energy that GPR transmits into the soil will be (partly) reflected when contrasts in soil permittivity are encountered. Figure hereunder shows an idealized GPR section measured with surface radar and a fixed antenna separation (single offset) over an anomaly (e.g., a water-filled pipe) having a different permittivity than the host material.
As GPR emits waves in all directions, reflected energy is measured before the GPR is directly over it. The reflected events in the radar section trace out a hyperbola because the reflected energy of the GPR measurement directly above the anomaly has the shortest travel distance (time) and all other waves will have a larger distance to travel. The average wave velocity in the soil determines the convexity of the reflection hyperbola; i.e., it determines how much longer the waves need to travel the extra distance. The average velocity between the ground surface and the anomaly, vsoil, can be determined from a GPR transect by fitting the following hyperbola to measured arrival times at several positions x
where x is the position relative to the position of the scattering object (apex of the hyperbola), d is
the depth of the scattering object, and trw,x is the arrival time of the reflected wave at position x that has been zero time corrected. If the GPR section is measured with a significant antenna separation, a, this should also be included in the velocity determination as follows :
Most common GPR analysis software provides routines where the velocity can be determined interactively by manually fitting hyperbola to the limbs of the reflections of the scattering object. The velocity can then be used to calculate soil permittivity and soil water content.
Although it is simple and straightforward to determine velocity from scattering objects, it is a method that has not been used often for soil water content determination. The main drawback of this method is that it can only be used in soils where scattering objects can be observed in the GPR section. Even when scattering objects are present, this method only provides the average soil water content to the depth of the reflector; that is, the user has no control over the depth resolution of the soil water content measurements.
The accuracy of the single offset GPR reflection method for estimating water content under natural conditions is not yet well established. Some researchers have tested the concept of using GPR reflections under natural conditions to estimate water content using depth measurements to the reflectors obtained at discrete locations from, for example, noting lithologic transitions during drilling or water table observations. The use of single offset GPR reflection data for estimating spatially variable water content under naturally heterogeneous conditions at the field scale is a topic of active research.
Multi offset Reflection Methods :
Single offset measurements cannot be used to determine water content from reflecting soil layers if no information about the depth of the reflector is available. In that case, one can use a multi-offset GPR acquisition geometry to determine soil water content from radar reflections. Two commonly used multi-offset GPR acquisition geometries are called Common-MidPoint (CMP) and Wide Angle Reflection and Refraction (WARR) measurements as shown below:
In CMP acquisition, the distance between the antennas is increased stepwise while keeping a common midpoint. In WARR acquisition, the distance between the antennas is increased stepwise with the transmitter at a fixed position. A schematic outcome of a multi-offset GPR measurement is given as under:
Most of the commercial software provides solutions for multi offset recording of data and determination of permittivity of various layers encountered. Following is a typical profile shot using Common Depth Point technique.
Three signals are clearly visible in the given profile, namely:
1. Air wave path signal. Always appears as an inclined straight line.
2. Path of signal reflected from the first layer interface.
3. Path of signal refracted on the first layer interface and reflected from the second layer interface.
To calculate characteristics of a medium (wave velocity or permittivity) by hodographs of received signals, software provides easy input windows:
Initial values are input are inserted by the user and then the software performs the calculations providing permittivity and thickness of various layers.
The numerical calculated results will be displayed for each layer, namely: layer thickness in meters, electromagnetic wave velocity in cm/ns, permittivity, and mean root square value of approximation error. The less is the last value, the more accurate is the result.
Although multi-offset measurements are widely used in GPR data processing for determining velocity profiles with depth, there are some distinct disadvantages to soil water content determination with this method:
1. As with soil water content determination from single offset measurements, there is no control over the measurement depth resolution.
2. Multi-offset measurements are cumbersome to make and do not allow reconnaissance studies of soil water content variation.
3. For heterogeneous media, soil water content determined from multi-offset measurements is biased toward the common midpoint in the case of CMP acquisition geometry and toward the position of the fixed antenna in the case of WARR acquisition geometry.
Soil Water Content Measurements with the Ground Wave
The ground wave is the part of the radiated energy that travels between the transmitter and receiver through the top of the soil. The ground wave is detected by the GPR receiver, even in the absence of clearly reflecting soil layers. The evanescent character of the ground wave measured by the GPR receiver antenna at the soil surface requires that both the transmitter and the receiver be placed close to the soil surface.
The ground wave can easily be recognized on data collected using a multi-offset GPR acquisition geometry, by the observed linear relationship between antenna separation and ground wave travel times, which starts at the origin of the multi-offset measurement set. The slope of the ground wave in a multi-offset measurement is directly related to the ground wave velocity and can, therefore, be used for soil water content determination. The accuracy of this soil water content measurement technique was determined by researchers by analyzing a set of 24 multi-offset measurements collected using 225-MHz antennas and independent gravimetric samples, and was found to be fairly accurate.
Wide angle reflection and refraction (WARR) measurement recorded on loamy sand with the 225-MHz antennas. The velocity of the ground wave is v and of the air wave is c, both in meters per nanosecond.
Estimation of soil water content using multi-offset GPR measurements is cumbersome and time-consuming, as was mentioned before. The ground wave velocity can also be determined from a single offset GPR measurement, provided that the approximate arrival time of the ground wave is known from a multi-offset GPR measurement. Following procedure for soil water content mapping with the ground wave of GPR has therefore been suggested:
1. Identify an approximate ground wave arrival time for different antenna separations in a multi-offset GPR measurement,
2. Choose an antenna separation where the ground wave is clearly separated from the air and reflected waves and
3. Use this antenna separation for single offset GPR measurements and relate the changes in ground wave arrival time to changes in soil permittivity.
Although the results with ground wave data generally are promising, there are still some uncertainties associated with this method. An important, but unresolved issue is the effective measurement volume over which the ground wave averages. It is suggested that the influence depth is approximately one-half of the wavelength [ = c/(f )1/2], which would, for example, mean that for a central frequency of 225 MHz, the depth of influence could vary from 0.50 m ( = 4.0) to 0.22 m ( = 20.0).
Other drawbacks of using the GPR ground wave to estimate soil water content include the following:
1. It might be difficult to separate the ground wave arrival in the clutter of critically refracted and reflected waves.
2. It might be difficult to choose an antenna separation for which the arrival times of the air and ground wave can consistently be picked despite moving the antennas across a field with varying soil water content.
3. The ground wave is attenuated more quickly than other waves, which limits the range of antenna separation at which the ground wave can be observed.
Soil Water Content Measurements with Borehole GPR
For borehole GPR applications, the transmitting and receiving antenna are lowered into a pair of vertical access tubes. In the zero offset profile (ZOP) mode, the antennas are lowered such that their midpoints are always at the same depth. The arrival time of the direct wave between the boreholes and the known borehole separation is used to calculate the velocity and soil permittivity. The ZOP mode is an attractive approach for measuring the soil water content profile of the vadose zone with a high spatial resolution and a large sampling volume. Furthermore, each borehole GPR measurement requires only a couple of seconds and, therefore, the ZOP method is potentially capable of measuring transient processes within the unsaturated zone.
Soil water content can also be determined from a multi-offset profile (MOP). The first arrival times of all multi-offset measurements can be used to reconstruct a (tomographic) two-dimensional image of the soil water content distribution between the boreholes. To extract high-resolution, quantitative information from radar tomographic data, it is important to process the data as accurately as possible. The procedure for inverting radar tomographic data has been evolved, including methods for recognizing and correcting for errors caused by incorrect station geometry, incorrect zero time and zero time drift, geometric spreading, transmitter radiation pattern, transmitter amplitude, and high angle ray paths. The two-dimensional tomogram is obtained by discretizing the area between the boreholes in rectangular cells of constant velocity and determining the velocity of each cell by minimizing the difference between measured arrival times and arrival times calculated for raypaths passing through these cells. When necessary, three-dimensional tomograms can also be reconstructed. The drawback of acquiring the two-dimensional tomogram is the much longer time (typically several hours) required to obtain all the required measurements. Therefore, the MOP mode is best suited for steady-state water content conditions.
A comparison between estimates of soil water content (SWC) obtained from simple transformations of 200- MHz zero offset profiling (ZOP) and multi-offset profiling (MOP) borehole ground penetrating radar (GPR) as well as cone penetrometer (CPT) data at the DOE Hanford Site in Washington (modified from Majer et al., 2002).
Recent publications show that borehole GPR is quickly becoming an important site-specific investigation tool in hydrogeological studies. An obvious but important difference from the other GPR methods presented in this review is that borehole GPR is not suitable as a reconnaissance tool. Despite the strong increase of borehole GPR applications, there are several points that require attention when using this technique:
1. To obtain quantitative information from borehole radar data, it is important to recognize and correct for errors caused by acquisition procedures and transmission characteristics, and to recognize the accuracy limitations.
2. It is important to consider the impact of refracted waves, especially those traveling in the soil, on the accuracy of soil water content measurements with borehole GPR.
3. The length of the borehole GPR antenna, the borehole separation distance, and the antenna frequencies have an impact on the maximum spatial resolution that can be achieved;
4. Soil heterogeneity affects the sampling volume of borehole GPR.
Soil Water Content Measurements with Surface Reflections
The measurement principle of soil water content measurements with air-launched surface reflections is illustrated as under.
The GPR antennas are operated at some distance above the ground by mounting them on a vehicle or a low-flying air platform. The soil property being measured is the reflection coefficient of the air soil interface, R, which is related to the soil permittivity, soil, by
The reflection coefficient is determined from the measured amplitude, Ar, relative to the amplitude of a "perfect" reflector, Am, such as a metal plate larger than the footprint of the radar:
The determination of soil water content from the reflection coefficient requires accurate amplitude measurements. Clearly, the impact of surface roughness and soil water content profiles on the surface reflection coefficient are two key issues that need to be addressed when applying this technique. It is interesting to note that these problems are similar to those in active remote sensing, and it is possible that this soil water content measurement technique can profit from developments in the highly sponsored field of remote sensing.
Following picture shows a sample of data acquired with the 500-MHz GPR surface reflection measurement set up. Generally, the soil water content measured with GPR is similar to the soil water content measured with 0.20-m-long TDR sensors. However, there seems to be quite a large amount of very short distance variation in soil water content. Three likely explanations for the observed large variation of 0.10 m3 m-3 in soil water content include (i) the impact of the soil water content profile with depth on the reflection coefficient, (ii) the impact of surface roughness on the reflection coefficient, and (iii) reliability and accuracy of amplitude measurements.
Water content profiles measured with air-launched 500-MHz ground penetrating radar (GPR) surface reflectivity method compared with measurements obtained using 20-cm-long time domain reflectometry (TDR) probes.
The paper has discussed in details capabilities and limitations of using Ground Penetrating Radar for determining soil water content. Various examples from work carried out by researchers across the world clearly demonstrate that GPR can be used by experienced researchers to provide reliable estimates of subsurface water content. The exception to this is the GPR surface reflectivity method that, although promising, does not yet provide the accuracy and robustness of the other methods. In our opinion, the complexity of data acquisition and processing currently limits the applicability of GPR for water content estimation to the research community. However, continued development of specific and easy-to-use GPR instrumentation, coupled with increased experience and application of GPR for water content estimation, should help to bridge the gap between the advances made in the research community and the practical need for straightforward tools to accurately measure water content in high resolution and over large areas. Most likely, the borehole GPR method will continue to be used as an important research tool for studying infiltration processes and contaminant transport in the saturated and unsaturated zones. There may even be potential applications outside the research community for monitoring subsurface water content as part of groundwater protection strategies.
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