There are a selection of geology tutorials from introductory to advanced levels covering a range of topics which together form a free online textbook.

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Below is the abstract of an example tutorial.

Gravity Surveying

Gravity surveying involves looking at the subsurface structure based on the differences in densities of the subsurface rocks. The idea is based around a causative body - one which produces a gravity anomaly. Assessment of gravity anomalies can give ideas about depth, size and shape of the causing body. Gravity anomalies are measured in ms-2. As the anomalies are very small compared to the Earth's field (9.81ms-2), they are commonly expressed in gravity units (g.u.), where 1 gu = 1μms-2.

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