Attenuation of P‐ and S‐waves in limestones

Abstract

Ultrasonic compressional‐ and shear‐wave attenuation measurements have been made on 40, centimetre‐sized samples of water‐ and oil‐saturated oolitic limestones at 50 MPa effective hydrostatic pressure (confining pressure minus pore‐fluid pressure) at frequencies of about 0.85 MHz and 0.7 MHz respectively, using the pulse‐echo method. The mineralogy, porosity, permeability and the distribution of the pore types of each sample were determined using a combination of optical and scanning electron microscopy, a helium porosimeter and a nitrogen permeameter. The limestones contain a complex porosity system consisting of interparticle macropores (dimensions up to 300 microns) and micropores (dimensions 5–10 microns) within the ooids, the calcite cement and the mud matrix. Ultrasonic attenuation reaches a maximum value in those limestones in which the dual porosity system is most fully developed, indicating that the squirt‐flow mechanism, which has previously been shown to occur in shaley sandstones, also operates in the limestones. It is argued that the larger‐scale dual porosity systems present in limestones in situ could similarly cause seismic attenuation at the frequencies of field seismic surveys through the operation of the squirt‐flow mechanism.