"Introduction to RAMACGPR borehole radar"
Introduction to RAMAC/GPR borehole radar MALÅ GeoScience 2000-03-31 INTRODUCTION Borehole radar is based on the same principles as ground penetrating radar systems for surface use, which means that it consists of a radar transmitter and receiver built into separate probes. The probes are connected via an optical cable to a control unit used for time signal generation and data acquisition. The data storage and display unit is normally a Lap Top computer, which is either a stand-alone component or is built into the circuitry of the control unit. Borehole radar instruments can be used in different modes: reflection, crosshole, surface-to-borehole and directional mode. Today’s available systems use centre frequencies from 20 to 250 MHz. Radar waves are affected by soil and rock conductivity. If the conductivity of the surrounding media is more than a certain figure reflection radar surveys are impossible. In high conductivity media the radar equation is not satisfied and no reflections will appear. In crosshole- and surface-to-borehole radar mode measurements can be carried out in much higher conductivity areas because no reflections are needed. Important information concerning the local geologic conditions are evaluated from the amplitude of the first arrival and the arrival time of the transmitted wave only, not a reflected component. Common borehole radar applications include: • Geological investigations • Engineering investigations • Environmental investigations • Hydropower dams investigations • Fracture detection • Cavity detection • Karstified area investigation • Salt layers investigations DIPOLE REFLECTION SURVEYS In reflection mode the radar transmitter and receiver probes are lowered in the same borehole with a fixed distance between them. See figure 1. In this mode an optical cable for triggering of the probes and data acquisition is necessary to avoid parasitic antenna effects of the cable. The most commonly used antennas are dipole antennas, which radiate and receive reflected signals from a 360-degree space (omnidiretionally). Borehole radar interpretation is similar to that of surface GPR data with the exception of the space interpretation. In surface GPR surveys all the reflections orginate from one half space while the borehole data re- ceive reflections from a 360- degree radius. It is impossible to determine the azimuth to the reflector using data from only one borehole if dipole Figure 1 antennas are used. What can be determined is the distance to the reflector and in the case where the reflec- tor is a plane, the angle between the plane and the borehole. As an example, let ‘s imagine a fracture plane crossing a borehole and a point reflector next to the same borehole (figure 1, left). When the probes are above the fracture reflections from the upper part of the plane are imaged, in this case from the left side of the borehole. When the probes are below the plane, reflections from the bottom of the plane are imaged, in this case the right side of the borehole. The two sides of the plane are represented in the synthetic radargram in figure 1. They are seen as two legs corresponding to each side of the plane. When interpreting borehole radar data, it is important to remember that the radar image is a 360-degree representation in one plane. A point reflector shows up as a hyperbola, in the same way as a point reflector appears in surface GPR data.Interpreting di- pole radar data from a single borehole, the interpreter can not give the direc- tion to the point reflector only the distance to source can be interpreted. In order to estimate the direction to the reflection, data from more than one borehole need to be interpreted. Figure 2: Dipole reflection measurement in granite. The antenna centre frequency used was 100 MHz. In granite, normally several tens of meters of range are achieved using this antenna frequency. CROSSHOLE SURVEYS Crosshole measurements are conducted with borehole radar systems devel- oped using separate probes for the transmitter and the receiver. In crosshole mode the transmitter and the receiver are lowered into different boreholes. In order to minimize gemetrical noise and other problems, the two boreholes must be in the same 2-dimensional plane. The investigated section is, of course, the media between the boreholes. In comparison to dipole reflection measurements, crosshole surveys are more Figure 3: Principle of tomographic survey (left)and results after completed tomographic inversion. time consuming because of the number of recordings needed. While the transmitter is fixed at one position in one borehole the receiver scans the complete length of the other borehole. Then, the transmitter is moved one step and the receiver scans the complete adjacent borehole again. This proce- dure is repeated until the transmitter has covered the whole length of the first borehole, figure 3. The crosshole survey mode is also referred to as the tomography mode. Tomography inversion can be made using two types of recorded data, the amplitude of the first arrival and/or the ratio between the time it takes for the first wave to arrive in the other borehole and the calculated arrival time in homogeneous media. Travel time tomography is an excellent surveying and processing method to determine areas between the boreholes containing high water content (e.g. water filled fractures and cavities). This because the travel time is heavily affected by the high dielectric constant of water. Figure 4: Velocity tomography is also called “slowness tomo- graphy”. This example is from a hydropower dam investigation. The dark area in the image corresponds to slow velocities and indicate a leakage in the dam. SURFACE TO BOREHOLE SURVEYS Standard GPR systems can be used for surface to borehole surveys. The standard surface GPR transmitter is positioned at different distances from the borehole on the ground and the borehole receiver probe is lowered into the borehole. See figure 6 below. It is possible to compile images, both ampli- tude and velocity tomographic images of the media between the borehole probe positions and the surface transmitter positions. However, surface to borehole measurements are more commonly used for velocity surveys. Planes can be created in different directions e.g. N, S, E and W. Figure 5: Surface to borehole set-up DIFFERENCE TOMOGRAPHY SURVEYS Tomography can also be used to perform difference measurements. Data acquisition is made twice, once with existing subsurface conditions and a second time after introduction of a tracer. Difference tomograms are typically made after injection of a saline tracer in a fracture zone resulting in a marked difference in the amplitude data before and after the injection (figure 6a-d). Figure 6a: Fracture system between Figure 6b: Initial tomography scan. boreholes. Amplitude map generated. Figure 6c: Tracer injected in to Figure 6d: Second tomography fracture system. scan. Amplitude map obtained, subtracted from first scan. Figure 7 shows a difference tomogram study performed in a granite.Amplitude inversion is normally used for difference tomography. Difference tomography shows the difference between two measurements. Note that the resulting image gives more information regarding the dip of the fracture zone, permaeability and if the fracture is transporting water or not. Figure 7a: Amplitude map generated from the first tomography scan. Figure 7b: After the saline tracer been implemented and the second amplitude map been sub- tracted from the first. The final, resulting difference tomogram is generated.