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Last edited 14 Jun 2021
A geophysical survey is a cost-effective, non-intrusive and relatively efficient means of detecting and assessing sub-surface features. Geophysical surveys are capable of covering large areas at low cost by taking readings between widely spaced borings to detect and locate changes in sub-surface materials or stratification. They are useful for extended sites where conditions are generally favourable and are widely-used in the support of planning applications.
There are a wide variety of geophysical techniques available for measuring different physical properties, with different levels of sensitivity to different types of feature. The site-specific conditions and the type and composition of likely features will determine the suitability of a particular technique.
The most commonly-used geophysical survey techniques:
- Electrical resistivity.
- Seismic refraction and reflection.
- Ground-penetrating radar (GPR).
Electrical resistivity surveys detect the difference in electrical resistance between different rock or soil types. They are able to detect features such as swallow holes or underground cavities, and analysis of the results can establish the thicknesses and depths of the various sub-strata.
The method involves passing a small electrical current through equally-spaced electrodes on the surface. By varying the electrode centres, the penetration depth can be varied. The recorded differences in relative resistance are used to map features such as voids, ditches, pits and structural features. This is useful in situations where magnetometry is unsuitable, however, it can be more time-consuming and problems can occur if conductors such as pipes and cables are present, as these can make the survey results unreliable.
 Seismic refraction and reflection
Seismic refraction and reflection involves propagating vibrations with a ‘sledgehammer’ and measuring the time taken and the distance covered by the shock waves. Shallow exploration tends to favour the refraction method, whereby seismometers are spaced at increasing distances from the vibration source with the shock waves plotted against a time scale.
The reflection method is suited to exploration depths of greater than 150 m. Both methods are useful for determining the change in soil type and for surveying sites for tunnels, dams and harbour works.
Magnetometry is based on measuring the variations in intensity and direction of the earth’s magnetic field and is capable of providing a rapid, detailed assessment of a large area. Anomalies in the vertical component of the earth’s magnetic field can be detected, which can identify and locate underground cavities, building remains, pits, and buried objects such as cables and pipelines.
 Ground-penetrating radar (GPR)
A radar signal is directed at the ground, and the time take for reflection to reach the detector can be used to map the depth of sub-surface features. It can be used for deep objects and can identify small features, but is generally best suited to good ground conditions.
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