SEISMOELECTRIC SURVEY

A geophysical method used to measure permeability or hydraulic conductivity, ransmissivity, porosity and to some extent estimated yield of an aquifer.

A method of obtaining electrical responses from seismic signals called seismoelectrics, has been periodically investigated since the 1930’s with very little reported development. During the 1990’s, an English company successfully developed and patented a practical SES site investigation system. This system can measure detailed SES signals from porous rocks to a depth of several hundred metres. SES soundings and profiles have been carried out to delineate the water table and permeable shallow aquifers, providing good resolution.

Unlike nearly all other geophysical techniques, if there is no fresh water filled layer, there is no reproducible signal.

Baylis Bros Ltd arrive to drill a few holes on the whenua.

Alex Johansen (Bay Geological Services Ltd) examining the soil removed from the test site.

A small sharp seismic pulse is generated by a small explosive charge.

The resulting wave front travels into the ground at the velocity of sound. When it encounters an aquifer, the water being less compressible, is forced to move as the rock matrix (more compressible) is deformed. The water carries an ionic charge with it away from ions of opposite charge, which adhere to the rock surface. This charge separation disturbs the electromagnetic field and that disturbance propagates to the surface at the speed of light where it is detected by a pair of electrodes. The rise time of the signals is proportional to permeability, the easier the water can move, the faster the response.

Where several shots have been fired in a line, a profile can be drawn.

EXAMPLE PERMEABILITY PROFILE LINE