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August 12, 2021

GPR EM Mag Comparison

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GPR is often believed to be useful only for the location of larger or geological features. However, it is strangely also used for utilities and smaller targets. GPR is also often considered complimentary to a survey, after magnetometry or EM surveys have been conducted first (Schmidt, et al., 2015). Clearly there is a discrepancy with how it is perceived and GPR’s full capabilities (in the right hands) have been misunderstood.

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    The many geophysical techniques available on the market all have their individual strengths and weaknesses. However, the industry stereotypes have meant that Ground Penetrating Radar is somewhat misunderstood.

    A study of an Iron Age hillfort in Hampshire, in which KB GPR Surveys was involved, recently revealed a perfect opportunity to display the discrepancies between Magnetometry, EM and GPR surveys over the same area.

    Magnetometry surveys have long been held as a fast and effective method for wide area geophysical investigation.

    Figure 2 EM Survey of Iron Age Hillfort

    Figure 2 EM Survey of Iron Age Hillfort (Green, 2018)

    This technology involves employing a magnetic gradient. This technology is used because magnetic particles are produced/altered through different processes in nature. Some of these processes are natural in origin e.g. bush fire, however some of the processes have human origins e.g. Kiln ovens. Different activity causes magnetic particles of varying strength to be deposited in the Earth after the event. If this event is anthropogenic (human) in nature, then it would display as intense ‘hotspots’ in the ground. Over time, these areas of hotspots wash away into ditches or pits. Magnetometry can be used to map areas of human activity as a by-product of heat generated magnetic variance in the soil.

    Magnetometry is not, however, effective at mapping archaeological stratigraphy, discriminating between layers or use near the geomagnetic equator without in depth knowledge on how to compensate. It is also not good at discovering wall foundations unless the wall is composed of magnetic materials (Schmidt, et al., 2015).

    Magnetometry survey results are indications of certain features (burning or increased magnetic responses) but do little to map other features or areas where the geology can counteract the Iron elements and create negative results e.g. limestone (Fassbinder, 2017).

    Magnetometry also, of course, only works if there has been burning or a change in the magnetic field of certain areas.

    Overall, Magnetometry is fast and effective at finding magnetic shifts, but does not produce the same resolution or discrimination as GPR. It can not be used for small detail or stratigraphy.

    EM surveys, otherwise known as Conductivity Surveys, measure the ability of the soil to conduct an electric current (Clay, 2005). An EM device induces an electromagnetic field through its primary coil and receives the returning EM field through its secondary coil. A comparison of the transmitted and received data sets is converted into an EM conductivity map of the area. As with many geophysical techniques, there are different devices for different applications, but broadly speaking this technique is another wide area method which lacks high resolution detail. EM is primarily used in search of large area features such as landfills, buried metal objects and locating water fractures deep underground.

    Strengths of EM surveys are often considered to include speed of application and coverage, variety of ground conditions including wet and dry, it has the ability of magnetic susceptibility and can somewhat measure the vertical stratigraphy in the ground (Clay, 2005).

    However, this technology is not without its weaknesses. These include a sensitivity to a wide range of metals which could cause interference, difficulty in depth discrimination, electromagnetic interference and the technology is becoming less and less competitive compared to the alternative survey techniques on the market. This is due to increased technological advances which enhance the performance and capability of the other techniques, whereas EM is relatively limited in its future development because it is more or less already at the limit of the technology.

    Ground Penetrating Radar (GPR) has been employed in geophysical surveys for decades. GPR has the ability to be used in wide area and small-scale applications using a variety of frequencies to produce an array of resolutions, depth penetration and subsurface mapping requirements. The GPR transmits an electromagnetic signal into the ground (or wall) which then receives any reflected signal. The GPR essentially maps subsurface reflectivity of different features, from pipes to coal mines.

    GPR can be used in small spaces for near surface detection, e.g. Rebar, and wide-open areas such as Archaeological prospection, utility surveys or void detection. This technology can detect buried features such as pits, walls, voids, rebar, stratigraphy/layers and much more.

    Figure 3 GPR Survey of Iron Age Hillfort

    Figure 3 GPR Survey of Iron Age Hillfort (Howard, 2018)

    Usually GPR requires being in contact with the surface (unless using an air-launched antenna) and certain materials can cause interference, e.g. clay or metal. GPR is relatively ‘geologically dependent’, meaning that the velocity of the signal transmitted and received varies greatly on subsurface conditions and mediums. For example, a very dry concrete medium will induce faster speeds of signal transmission. However, this velocity can be accounted for and factored in to data processing when being analysed. In fact, it is the features themselves which allow for the speed calibration in most cases as a hyperbolic speed calibration is used to detect and adjust data.

    Of the techniques used on the site mentioned at the beginning, it was clear that GPR produced the most detailed results. Also, the application of the GPR on-site was done through towing behind an ATV with GPS coupling so the survey was done quickly and accurately. The data produced features which had never been seen in 25 years of geophysical investigation on this site, resulting in a greatly improved understanding of this site and the potential for further study.

    This is just one example, but GPR is continually improving and being developed in a highly competitive market to create incredible results and capabilities. It is no longer an expensive, slow and inaccurate technology, quite opposite in fact. GPR has so much versatility in its application that, as seen here, it out competes the closest of geophysical competitors.

    Clay, R. B., 2005. Conductivity (EM) Survey: A Survival Manual. [Online]
    Available at: Conductivity EM Survey A Survival Manual [Assessed 05 11 2018].

    Fassbinder, J., 2017. Magnetometry for Archaeology. In: Encyclopedia of Geoarchaeology. Munich: Springer Science and Business Media, pp. 499-514.

    Green, F., 2018. Buckland Rings Project Report, Lymington: New Forest National Park Authority.

    Howard, B., 2018. Buckland Rings Archaeological Report, Southampton: KB GPR Surveys Ltd.

    Schmidt, A. R. et al., 2015. EAC Guidelines for the use of Geophysics in Archaeology: Questions to Ask and Points to Consider.. Belgium: Europae Archaeologia Consilium.

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