Separating Local and Regional Gravity Anomalies
Because Regional Anomalies vary slowly along a particular profile and Local
Anomalies vary more rapidly, any method that can identify and
isolate slowly varying portions of the gravity field can be used to
separate Regional and Local Gravity Anomalies.
The methods generally fall into three broad categories:
- Direct Estimates - These are estimates of the regional gravity anomaly determined
from an independent data set. For example, if your gravity survey is conducted
within the continental US, gravity observations collected at relatively large
station spacings are available from the National
Geophysical Data Center on CD-ROM.
Using these observations, you can determine how the long-wavelength gravity
field varies around your survey and then remove its contribution from your
data.
- Graphical Estimates - These estimates are based on simply plotting the observations,
sketching the interpreter's estimate of the regional gravity anomaly, and
subtracting the regional gravity anomaly estimate from the raw observations
to generate an estimate of the local gravity anomaly.
- Mathematical Estimates - This represents any of a wide variety of methods for determining
the regional gravity contribution from the collected data through the use of mathematical procedures.
Examples of how this can be
done include:
- Moving Averages - In 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.
Averaging gravity values over several observation points enhances the long-wavelength contributions
to the recorded gravity field while suppressing the shorter-wavelength contributions.
- Function Fitting - In this technique, smoothly varying mathematical functions are fit to the
data and used as estimates of the regional gravity anomaly. The simplest of any number of possible
functions that could be fit to the data is a straight line.
- Filtering and Upward Continuation - These are more sophisticated mathematical
techniques for determining the long-wavelength portion of a data set.
Those interested in finding out more about these types of techniques can
find descriptions of them in any introductory geophysical textbook.
