Q Measurements under Confining Pressure

Abstract

Birch and Bancroft [1] demonstrated the feasibility of resonating a rock sample under hydrostatic pressure by vibrating the fully encapsulated samples in the torsional mode. They used this approach to obtain some Q data and extensive data on the modulus of rigidity as a function of pressure for a wide variety of rock samples. We have extended their approach with the additional stipulation that the rock sample environment approach lunar conditions; i.e., a vacuum. The motivation has been to provide guidance and aid in the interpretation of lunar seismic experiments which have yielded anomalously low attenuation values. Our technique and some of the results for our torsional mode measurements are summarized below. Also described are preliminary results on measurements in the extensional (Young’s modulus) mode. Most of our Q measurements have been carried out in the pressure range 0 to 2 kbar, because this is the range where partial crack closure produces the most dramatic increases in the velocity and where any changes in Q might also be expected to be most pronounced.