Gravity Mesurement
The acceleration of gravity varies with one's location on the surface of the earth depending on such factors as elevation, density of surrounding bedrock, surrounding topography, and latitude. Topography and density variations and inhomogeneities lead to local variations in the surface gravity referred to as gravity anomalies (unit : mGal). We usually observe large topographic features with low-density crustal roots. The excess mass of topography produces a positive gravity anomaly, the low-density root produces a negative anomaly in contrast.

Geomagnetic Mesurement
The geomagnetic field varies in both direction and intensity over time scales from milliseconds toward millennia. Rocks contain small amount of magnetic minerals which can acquire a weak permanent magnetism when they are formed. This special variation is known as the magnetic anomaly. Regions of high magnetic field show a positive magnetic anomaly concentrated on magnetic minerals in the earth's crust. Marine magnetic surveys have shown magnetic anomaly lineations which indicate continuous parallel zones of positive anomalies and negative anomalies (bands). These magnetic bands are extremely useful in determination of the age of the oceanic crust. Shipboard magnetometers have been used to obtain and create maps of magnetic anomalies over most of the world's oceans.

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