Geophysical Borehole Logging In Massive Carbonates Whitmoyer Laboratories Superfund Site Meyerstown, Pa

Correlation within massive carbonate sequences like those present in central Petmsylvania’s<br>Lebanon Valley is often difficult because the units contain few distinct marker beds and the<br>contacts between units are frequently gradational. This project demonstrated that correlations<br>among boreholes can successfully be made using a combination of geophysical logs,<br>particularly natural gamma, high resolution density and sonic. The technique involves<br>positioning logs from several of the deepest holes on cross sections at the best estimated<br>structural elevation (correcting for regional dip), comparing the general shape of the natural<br>gamma curve and adjusting the logs positions for a best fit. Once a datum has been<br>hypothesized, (all) the logs are examined for anomalies that might be traced through. The high<br>resolution density proved capable of identifying small fractures which appeared to correlate<br>along strata lines with the gamma. These hypothetical “marker beds” are projected onto as<br>many logs as possible to verify the anomaly. Once an anomaly (bed) is identitied on several<br>logs over a sutXciently wide area, three point strike and dip calculations are performed to verify<br>that the correlations fit the regional pattern.<br>This technique is not particularly new, it has been used in subsurface investigations for<br>decades. The application to massive units with only the most subtle marker beds, however, has<br>not been particularly successful. It proved successful at this site because of the use of sensitive<br>gamma and density sondes, and by a large number (62) of boreholes, a third of which penetrate<br>over 400 feet of section. (Although only two wells intersected the Ontelaunee-Amiville contact.)<br>Only very slight density (lower) and velocity (faster) differences were noted between the<br>dolomitic limestone (bioherm) of the Ontelaunee Formation and the limestones of the Amwille<br>and Epler Formations. These differences were so slight as to not be noticed until the logs were<br>positioned correctly on the sections by gamma and density log correlations.