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 catagories;

• 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 continential US, gravity observations collected at relatively large station spacings are available from the National Geophyiscal 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 Esimates - These estimates are based on simply plotting the observations, sketching the interpreter's esimate 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 sophisicated 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.