C+S March 2020 Vol. 6 Issue 3 (web)

Antennas used with ground penetrating radar (GPR) come in different shapes and sizes. The largest antennas typically radiate lower frequen- cies necessary to detect the deepest targets. The smallest antennas radi- ate the highest frequencies that provide the greatest resolution required to detect small, shallow targets. The “best” antenna for a job is the one with the highest frequency that can still detect objects at the desired depth. Mark DeSchepper, President of Kansas-based Echo GPR Ser- vices, has more than 12 years of GPR experience and offers some great examples of how to select the right antenna. The Importance of selecting the right antenna for the de- sired depth Selecting the right antenna for the depth of the particular target is critical. If the antenna frequency is too high, the maximum detection depth may be less than needed to find the desired target. The GPR image quality deteriorates with increasing depth to the point where no reflections can be observed. Conversely, if the antenna frequency is too low, fine details are lost. For example, if using the wrong antenna, individual closely-spaced shallow targets cannot be identified because they may appear as a continuous reflection band. When assessing which antenna to choose for a job it is essential to consider the surface cover and soil present. One important parameter is the material’s conductivity—how well does the soil, asphalt, or con- crete let an electrical current pass through it? The higher the material’s conductivity, the more the radar waves are absorbed into the material as they are moving through it. In other words, the radar waves’ depth penetration progressively decreases as the conductivity of a material increases. For example, asphalt typically has low conductivity, while concrete has varying degrees of conductivity depending on its curing state, wetness, and composition. Soil conductivity varies wildly, from radar-friendly sandy soils to radar-foe clay soils. For this reason, antenna depth specifications are based on the material and application. Antenna specifications for soil-based applications use soil information, while concrete-based applications use typical con- crete parameters. Other factors to consider when assessing depth penetration include the presence of metal reinforcing between the surface and the target. The tighter the rebar or mesh spacing (for example, in a concrete slab), the more difficult it is to achieve the desired depth penetration. Choosing the Right Antenna for GPR Investigations Look for the highest frequency that can detect objects at the desired depth By Jami Harmon Geospatial

Local supermarket hired Echo GPR to conduct a GPR scan to avoid losing power to cash registers or cutting refrigeration pipes in recent renovation project.

A closer look at antenna bandwidth ranges GPR antennas send out a range of radio frequencies and generally have an ideal frequency at which they transmit most efficiently. This value is based on the size of the antenna’s transmitter and receiver elements and is often referred to as the antenna’s “center-frequency.” Another often-used term is the antenna’s bandwidth, the range of frequencies that the antenna transmits at power levels that are within half the power level radiated at the antenna’s center-frequency. A typical GPR antenna’s bandwidth is about the same as its center frequency. For example, a 400 megahertz (MHz) GPR antenna will have a center- frequency of 400 MHz and a range of frequencies radiated at power levels within half the power level at 400 MHz – from 200 MHz to 600 MHz. The bandwidth would therefore be 600 – 200 = 400 MHz. Like- wise, a 900 MHz GPR antenna could have a bandwidth that extends from possibly 450 MHz to 1350 MHz. This bandwidth range means that small changes in center-frequency provide minimal enhancement in penetration depth. That is why GPR antennas are not typically available in small frequency increments. More commonly, antennas are available in center-frequencies that are doubled, such as 2000 MHz, 400 MHz, 800 MHz, 1.6 MHz and 2.6 MHz. Selecting the right antenna Selecting the right antenna frequency for a GPR job depends on the


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