GPR

	Global Geophysical

Ground Penetrating Radar (GPR)

Introduction

Ground Penetrating Radar (GPR) is an electromagnetic technique in which electromagnetic waves are emitted into the ground surface which reflects from structures/targets with different dielectrical properties. GPR is similar to seismic reflection with an electromagnetic pulse being used instead of a pulse of seismic energy. GPR typically operates in the frequency range of 10 to 1000 MHz. A typical system consists of two antennas, spaced a fixed distance apart, with a data recorder connected to the antennas with communication cabling (Fibre-optic or copper). GPR is an extremely attractive geophysical method as it has the highest resolution of available geophysical techniques for imaging the subsurface.

Method of Operation

A transmitter antenna is used to transmit a high energy pulse into the subsurface with reflections measured by the receiver antenna. While pulses are continuously transmitted, the whole system is moved continuously, or in fixed station spacing, while readings are being taken. This allow for the construction of a pseudo cross section of reflections as a function of depth and chainage. A GPR system can be operated in stacked and continuous modes. In stacked mode, a single reading is stacked (averaged) typically 128 times, after which the system is moved a fixed distance, and then another reading is taken. The distance depends on the dimensions and depth of the target of interest. Stacked data gives better signal-to-noise ratio than continuous readings, and is normally employed where the target is relatively massive (for example bedrock) and relatively deep (greater than 10 meters). Continuous measurements are taken where targets are shallow with small dimensions (for example cracks and services/pipes).

Choice of antenna depends on the required resolution, depth-to-target and site conditions. A 1000 MHz antenna gives cm resolution for imaging targets in the 0-0.5 meter depth range, but will not give penetration beyond those depths. A low frequency antenna (100 MHz) will have improved penetration, but with the lower resolution cannot image small and shallow targets.

Penetration is to a large extent determined by the conductivity of the subsurface. The more conductive the subsurface, the lesser penetration is achieved. In general GPR does not work in areas where a combination of any two of water, salt or clay is present. For example, GPR does not work in seawater.

GPR allows for data interpretation on-site as data can be processed relatively quickly. An assessement of the effectiveness of the technique for the particular application can also be made during data acquisition, allowing Data Quality Control. GPR is a rapid method, and large distances can be covered in a single day. Systems can be vehicle mounted, or towed behind a vehicle. The specific system setup depends on the site logistics, targets of interest, etc.

Applications

GPR is commonly used in the following applications:

Delineation of subsurface pipes and services
Mapping of leaks
Mapping of subsurface cavities and undermining
Mapping of buried objects such as drums and tanks (both metallic and non-metallic)
Mapping depth to water table
Mapping depth to bedrock
Road Investigations
Underground mining: Roof Integrity Investigations
Underground mining: Mapping of geological structures
Mapping rebar in concrete