Attenuation of P- and S-waves in limestones

Ultrasonic compressional- and shear-wave attenuation measurements have been made on 40, centimetre-sized samples of water- and oil-saturated oolitic l imestones at 50 MPa effective hydrostatic pressure (confining pressure minu s pore-fluid pressure) at frequencies of about 0.85 MHz and 0.7 MHz respect ively, 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 porosi meter and a nitrogen permeameter. The limestones contain a complex porosity system consisting of interparticle macropores (dimensions up to 300 micron s) and micropores (dimensions 5-10 microns) within the ooids, the calcite c ement 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 show n to occur in shaley sandstones, also operates in the limestones. It is arg ued that the larger-scale dual porosity systems present in Limestones in si tu could similarly cause seismic attenuation at the frequencies of field se ismic surveys through the operation of the squirt-flow mechanism.