Survey of site with solution features.JPG

GEOPHYSICAL
ENGINEERING

Microgravity

Gravity surveys measure variations in the earth’s gravitational field (specifically changes in the acceleration due to gravity) and have been used to identify large scale geological variations and features since the early part of the twentieth century.

Over the last few decades improving equipment sensitivity has enabled small localised variations to be detected which allows the technique to be used for engineering and environmental applications. To detect features on this local scale instrumentation that is capable of measuring the Earth’s acceleration due to gravity (g) to 1 part in 1 billion is required. These are termed microgravity surveys as the variations are usually measured in micro Gals (µGal).

These small variations in the acceleration due to gravity, caused by changes in the near-surface geology, including variations caused by sub-surface features (coupled with the effects from the sun and the moon) allows geophysicists to detect features such as:

Mineshafts
Mine workings
Solution features
Voids
Cavities
Tunnels and culverts
Buried structures

The Method

The instrument used is usually a Scintrex CG-6 which is an automated gravity meter with a resolution of 1 µGal. The measurements are usually acquired on a regular grid. The grid spacing determines the resolution of the survey and is dependent on the survey objectives.

A base station is established and readings are taken at this position at the start and end of the survey and also at regular intervals during the course of the survey.

Each survey station must be accurately recorded in both plan and elevation, relative to the base station, to allow accurate processing of the data.

Above: A mine adit.

Culverted watercourse_washout detection.JPG

Above: Culverted watercourse washout detection.

Residual Bouguer Anomaly over solution features (blue circular areas).jpg

Above: Residual Bouguer Anomaly over solution features (blue circular areas).

Processing and interpreting the collected data

The data is stored within the microgravity instrument and downloaded to specialist software for processing. The measurements recorded are subject to several external effects that are not related to the sub-surface geology or features and must be removed. The process of correcting for these effects is often called the reduction of data.

The end result of gravity data reduction is called the Bouguer Anomaly which is presented as a 2D contour plot. A further processing step can be applied to the data which results in the ‘residual Bouguer Anomaly’. It is variations in this data that can highlight changes in the sub-surface density.

The data is analysed in detail by our experts to identify anomalies relative to the general background that could indicate the presence of sub-surface features. Survey results are presented as co-ordinated plans in a CAD format with an accompanying report.