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Module 3.3: GPR Penetration

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Factors affecting Ground Penetrating Radar penetration

Ground Penetrating Radar penetration is affected by four main factors:

  • Transmitted power,
  • Dynamic range of the receiver,
  • Attenuation of the ground material,
  • Centre frequency of the transmitting antenna.

If we look at each of those factors individually.

GPR Penetration: Transmitted Power

Transmitted power is limited by EC and FCC rules and almost all GPR already transmit at the maximum allowed power. Significant increases in the transmitted power would result in much more power consumption within the GPR and require bigger batteries, therefore trolley design would be impacted and it would be less easy to operate on-site – but all of this effort would only result in relatively small increases in penetration.

With all that in mind, increasing the power is both illegal and impractical, and beyond the reach of GPR operators.

It can be discounted as a viable option.

GPR Penetration: Dynamic range of the receiver

The dynamic range of the receiver is a clever sounding way of saying ‘how good the GPR’s hearing is’ or the ‘smallest reflection a GPR can receive’.

This is purely a consideration in the design and manufacture of a GPR, choice of components, efficiency of design, size of antenna, and so on. It is possible for there to be some variation between different manufacturers and it is possible for advances in technology to result in improvements in dynamic range (with respect to that manufacturers previous design).

But, most modern GPR’s are well designed and this variation will typically be small. Expect differences of a few % at most and beware claims that one GPR has significantly more penetration than another of a similar frequency. These claims are almost certainly lies.

GPR Penetration: Attenuation of the ground material

Attenuation is another word for resistance, so what we are talking about now is the resistance to the GPR signal due to the ground material.

This factor is clearly beyond a Ground Penetrating Radar operators control, but it does have a very significant influence on the maximum penetration that can be achieved using a GPR.

The attenuation in the ground is primarily caused by three main factors: conductivity losses, heating losses and scattering losses, with conductivity being the greatest cause.

GPR Penetration: Losses on a GPR signal due to conductive ground

Conductive materials allow the GPR energy to be dissipated and so reduce the penetration rapidly, in particular metal cannot be seen through but other materials such as clay become poor conductors when an electromagnetic signal is applied to them.

*Clay contains a number of free moving particles which are electrically charged. When an electromagnetic field is applied those particles line up like in a magnet and become a very poor conductor which dissipates the electrical charge. Moving those particles also consumes energy which adds to the resistive effect of this type of soil.

Increased water content also increases conductivity, as does salinity (salt). Wet ground will have reduced penetration and ground saturated with salt will be impervious to GPR.

GPR Penetration: Heating losses

Another cause of loss or resistance to a GPR signal is the heating effect. This is similar in practice to a microwave oven.

When the electromagnetic signal is applied to a material, the molecules may absorb some of the energy and become ‘agitated’, starting to move more aggressively. This process causes a heating effect.

The heating effect is more significant with higher frequency antennas and also with increased water content.

GPR Penetration: Scattering losses

Scattering losses are caused by irregular materials in the ground causing reflections to be sent in random different directions other than further into the ground or directly back to the GPR.

This energy is lost and since the amount of energy available is limited, lost energy reduces penetration.

This effect is typical in brown fill sites backfilled with rubble and can be very significant.

Below is a simple table showing relative maximum penetration by material for the same GPR. Please note that these numbers are approximations to indicate the differences between different materials for the purposes of this course and not accurately calculated.

Factors affecting Ground Penetrating Radar penetration

Ground Penetrating Radar penetration is affected by four main factors:

  • Transmitted power,
  • Dynamic range of the receiver,
  • Attenuation of the ground material,
  • Centre frequency of the transmitting antenna.

If we look at each of those factors individually.

GPR Penetration: Transmitted Power

Transmitted power is limited by EC and FCC rules and almost all GPR already transmit at the maximum allowed power. Significant increases in the transmitted power would result in much more power consumption within the GPR and require bigger batteries, therefore trolley design would be impacted and it would be less easy to operate on-site – but all of this effort would only result in relatively small increases in penetration.

With all that in mind, increasing the power is both illegal and impractical, and beyond the reach of GPR operators.

It can be discounted as a viable option.

GPR Penetration: Dynamic range of the receiver

The dynamic range of the receiver is a clever sounding way of saying ‘how good the GPR’s hearing is’ or the ‘smallest reflection a GPR can receive’.

This is purely a consideration in the design and manufacture of a GPR, choice of components, efficiency of design, size of antenna, and so on. It is possible for there to be some variation between different manufacturers and it is possible for advances in technology to result in improvements in dynamic range (with respect to that manufacturers previous design).

But, most modern GPR’s are well designed and this variation will typically be small. Expect differences of a few % at most and beware claims that one GPR has significantly more penetration than another of a similar frequency. These claims are almost certainly lies.

GPR Penetration: Attenuation of the ground material

Attenuation is another word for resistance, so what we are talking about now is the resistance to the GPR signal due to the ground material.

This factor is clearly beyond a Ground Penetrating Radar operators control, but it does have a very significant influence on the maximum penetration that can be achieved using a GPR.

The attenuation in the ground is primarily caused by three main factors: conductivity losses, heating losses and scattering losses, with conductivity being the greatest cause.

GPR Penetration: Losses on a GPR signal due to conductive ground

Conductive materials allow the GPR energy to be dissipated and so reduce the penetration rapidly, in particular metal cannot be seen through but other materials such as clay become poor conductors when an electromagnetic signal is applied to them.

*Clay contains a number of free moving particles which are electrically charged. When an electromagnetic field is applied those particles line up like in a magnet and become a very poor conductor which dissipates the electrical charge. Moving those particles also consumes energy which adds to the resistive effect of this type of soil.

Increased water content also increases conductivity, as does salinity (salt). Wet ground will have reduced penetration and ground saturated with salt will be impervious to GPR.

GPR Penetration: Heating losses

Another cause of loss or resistance to a GPR signal is the heating effect. This is similar in practice to a microwave oven.

When the electromagnetic signal is applied to a material, the molecules may absorb some of the energy and become ‘agitated’, starting to move more aggressively. This process causes a heating effect.

The heating effect is more significant with higher frequency antennas and also with increased water content.

GPR Penetration: Scattering losses

Scattering losses are caused by irregular materials in the ground causing reflections to be sent in random different directions other than further into the ground or directly back to the GPR.

This energy is lost and since the amount of energy available is limited, lost energy reduces penetration.

This effect is typical in brown fill sites backfilled with rubble and can be very significant.

Below is a simple table showing relative maximum penetration by material for the same GPR. Please note that these numbers are approximations to indicate the differences between different materials for the purposes of this course and not accurately calculated.

Table showing differences in GPR penetration between ground materials
Table showing differences in GPR penetration between ground materials

GPR Penetration: Centre frequency of the antenna.

The final significant factor affecting the penetration capability of a Ground Penetrating Radar is the centre frequency of the antenna. This factor is particularly important because it is the only one over which the GPR operator has any control.

The general rule with GPR is that lower frequency antennas have greater penetration but less resolution.

And conversely, higher frequency GPR antennas have less penetration but greater resolution.

(We will talk about GPR resolution in ‘KB GPR Training Module 3.4: GPR Resolution’)

  • A higher frequency electromagnetic signal has a smaller wavelength which interacts more strongly with moisture in the ground causing increased heating losses.
  • Signals with a smaller wavelength (higher frequency) are also attenuated more strongly due to increased scattering.
  • Permittivity (the measurement of the resistive properties of a particular ground) is a function of frequency and increases with frequency.

The table below offers some approximate depths for maximum penetration by antenna frequency. Please note that this is for the same manufacturers of GPR on exactly the same ground, and as with the previous table all of the numbers are approximations and not accurately measured of calculated.

Table showing differences in GPR penetration between antenna centre frequencies
Table showing differences in GPR penetration between antenna centre frequencies
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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