Ground Penetrating Radar
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Ground penetrating radar (GPR) is a relatively new method of interpreting stratigraphic sequences and profiles. The GPR process involves shooting electromagnetic energy (radar) into a section of ground being studied using portable dipole transmitting antennae which are placed at ground level over the site. The radar waves are sent into the ground and portions of this energy is reflected back upwards to a receiving antennae and is then processed using appropriate software within a computer to create a side profile of the sedimentary layers. Profiles can then be analyzed to determine stratigraphic content and boundaries, water table elevations, and the presence of archaeological items. GPR has been proven to be an effective means of accomplishing these tasks.
GPR is a method used to analyze stratigraphic sequences by sending high frequency electromagnetic pulses of radar energy to into the ground to detect subsurface formations. These high frequency radar waves reflect off of the subsurface formations and return to the surface at different time intervals, thus enabling a computer program to process a two-dimensional or three-dimensional profile of the below-ground stratigraphy. Ground penetrating radar works on the premise that different stratigraphic materials reflect the radar waves back towards the surface at different rates.
The process of using ground penetrating radar is amazingly simple even though it is intimidating at first glance. There are four main components that make up a working GPR system. The first component is an antennae that transmits the high energy radar frequency used to penetrate the ground. This antennae is placed at or near ground level over the site that is being analyzed. Radar waves are created by focusing high voltage toward the center of a bow-tie shaped copper plate in regulated pulses. An electromagnetic field is created around the copper plate as the current travels from the plate center to the plate edges and back again. The radar waves are then transmitted into the ground at a pre-determined frequency to be reflected back to a the second component of the GPR system; a receiving antennae. The receiving antenna is placed at an appropriate distance away from the transmitting antenna determined by the frequency used. If the two antennae are too close together severe distortion can occur in the received data due to interference caused by the resonation of the copper plate. The receiving antennae records the reflected radar waves and the data via fiber-optic cables to the third component, a console. The control unit then digitizes the information and displays it on the fourth component, a computer or a printout.
Under normal conditions, the most dominant factor in the collection of good data while using GPR is choosing the right frequency. There is a wide range of antenna frequencies to choose from, and since the site itself is the determining factor of which one to use, the choice can get complicated very quickly. There are two very important factors that must be considered when choosing a frequency. The first factor is the depth of the study in question. Different frequencies penetrate to different depths with different results (see figure below). Characteristically, lower frequencies(< 100 MHz) penetrate to depths of 30 meters or more, but return with a rather low-resolution representation of a profile. Higher frequencies(>300 MHz), return a relatively high-resolution representation, but only penetrate to depths of 5 meters or less. This shows that depth is a very important factor to consider. The second factor that must be considered is the material present at the location. Certain sediments or materials cause problems when using GPR. For example, radar energy is weakened much more rapidly by clays than by other sediments such as sand or silt. Magnetic properties of materials can also interfere with the GPR equipment and cause distortion in the data. The frequency chosen when using GPR will have a serious impact on the data collected, so all factors must be considered the first time so as to avoid repetitive work.
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Common Middle Point
The common middle point (referred to hereon as CMP) is a method used to determine the velocity at which the radar waves are penetrating the subsurface stratigraphy. The CMP method involves shooting radar at pre-determined increments outward from a common middle point until a side profile such as the below figure is produced. The method is dependent on the simple physics equation, distance (meters) = velocity (meters/ns) x time (ns). The average velocity is found by determining the average slope of the profile created. This slope is calculated by dividing the distance (m) traveled from the midpoint, by the deepest time (ns) achieved during the test. This number is equal to the slope of the line on the profile, as well as the average velocity of the radar waves in meters/nanosecond.
A Common Middle Point (CMP) profile of Russell Spit in Western Nevada. The average velocity is calculated by determining the horizontal distance in meters from the CMP, and then dividing it by the deepest time in nanoseconds.