A study of the difference between gravimetric and gamma-gamma density near the surface in northern Yucca Flat

Although the mean and median of the difference between average density from 50 to 200 m below the surface in 43 holes in northern Yucca Flat computed from a density log and that computed from borehole gravity using the measured free air gradient is very close to zero, the range of values is very large. Many individual values of the difference are significantly different from zero. When the gravimetric density is computed using a free air gradient calculated from the surface gravity on an irregular but approximately 300 m grid, the mean and median of the difference between gravimetric and gamma-gamma density is slightly closer to zero than when the measured free air gradient is used. The range of values is not significantly decreased. Proposed explanations for this large range fall into two categories: anomalies in the gravity field or its horizontal derivative at the sites with large difference range, causing changes in the free air gradient or the near-surface gravimetric density, and mechanisms that affect the bulk density near the borehole, causing changes in the gamma-gamma density. We have investigated many proposed explanations and have found no single mechanism that is consistent with the data. These results indicate thatmore » the choice of method of determining free air gradient causes no significant change in the difference between near-surface gamma density and gravimetric density, and that there is no obvious method of eliminating the range. Because calculation from surface gravity is much less expensive than tower measurement, and there is no evidence that it is less accurate, we consider it the logical method for routinely obtaining free air gradient until some evidence is found to suggest the contrary. We are attempting to optimize the calculational procedure. The tradeoff is between the accuracy of the calculated free air gradient and the amount of surface gravity data required. 8 refs., 19 figs., 1 tab.« less