Ground Penetrating Radar For The Masses

Radar is a useful tool with familiar uses such as detecting aircraft and observing weather. It also has some less known applications, such as a technology known as ground-penetrating radar (GPR). Despite the difficulty of sending and receiving radio waves through solid objects, with the right equipment it’s possible to build a radar that works underground as well.

GPR is used often for detecting underground utilities, but also has applications in other fields such as archaeology and geology. For those people in these fields, a less expensive GPR was the priority of a group presenting at a 2017 National Institute of Telecommunications of Poland conference (pdf warning). The presentation goes into specific detail on how to build a GPR for around €600, much less than commercial offerings.

The presentation begins by highlighting the basics of GPR, then details the hardware bill of materials for the transmitting circuit, receiving circuit, and the DC power supplies. It also details the theory behind the software needed to get the circuit running properly, and has code as well. The processing is done on a 32-bit Mbed platform, and the rest of the GPR is built with easy-to-source components as well.

It’s always good to see useful hardware projects that bring costs of traditionally expensive equipment down to the grasp of average people. Even traditional radar systems are now available for hundreds of dollars, and we’ve even seen attempts at other GPR systems before as well.

Thanks to [Stefan] for the tip!

42 thoughts on “Ground Penetrating Radar For The Masses

    1. They’re bow tie antennas, which are very wideband. You need to understand that being in contact with the ground reduces the wavelength of all frequencies (like the impedance of stripline). Might be 3:1 over typical ground.
      I’ve built 40 foot antennas which worked down to about 1.5 MHz.

      We have 3D images of mine tunnels down 150 feet.

        1. Why, so they are.

          After fooling around with log periodic and yagi antennas aimed at the ground, we found that antennas work one way in free space and completely differently when pointed at the ground, a small multiple of a wavelength away. It detunes your short elements (at the point) and your signal might go up, not into the ground. The dielectric constant of the air-ground interface is what causes it.

          If you pull them far enough away from the ground so they don’t detune, you get a big ground bounce. So bow ties laying right on the ground seem to be the smoothest match.

  1. It’s 1.3-2.6GHz… expensive Minicircuits plumbing modules screwed together, it’s basically identical to Greg Charvat’s MIT radar.

    I’ve never heard of somebody making PCBs with AZ1518 photoresist and a mask aligner before… that sounds super expensive to make a chip mask in the yellow room just to make a PCB.

  2. The true challenge of a ground penetrating radar is not the transmitter/receiver- it is the processing- the focusing algorithms that allow you to figure out the size, shape, depth, and composition of things without having to interpret hyperbolas or simple time domain stripcharts. I was building/testing/field testing with virtually the same technology(*) (**) in the early 90’s and the vast majority of the interpretation methods are the same 25 years later. Collecting the data is maybe 2% of the effort, and the methods have long been solved.

    (*) Keith Sturgess, Matthew Bennett, Marc Ressler, Ted Grosch, Army Research Laboratory Ultra-Wideband Boom-SAR: System Overview and Minefield Detection and Recognition Results, presented at the Symposium for the Application of Geophysics to Engineering and Environmental Problems (SAGEEP ‘95) April 24 – 27, 1995 in Orlando, Florida.

    (**) Lynn Happ, Marc A. Ressler, Keith Sturgess, Matthew Bennett, Lawrence Carin, and S. Vitebskiey, Army Research Laboratory ultrawide-band testbed radar and comparisons of target data with models, Proc. SPIE Int. Soc. Opt. Eng. 2496, 42 (1995)

  3. Depending on your country and regulations, 1.3-2.6 GHz seems like a big wide chunk of spectrum that you can’t legally use.

    You’re stepping on GPS, aircraft navigation radar, radio astronomy and a host of other things.

  4. Ok, stupid question from someone who has been working in RF for the last 30 years:

    How can this possibly work? You are launching a low-power wave directly into absorbent / reflective earth, and looking for some sort of return? I would guess all that you are seeing is the atmospheric conditions above your angle of incidence. To make a much simpler analogy, imagine pointing a bright flashlight into a dirty mirror, and looking for reflections. How can you pernitrate the earth with anything in the MHz range? It seems to me that this would best be served with large acoustical impulses into the dirt, exactly how the oil prospectors do it?

    1. Sonar works, but sometimes you don’t want to make so much noise. And sonar has the same problems as GPR.

      GPR works just like radar in air. RF does penetrate into the ground, and it does reflect off features that are different than the surrounding medium. The attenuation is much greater than air, and it works both ways, so you are greatly limited in range. But the attenuation varies with frequency, so you can penetrate deeper with lower frequencies. But … the resolution is proportional to bandwidth, so at lower frequencies your resolution gets worse. Also, antennas get bigger and/or less efficient at lower frequencies and higher bandwidths. If you are looking for a tank 3 feet down, or a salt dome 500 feet down, you use different frequencies.

      Another big problem is isolation. If you use an FMCW radar, and your transmitting antenna is near your receiver, you need a lot of dynamic range to cancel out the transmitted signal from your received signal, so you can see those targets waaaay down in the mud (electrically, and physically). If you use a pulsed radar, you need a short enough pulse to do this cancellation, so you need a higher power or PRF, but there’s limits on that too.

      Simple pulsed radars are used to find buried tanks, pipes, and the like. Ground contact is commonly used to reduce ground bounce. Designing antennas for these is not fun. They are electrically shortened by the effective dielectric constant of the ground/air half plane (kind of like stripline). So pointing a long log periodic down is not going to work right. Bow ties flat on the ground are good, but for wide bandwidth they need to be electrically long. Loaded dipoles are easier to move around but inefficient.

      It’s an engineering problem. If you are looking for smaller targets, deeper, through a more dissipative or dispersive medium, sooner or later you run out of dBs, and money to buy A/D converters.

  5. It was mentioned that GPR is often used for finding underground utilities. Is this something that would be helpful before you begin digging in your yard? We are wanting to build an underground pool, and want to make sure we don’t hit any water lines in the process. I will have to let my husband know about GPR, and see what he thinks.

    1. In most places there are laws that require you to contact utilities to see what’s buried where; they will come and mark them. I’d expect you’re not doing the digging yourself, so make sure your pool installer does this. It will cost you time and money, but the fines are high if you don’t. And if you hit a sewer, water, power, or internet line, that would be very bad.

      Finding a thin pipe buried below the frost line with GPR is possible but don’t count on it. Plus, renting a radar and somebody who knows how to run it is not cheap. If it’s something you or a previous homeowner put in, better to start digging (carefully) and see what turns up. You might run across an old well or septic tank. I found a foundation for a building that was gone by 1900. Do some detective work, looking for clues like old pipes coming out of the ground or oddly settled ground. Use a metal detector if you have one; much cheaper than a GPR.

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