"GPR - SOEST"
GG 450 Lecture 19 February 26, 2006 Ground Penetrating Radar Background: Contrary to popular belief, GPR or Ground Penetrating/Probing Radar, is not a new technology. The first uses were in Austria in 1929, but the technology was largely abandoned until the late 1950's when U.S. Air Force radars were seeing through ice as planes tried to land in Greenland, misreading the altitude and crashing into the ice. This started investigations into the ability of radar to see into the subsurface not only for ice sounding but also mapping subsoil properties and the water table. GPR systems have been in commercial use for over 30 years. It is only recently that the environmental, construction and utility industries have discovered the multiple uses and benefits of performing GPR surveys to gain forehand knowledge of what's underground and in walls. GPR surveys are now being specified into engineering designs, environmental assessments and maintenance programs. Description: Ground penetrating radar has many similarities with wave propagation methods in subsurface imaging for oil exploration. This analogy has been used to transfer technology from the petroleum industry to the geotechnical arena. The approach does have its limits, as the physical processes involved in signal transmission are very different. Seismic methods use acoustic waves, radar uses electromagnetic waves. Nonetheless, GPR has become one of the instruments of choice for many small site investigations where a metallic object that is shallowly buried, such as an underground gasoline storage tank, must be located. Ground penetrating radar (GPR) has established itself as a successful technique for a wide range of shallow (< 50 m) subsurface evaluations. How does GPR work? • Ground penetrating radar uses electromagnetic wave propagation and scattering to image, locate and quantitatively identify changes in electrical and magnetic properties in the ground. It may be performed from the surface of the earth, in a borehole or between boreholes, from aircraft or satellites. It has the highest resolution in subsurface imaging of any geophysical method, approaching centimeters under the right conditions. A transducer generates a broadband (10-1000 MHZ) electromagnetic wave (impulse). A specially directed antenna emits the pulse into the ground. As the wave travels through the ground, it is reflected, deflected and absorbed by varying degrees of the material (soil, water) through which it travels. As the radar reflects off of materials it ‘echo locates' materials, or objects, of different electromagnetic conductivity within a matrix, for instance, a pipeline, storage tank, contaminant or re-bar in a matrix of soil or concrete. The receiver in the antenna will pick up the return signal to be processed by the radar unit. The radar unit will then plot a mark on a vertical scale based on the time it took for each signal to return. The radar unit will also analyze the characteristic properties of the waves, mainly the amplitude. On the same plot, the radar unit will assign a color to the vertically-scaled mark based on the severity of change in the return signal's amplitude and the emitting signal's amplitude. This severity of change in amplitude of the transmitted signal is based on the conductivity and dielectric properties of the reflective target. The waves reflect off the subsurface interfaces as if they are mirror-like. Because of this, the image produced will not be a direct replica of the subsurface – sloping reflectors will appear to slope less than they really do, and point or circular reflectors will appear as hyperbolas. The pulse has to travel through the substrate before it gets to the reflector, and again through the substrate to get to the receiver. Anything in the substrate that may block the beam will affect the data. Because the beam is a 45° cone, reflectors angled at greater than 45° cannot be seen. Objects within the matrix, such a pipeline or re-bar, show up quite clearly as hyperbolas with amplitudes depending on their conductivity contrast. Because the propagation of electromagnetic energy at radar frequencies is controlled by dielectric properties in geologic materials, the method is sensitive to changes in dielectric permittivity of the bulk material. The dielectric permittivity of a material is strongly related to its resistivity. The higher the resistivity, the higher the dielectric permittivity, and the farther an electro-magnetic wave will propagate through that material without absorption. The bulk dielectric permittivity of a rock formation is highly dependent upon the dielectric value of any pore fluid present, the degree of saturation, and the porosity. The presence of water filled pores increases the bulk dielectric permittivity from the value associated with the unsaturated state. This characteristic allows GPR to detect the water table under certain conditions. If pore water is replaced by organic compounds, which typically have a dielectric constant less than water, electromagnetic energy will be reflected. Depth of Investigation varies from less than a meter to over 5,400 meters, depending upon material properties. Detectability of a subsurface feature depends upon contrast in electrical and magnetic properties, and the geometric relationship with the antenna. Quantitative interpretation through modeling can derive from ground penetrating radar data such information as depth, orientation, size and shape of buried objects, density and water content of soils, and much more. (http://www.g-p-r.com/introduc.htm) HIGH RESOLUTION REQUIRED? YES: use high frequencies DEEP PENETRATION REQUIRED? YES: use low frequencies Sander et al.  and Greenhouse et al.  describe the 1991 Borden experiment, in which GPR was used, along with other geophysical techniques, to monitor a controlled spill of percholorethylene (PCE), a dense nonaqueous phase liquid (DNAPL). This study points out the need for time-differential measurements to remove background effects to allow the detection of small dielectric changes. This technique will be most useful for monitoring contaminant movement during remediation efforts. GPR Data Collection: In order to generate an "image" of a buried object , a GPR profile must be obtained. A GPR profile is generated when the antenna is moved along the surface. This can be done by hand, by vehicle, or even by air. The radar unit emits and receives reflected signals up to a thousand times per second. As a result, not only do the relative depths and "strengths" of the targets appear, but the image or shape of the target is "seen" on the monitor. A number of these transect lines need to be acquired to gain a precise location of the target in one direction. The same process must be done in the perpendicular direction to get a full picture of where objects are in the matrix. The reflected energy pulses are acquired only in a narrow line directly below where the transects are taken and the positions of objects have to be correlated from line to line. The data can also be utilized in a 3-D program to yield a sub-surface profile of the area surveyed. An obvious problem with GPR data acquisition is site accessibility. Since the GPR antenna has to be moved over the area to be investigated, the search area has to be physically accessible. Heavily wooded sites or areas containing cars, debris piles, sharp inclines, etc. all limit the accessibility of GPR data acquisition. A good analogy when considering the accessibility of a GPR investigation (for most applications) is to use Geo-Graf's rule of thumb, " The desired search area has to be clear enough so that you could push a shopping cart through it." In addition to the medium through which the GPR pulse travels, the frequency of the wave is a contributing factor in depth of GPR signal penetration. Typically, within the range of GPR antenna frequencies, the lower the frequency of the pulse, the deeper the signal penetration, but at the "cost" of data image resolution. Conversely, the higher the frequency, the greater the image resolution, but at the "cost" of signal penetration. This is due to the inherent properties of the Earth, that typically allow lower-frequency waves to travel farther within the subsurface. The type of antenna used will depend on the particular targets-of-concern. For instance, in measuring concrete floor thickness or rebar spacing, a 900 to 1500 MHz antenna would provide the best data. However, if the desired target is a UST or bed rock layers, a 120 MHz or 80 MHz antenna would be best. What’s the wavelength of the signal at 100 MHz? Velocity = distance / time Wavelength = distance / cycle Frequency = cycles / time Wavelength = velocity / frequency = 3*108 m/sec / 108 cycles/sec = 3 meters GPR works best in dry coarse-grained materials like sand and gravel. It works poorly in moist fine- grained sediments. Penetration in course grained sediments may be as much as 20 m and as little as 2 m in fine-grained materials. Usually GPR can be used with several antennae sizes that produce waves of different frequencies. High frequency antennas (200 to 400 MHz) produce the highest resolution images, but penetrate only to shallow depths because waves are quickly attenuated. Low frequency (80 MHz) antennae produce poorer resolution images, but can penetrate more deeply into the subsurface. The Radargram GPR data are presented as a radargram. As the antennas are moved across the surface, the Transmitter radiates short sharp pulses, and the Receiver records the echoes. The radar system constructs amplitude vs. time traces as the antennas are moved across the subsurface, very much like a seismic reflection profile. These traces are plotted next to each other showing recorded amplitudes vs. distance along the profile, and time (depth) into the ground. The resulting radargram appears in the form distance (horizontal axis) vs. time (vertical axis). The simplest conversion from time to depth requires that one know (or estimate) the velocity of the pulse in the ground. Typical time- to-depth conversion factors are given in the next table: Medium Time-to-Depth Conversion, (two- way travel-time) Air 6.6 nanoseconds/meter Dry geological 12 - 20 ns/m materials Damp geological 20-35 ns/m materials Water 60,000 ns/m APPLICATIONS ENVIRONMENTAL & ARCHAEOLOGICAL SURVEYS mapping extent of contaminant plumes determining direction of contaminant migration locating buried storage tanks locating buried artifacts, ruins or treasures delineating boundaries of ancient cemeteries and locating burial plots Mapping gravel and sand deposits, determining depth and quantifying volumes Glacial ice thicknesses - in cold ice bedrock-ice contact can be mapped. Finding rebar or culverts during highway construction. Finding caves or sinkholes. Oil & Gas: Location of pipelines, utility lines, water and sewer pipes for gas / oil facility surveys Assessing depth of sediment cover over pipeline river crossings and rights of way Determining depth to bedrock for proposed pipeline rights of way Aerial reconnaissance Survey depths up to 4 meters or more (depending on soil conditions - see chart) GPR can be tied to a GPS to yield precise locations 3-D software allows results to be obtained with x, y and z coordinates CIVIL SURVEYS Accurate location of in-slab: structural steel (re-bar) stress cables electrical and communication conduits - including PVC, fiber optics, telephone wiring and other non- ferrous materials water and sewer pipes It is essential to avoid hitting these features when coring or drilling through a concrete slab during construction renovations. Advantages of GPR As opposed to other locating techniques that are capable of detecting only metallic or conductive utilities and underground targets, GPR can locate and characterize both metallic and non-metallic subsurface features. It is completely nonintrusive, nondestructive and safe. GPR can be thought of as a Subsurface Imaging System, similar to sonar used for underwater applications. With GPR, surface conditions are not a major factor. Targets can be "seen" beneath reinforced concrete, asphalt, gravel, and most other common surfaces. High-resolution data in certain cases. Non-destructive and quiet. Requires only one or two people for field work. Fast and economic . Wide spatial coverage may be obtained, can be towed by a truck. Disadvantages of GPR Equipment is expensive. Limited penetration depths. Can be used in only specific sediment-bedrock terrains. Requires trained people for data collection and interpretation. Post-processing of data requires sophisticated computer software Information about dialectric properties must be known in order to convert to wave return times to depths. Safety and Interference Concerns During investigations, especially civil surveys, GPR surveys are often performed near sensitive electronic equipment or tenant occupied spaces. To address safety and interference concerns GPR technology is quite benign. The energy source is, as the name implies, Radar, or radio frequency. It is relatively low power so there are no deleterious effects from destructive radiation and no need to do locates after hours. The antennas used in civil surveys are fully shielded to direct all the transmitted energy into the ground and to eliminate surface reflection artifacts and radio frequency interference common to an unshielded system The radar signal reflects off of any objects with a difference in conductivity so materials such as plastics or air voids, as well as steel, can be resolved. Distinguishing between different materials can only be done in a relative sense, and because concrete varies a great deal, a direct calibration must be done to get accurate depth measurements. The reflected signal from 3 mm steel reinforcement mesh can be more pronounced than 30 mm PVC conduit. GPR can accurately resolve objects such as re-bar, stress cables and conduit in concrete to a depth of 450 mm depending on how many other there are in between. Commercial daily rate: ~$3,500.