Ground penetrating radar experiments for high school students

Autumn academy 2016 for high school students (flyer here)

 14 October 2016 – Westfälische Wilhelms-Universität in Münster (Germany)

The autumn academy is an event addressed to high school students, which takes place every two years at the Westfälische Wilhelms-Universität in Münster (Germany). It is supervised by the Department of Physics and aims to provide insights into the world of physics and scientific research.

By attending to interesting and stimulating lectures, laboratory and outside experiments, high school students interested in physics can have a closer look at the world as seen by physicists.

Different topics were proposed to the students, ranging from applied physics using optical imaging processing and positron emission tomography, to astrophysics and astronomy using detection of cosmic radiation and optical spectroscopy to study the Sun.

The students use this kind of experiences to better find out what to do after the high school. Our goal was to make them as interested as possible in Geosciences, by introducing them to the versatile and powerful technique of Ground Penetrating Radar (GPR) for geophysical applications.

I had the opportunity to join this activity and, together with my office mate, we showed, to a small group of high school students, how and why to use the GPR method to probe and detect very shallow (up to few 10s of meters) underground structures with very fine resolution.

Before going outside and start doing measurements, we explained to the students what the GPR is used for and why it is important in some geophysical branches.

The physical phenomena used in GPR, which images the underground directly behind our feet, is the propagation of electromagnetic (EM) waves, which speed depends on the dielectric properties of the underground rocks and soils, their composition, and structures, including man-made objects. In Earth materials, the velocity of an electromagnetic wave in the ground is found to be about 0.2-0.6c, where c is the light speed in the vacuum (ca. 300000Km/s).

The physical principles behind the GPR method are therefore similar those used in seismic reflection methods for oil exploration. The method is schematically depicted in figure 1.

spr-1

Figure 1. Cartoon picturing the functioning of the GPR. The GPR instrumentation is led by an operator while the antenna is emitting radar waves into the ground. The waves are then reflected-back to the receiver as they hit a buried object. The collected data are processed afterwards to retrieve and locate the buried object.

By recording the back reflected radar waves that propagate into the ground, scientists are able to precisely reveal the electrical structure of the underground. By interpreting these data, they can :  get information about shallow geological and hydrogeological processes, locate man-made objects (like pipes and wires or even archeological remains and sites); GPR is also more and more used for forensic investigations.

The GPR signals are sensitive to the dielectric structure of the ground. Thus the GPR reliability to find the wanted targets depends on the soils humidity and soils/rocks water content. Ideal cases of surveying, unless the target is the underground water, concern dry soils/rocks with targets having a big contrast in dielectric response with respect to the surrounding materials.

The resolution and the sensitivity of the instrument, at a given depth, actually depend on the waves frequency used. Generally, the higher the frequency, the better is the resolution and the lower is the sensitivity at depth. At a radar frequency of 0.1-1GHz and with a wave speed of 108 m/s the resolution is in the range of 0.1-1 m. This allow imaging fine structures at shallow depths.

During the set up of the instrumentation, the students had the possibility to follow all the operations. They helped us to build the instrumentation set up, which consisted of:

 – a high frequency radio waves (0.1-1GHz) emitting and receiving antenna

 – a computer to real-time storing, visualizing, and raw processing of the data

 – a GPS system to locate the measurements

 – a moving platform on which the instrumentation was carried.

In this kind of instrument, the operator chooses the antenna (from which the emitting frequency depends on) he wants to work with, by taking into account some key parameters:

– the target’s size, composition, and depth

– the surrounding rocks or soils and their water content

– the desired resolution

After setting up the instrumentation and properly choosing the antenna for our purposes, we showed to the student how to use the GPR and make the measurements. Then the students started to cover a given path with the GPR (figure 2). The aim of this particular experiment was to recognize signals related to buried man-made pipes. The students were  able to observed the signal reflected from the ground on the display hold in their hands (figure 2). In this kind of equipment, the signal is usually displayed as travel time versus the covered path, as it is showed in figure 3.

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Figure2. Photos taken during the experiment showing the acquisition of GPR measurements. Two student are carrying the source-receiver antennas and checking the real time displayed signal.

By switching roles (either carrying the antenna or the computer) the students enjoyed the experience especially when, passing over a pipe, they detect a quite strong signal. They were also able to recognize the visible difference in signals when passing over either soils or concrete.

As most of the geophysical data, lots of processing is necessary to reveal the underground structures, but this was out of the purpose of the experience. What it is usually done in this kind of surveys is to convert the signal from time (the two-way travel time from the antenna to the ground and back to the receiver, figure 3a) to space (depth) using the EM wave propagation velocity (related to the electrical structure) of the underground. Once the process is done, the resulting model is then interpreted in terms of geological (figure 3b) or archeological features, artificial objects, etc…

lowrie

Figure 3. a) the processed GPR reflection signal in terms of depth (after the conversion from travel time) reveals (b) the pattern of geological features like fractures in granitic bedrock. Modified from Lowrie W., Fundamentals of Geophysics, (2007).

At the end of the experiments the students also asked some question about our own research projects and why they are important. They did not know about our specific projects but, surprisingly, they asked about the solid rocky Earth’s mantle and the presence of the liquid iron outer core. Me (working on the deep mantle rheology) and my office mate (working on early Earth’s magma ocean crystallization) answered to the questions they made. The students were also quite fascinated and curious when, during the conversation, they wanted us to tell something about the presence of the Earth’s magnetic field and how and when it began, which is actually a still quite controversial issue in geophysics.

Hoping the students have got interested in studying geoscience, we look forward to participating to the next appointment in autumn 2018.

Written by Angelo Pisconti

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