Gravity and Micro Gravity

Introduction

The gravity method measures the gravitational attraction exerted by the earth at a measurement station on the surface. The strength of the gravitational field is directly proportional to the mass and, therefore, the density of subsurface materials. Anomalies in the earth’s gravitational field result from lateral variations in the density of subsurface materials. Gravity acceleration is measured in microGals or sometimes in FGal for very high resolution microgal surveys. Gravity acceleration variations as a result of geological changes is very small compared to the average gravity acceleration measured and requires the need for very precise measuring and field techniques. Gravity surveys are often undertaken in areas where "cultural" noise precludes EM and seismic surveying. High resolution gravity surveys for engineering studies are often referred to as microgravity surveys. Gravity data is normally presented as Bouguer anomalies after a series of corrections including instrumentation drift, earth tidal effects, elevation corrections, changes in latitude and terrain effects in areas of significant topography.

Applications

Gravity works well in environments where there is a dramatic density contrast between the host and the target mediums like in the following applications:

• Mapping subsurface voids
• Mapping bedrock topography
• Mapping landfill thickness and extent of landfill

Limitations

• Gravity is only sensitive to density changes and could be ineffective as a single geophysical method.
• Surveying is cumbersome and prone to human errors. The cost of surveying could be therefore relatively high compared to EM and magnetic techniques.
• The resolution needed for some environmental and engineering applications could be out of range for some gravity meters.
• Very accurate elevation data is needed which increases the cost of data acquisition.

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