Geophysical methods of investigation offer a number of alternative means of detecting and mapping buried karst features, says TerraDat UK's director Nick Russill.
Microgravity, for instance, is a powerful tool for identifying the presence of voids and sinkholes, exploiting the fact that their density is significantly lower than the host bedrock.
However, in some instances it can prove difficult to distinguish between solution features and localised rockhead depressions using only the gravity method.
As a result it is best to use an additional geophysical method to confirm the interpretation. A key control factor is knowledge of the depth to bedrock, which can be derived using resistivity tomography or seismic methods.
Under favourable site conditions, resistivity can be used to map karst features in cross-section, although it is recommended that any identified anomalous zones are targeted using follow-up microgravity to determine whether they relate to fracture zones or broader cavities.
Without the additional gravity data, conclusive interpretation can be less certain, particularly in areas that have variable soil cover and/or laterally unhomogeneous rock resistivity due to water or clay content.
Seismic refraction surveys are generally not suited to mapping sub-rockhead solution features. The method provides a powerful means of accurately determining soil depths for input to the gravity modelling process or for identifying sinkholes beneath selected traverse lines, however.
In summary, individual geophysical methods can provide a total solution at some sites but more commonly a combination of two carefully chosen methods measuring different physical properties will provide a better result.