THE APPLICATION OF SPATIAL DISPLAY METHODOLOGIES AND ANALYSIS TO GRAVITY MAPPING IN PENNSYLVANIA

Gravity mapping projects are excellent examples of the integration of geophysical data collection, modeling and display. Besides subsurface geologic modeling, these projects require the spatial presentation and manipulation of large amounts of data.

Over the last fifteen years, we have been collecting gravity data in Eastern Pennsylvania as part of numerous class, research and senior thesis projects. More recently the project has expanded to involve the creation of a gravity map of Pennsylvania. In addition to the data that we have collected, we have been compiling gravity data from various sources including the National Geophysical Data Center and other academic institutions. These data have been corrected for compatibility and integrated into one large database. At the present time our gravity database for Pennsylvania has more than 13,000 observations. However, because the density of stations varies across the state, another 5,000 to 10,000 observation will be made in order to complete the gravity map of Pennsylvania,

Spatial data presentation and analysis plays a very large role in the interpretation of gravity data. The first step in the process of making the gravity map is the posting of the locations. This is used to help us determine where new observations should be competed. Once the data set is assembled, the primary goal is to create a contour map of the gravity data. This is usually the step that requires the greatest number of iterations since so many parameters can be varied including; gridding and contouring algorithms, contour intervals and display types. In addition to contour maps of the observed gravity, regional and residual maps can also be made. Regional maps essentially display the long wavelength anomalies in the data, generally reflecting deeper-seeded variations in geology. Residual maps display the difference between the observed maps and the regional-trend maps. The anomalies displayed on residual maps are in response to near-surface variations in lithologies. Examples of all of these maps as well as models and interpretation of subsurface geology will be taken from the preliminary gravity map of Pennsylvania.