GBG Group was asked to investigate a mineral processing plant overseas in Papua New Guinea (PNG) to test the ground beneath the processing plant. Concern about the integrity and life of the structures meant that remediation or replacement was required for the plant infrastructure. The material beneath the plant has a risk of being eroded by the acidic contents of the tank. Access below the tanks was limited to tunnels lined with corrugated iron or concrete-lined rooms. Concrete cores were taken using a drill to test the concrete and soil conditions.

Determining the size and location of areas that had been eroded chemically or physically required a geophysical approach. Areas of low density or voiding would impede the velocity of seismic waves. The geophones were placed at regular intervals along the length of the access tunnels – one row in the ceiling and another on the floor. Hammer blows were struck around the circumference of the tank at the surface. The travel times from the surface to each geophone were analysed and measured to determine variations.

By mapping the ray paths and comparing their known distances and the time for the wave to travel, the velocity was determined. Each ray path was then inverted to generate a model of the subsurface in GeoTomCG software. Because the elevation varied between the geophones, as well as the shot points, a 3D model could be generated. Areas of overlapping low bulk velocity ray paths were able to be modelled as discrete anomalies.

The geometry of this survey was unusual and required specific processing that can be applied to other jobs where traditional seismic surveys would be unable to generate an appropriate model.