Regional/Local Gravity Anomaly Separation Example
As an example of estimating the regional anomaly from the recorded data and isolating the local
anomaly with this estimate consider using a moving average operator.
With this technique, an estimate of the regional gravity anomaly at some
point along a profile is determined by averaging the recorded gravity values at several nearby
points. The number of points over which the average is calculated is referred to as the
length of the operator and is chosen by the data processor.
Averaging gravity values over several observation points enhances the long-wavelength contributions
to the recorded gravity field while suppressing the shorter-wavelength contributions.
Consider the sample gravity data shown below.
Moving averages can be computed across this data set. To do this the data processor
chooses the length of the moving average operator. That is the processor decides to compute the average over 3, 5,
7, 15, or 51 adjacent points. As you would expect, the resulting estimate of the
regional gravity anomaly, and thus the local gravity anomaly is critically dependent on
this choice. Shown below are two estimates of the regional gravity anomaly using moving
average operators of lengths 15 and 35.
Depending on the features of the gravity profile the processor wishs to extract, either of these operators
may be appropriate. If we believe, for example, the gravity peak located at a distance of about
30 on the profile is a feature related to a local gravity anomaly notice that the 15 length
operator is not long enough.
The average using this operator length almost tracks the raw data, thus when we subract the averages from the raw data
to isolate the local gravity anomaly the resulting value will be near zero.
The 35 length operator, on the other hand, is long enough to average out the anomaly of interest, thus isolating
it when we subtract the moving average estimate of the regional from the raw observations.
The residual gravity estimates computed for each moving average operator are shown below.
As expected few interpretable anomalies exist after applying the 15 point operator.
The peak at a distance of 30 has been greatly reduced in amplitude and other short-wavelength
anomalies apparent in the original data have been effectively removed.
Using the 35 length operator, the
peak at a distance of 30 has been successfully isolated and other short-wavelength anomalies
have been enhanced. Data
processors and interpretors are free to choose the operator length they wish to apply
to the data. This choice is based soley on the features they believe represent the local anomalies
of interest. Thus, separation of the regional from the local gravity field is an interpretive
process.
Although the interpretive nature of the moving average method for estimating the regional
gravity contribution is readily apparent, you should be aware that all of the methods
described on the
previous page
require interpreter input of one form or another. Thus, no matter which method is
used to estimate the regional component of the gravity field it should always be considered
an interpretational process.