Geophysical Investigation

Service 06

Boreholes give precise information at single points. Geophysical survey fills in what happens between them — the shape of the rockhead, the position of a fracture zone, a cavity, a contamination plume or a water-bearing layer — by measuring physical properties from the surface or down a borehole.

How geophysics is normally used

A survey is run first to identify where the ground changes, and the drilling programme is then positioned to confirm and calibrate what the survey shows. Used this way, geophysics reduces the number of unproductive boreholes and gives a continuous picture instead of a set of isolated points.

Seismic

Refraction

Measures the travel time of seismic waves refracted at layer boundaries. Commonly used to profile depth to rockhead and the degree of weathering along a line, which supports earthworks and foundation planning.

Reflection

Images deeper layering by recording waves reflected from boundaries at depth.

Electromagnetic

Sirotem

Transient electromagnetic survey used to detect conductive ground — for example saline groundwater or certain mineralised zones.

Very low frequency (VLF)

A rapid reconnaissance method that responds to steeply dipping conductive features such as fracture and shear zones.

Down-the-hole logging

Instruments run inside a borehole to log formation properties continuously with depth, tying surface survey results to the ground truth in the hole.

Potential field and electrical

Gravity

Detects density contrasts in the subsurface, including cavities in limestone terrain and buried structures.

Resistivity

Measures the electrical resistivity of the ground to map soil and rock layering, groundwater and anomalous zones — the workhorse method for groundwater exploration.