Gravity

Gravity surveys look for and map local variations in the earth’s gravity field The variations of interest in oil and gas exploration are caused by irregularities or differences (anomalies) in the density of different rock strata at the point at which the field is being measured.

Different rock lithologies have different densities. For example, sandstone is approximately 2.44 g/cc, the metamorphics average 2.62g/cc., and mafic basement averages 2.77g/cc. (Actual measurements of gravity are very precise, on the order of one in 100 billion.)

Conventional gravity surveys measure only the magnitude of the gravity field. Gravity anomalies can be used to infer differences in rock density and therefore the rock types.

Interpreters can use gravity data to identify structures such as anticlines where oil traps can form. It is especially useful where low density such as salt absorbs seismic energy and masks actual structure.

Expertise is critical to gather value from gravity studies. Gravity studies require extensive knowledge of the different types of data available, the tools used to process or condition the data, and the presentation of the gravity data for meaningful interpretations. Knowledge of the science, the data sources and regional/local geology are critical to getting full value from gravity studies.

Tensor Data

Gradient tensor data
Newer equipment has enabled survey companies to collect much-higher-resolution data about the magnitude of the change in gravity in all three dimensions. Because of the mathematics involved, this is called gradient TENSOR analysis. The change in gravity in each direction reveals different features.

Gradient tensor data is used in conjunction with seismic data. It serves to constrain seismic data, to restrict the interpretation of ambiguous data to a much smaller set of possibilities. Through a series of iterations, interpreters can build a structural model that is consistent with both the gravity and seismic data. -more-