Gravity data processing

Once the gravity data are measured the more demanding task is to be carried out – the data processing and interpretation.

The first step in the data processing is deleting of wrong gravity readings. During the the field measurements there is usually several gravity readings taken at every station. Now, the outliers are removed and the rest of gravity readings from every station are averaged. Next, the readings from the base station are taken to determine the drift of the instrument.

First, these data need to be corrected for the different heights of the tripod, the free-air correction – equation (2.6). Second, the drift should be estimated – usually the data are interpolated using the second or third-order polynomial (Fig. 2.14). Third, the readings at individual stations are corrected from the drift.

The drift is estimated from the fitted polynomial according to the time of the gravity reading.

Fourth, the drift corrected data are reduced again, now using the latitude, free-air and Bouguer reductions – see equation 2.8). If necessary, the density for the Bouguer slab is estimated (e.g. Nettleton’s method). There are also additional steps, which depends on the type of the survey and target structures. However, usually we want to suppress regional anomalies and enhance the local ones or vice versa. The regional anomalies is a general term depending on the size of target structures.

These anomalies are caused by large and deep structures, often larger than our survey area. In the data they usually represent the long-wavelength high-amplitude anomalies (Fig 2.15c, d). Sometimes they are also referred to as a trend (Fig. 2.15d). There are numerous techniques to remove the trend, the easiest are based on approximation by a polynomial. In this case we take the part of the data without our target anomaly and fit a polynomial through them.

This polynomial approximates the effect of large-scale regional structures and we can subtract it from our data leaving us with residual anomalies. The residual anomalies are, in an ideal world, anomalies caused only by our target structures.