An EM31 CMD survey is an electromagnetic geophysical survey carried out using an EM31 or CMD instrument. Both use a boom configuration to identify variations in ground conductivity.

GEOTEC predominantly uses EM31 and CMD surveys to detect evidence of tunnels, cavities, mineshafts and dissolution features.

The EM31 can penetrate to a depth of approximately 6m below the ground surface.

The CMD Explorer has three depth thresholds and provides three sets of data: shallow (2.3m), mid-range (4.2m) and deep (6.7m).

Because the instrument provides three datasets, each can be reviewed in isolation to help identify and reduce noise caused by interference or metal.

An EM31 CMD survey may be suitable when you need to locate or investigate something beneath the ground without excavation. GEOTEC will assess the target and site conditions to determine whether this or another geophysical survey method is most appropriate.

The CMD provides three different depth readings and therefore three times as much data, helping to reduce the impact of noise within the results.

Another benefit is the ability to survey a large area in a relatively short period of time.

This technique is most effective where there is no metallic interference or physical obstruction. Metal and obstructions can reduce the ability to collect reliable data across the full survey area.

Although the boom is approximately 4m long, data must be collected at consistent 1m or 2m intervals without gaps. Each pass forms part of the wider dataset, so even a small area of interference can affect the overall results.