Module 3.2: Reflections and Target Depths

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What causes reflections of GPR energy

Ground Penetrating Radar energy is reflected by ANY change in the electromagnetic properties of the ground, with a stronger reflection for the greater the change.

Conversely, relatively little or no GPR energy is reflected by minor changes in the electromagnetic properties of the ground.

GPR is therefore looking for contrast; the more electrical contrast between the materials, the greater the probability of detection.

The specific electrical parameters that cause reflections of GPR energy are:

  • Permittivity – the insulation / resistive properties of the ground
  • Permeability – the magnetic properties of the ground
  • Conductivity – the conductive properties of the ground

With permittivity being the greatest cause of reflections.

Some simple examples of materials that cause reflections of a GPR signal include:

  • Changes in the ground material such as layers of a road or bedrock
  • Air pockets (voids, gas pipes)
  • Water (water pipes, culvert, sewer)
  • Buried objects (stone, IED)
  • Metal (pipes and cables)
  • Disturbed ground (bodies, archaeology)

It’s important to remember that GPR energy is reflected by changes in the ground conditions, therefore, an object with a relatively similar electrical characteristics to its surrounding media will generate relatively little reflection (for example compacted layers of a similar material), whereas objects with relatively different electrical characteristics will generate large reflections (for example a large water pipe buried in soil).

Calculating target depths using GPR

Ground Penetrating Radar is able to locate the horizontal positions of targets by producing recognisable or interpretable shapes on two dimensional images called B Scans or brightness scans. We have already looked at two simple examples for a point and a linear feature.

The GPR has also recorded the vertical position of that target or feature by measuring the time delay between the GPR signal being transmitted and reflections being received.

Using the principle of: 2 x depth = speed / time

Where:

Distance refers to the depth between the surface and the feature.

  • Speed refers to the speed of the GPR signal travelling through that particular ground material
  • Time refers to the time delay accurately measured by the GPR.
  • ÷2 because the signal has to travel in two directions

Of the variables in the formula, speed is the one which we cannot accurately know. The GPR records the time itself using an accurate clock, and distance is what we are calculating, however, speed will vary depending on local ground conditions. The type of soil and the water content will be different in every location the GPR is used. In order to accurately calculate distance, we need to measure and calibrate the speed.

There are a few ways to calculate the speed of the GPR signal in a particular ground. Some of these include:

  • Taking a soil sample and sending it to a lab to be analysed; the lab will be able to calculate the speed for that particular material at that specific location.
  • Performing a ‘common midpoint’ measurement; this involves having your transmitter and receiver in separate antenna boxes, placing them above a visible features (such as a clear point target) and then moving the antennas away from the target systematically whilst recording data. It will then be possible to calculate the speed from this information.
  • Using an array to perform automatic velocity calibration – transmitting with one antenna and receiving with several antennas at the same time to recover comparative data.
  • Performing a ‘hyperbola fitting’ either on-site or using your GPR post processing software.
  • Recording a known depth using ground truth (for example, lifting a manhole and measuring a depth) then inserting this information onto your GPR either onsite or in the office.

Of the above methods, the last two are the most practical and commonly used.

Why perform a speed calibration using GPR

Accurate estimation of speed is essential to recording accurate depths using Ground Penetrating Radar.

The importance of performing a speed calibration in GPR cannot be understated. Common speeds can range from 6 or 7cm/ns in wet ground such as clay up to 13-14 in dry homogenous soils.

This was proven at the University of Leicester’s GPR test site in the UK, when we (I) was unable to detect a simulated roman ruin buried at 80cm depth using a 200MHz antenna. That antenna recorded 1.1-1.2m of penetration at the default propagation speed of 10cm/ns, this calculates to an actual speed through the ground of 6cm/ns or less, in order not to detect the ruin.

It had been raining heavily in the weeks before this test, and the ground was saturated.

This may not seem like a great issue – but the key point is the variation in possible speeds through the same ground. Imagine that you recorded 1.1-1.2m for a gas main using your uncalibrated GPR with its default setting of 10cm/ns (we are not talking about an extreme example with the speed set unusually high), your contractor now digs to that main using a mechanical digger and encounters it at 0.6m and puts his bucket straight through it – this is not an impossible scenario.

Calibrate the speed using your GPR, its easy and may save a life.

How to perform a speed calibration using GPR

Most (but not all) Ground Penetrating Radar that we have tested, have a feature that allows you to perform a speed calibration on-site and all GPR post-processing software incorporates this feature. Consult your manufacturer, dealer, or user manual to locate the feature on your particular software.

(Some GPR we can confirm offer this feature on-site incudes; Mala EasyLocatior HDR, IDS Detector Duo / Opera Duo / DS2000 / Hi Mod, and GSSI DF and HS.)

Once you have located the hyperbola fitting / speed calibration feature on your GPR or in your GPR post processing software, in most cases you will be given a curve which approximates the shape of a hyperbola.

Note that it is the angle of the ‘tails’ of the hyperbola which represent different speeds. Wider tails on the hyperbola represents a faster speed whilst narrower represents a slower speed.

Line up the software’s shape representing the hyperbola with a real one in your GPR data and adjust until the two shapes are as similar as possible. You software will have calculated the speed for that particular ground.

Our Most Common Questions

Frequently Asked Questions

If your question isn't answered here, please do contact us for more information. 
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What can a Ground Penetrating Radar survey detect?

With GPR, you can detect a wide range of objects below ground level, including both metallic and non-metallic objects such as plastic pipework. GPR will also identify and map any voids below the surface, such as air pockets or mine shafts, as well as any other irregularities including concrete and previously excavated or back-filled areas.

Will GPR compromise safety on my site?

GPR equipment emits an electromagnetic pulse into the ground and records the reflected signals from subsurface structures and voids. It is entirely non-destructive and will not break the ground’s surface or affect any objects below. What’s more, it doesn’t emit any harmful levels of radiation, nor are there any other by-products created throughout the process. This means it’s entirely safe to use by its operators, and on sites of any type, including those open to the public.

Is a GPR survey 100% accurate?

While GPR is one of the most effective methods of non-destructive testing available, it can never be 100% accurate. One factor that can adversely affect the accuracy levels include the type of soil being surveyed. Clay soils and soils that contain high levels of salt or minerals can obstruct the GPR reading. Another factor is the experience of the equipment’s operator: interpreting the data collected can be complex, which is why it’s beneficial to commission surveys from an expert firm.

Is GPR equipment difficult to use?

The equipment itself is not difficult to use, but the interpretation of the data recorded tends to be complicated. The results of a GPR survey aren’t automatically translated into an easy-to-understand picture of what lies below the surface; instead, it’s a series of lines and waves and it can take both training and years of practice to master the art of correctly reading the output. Often, it is the experience of the equipment’s operator that plays the most significant role in the accuracy of the results GPR can achieve.

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