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.

Attenuation from microseismic datasets by the peak frequency method benchmarked with the spectral ratio method

“…Nevertheless, the attenuation factors are reasonable and expected for carbonate sedimentary basin. Assefa et al (1999) shows that 40 P Q  is common value for limestone and 55 S Q  is within upper limits presented in that study. Note that the estimate of quality factor of S-waves is higher than the quality factor of P-waves.…”

Estimation of Source Parameters, Quality Factor (QS), and Site Characteristics Using Accelerograms: Uttarakhand Himalaya Region

“…Besides the divergence, seismic waves lose energy due to the attenuation caused by scattering and intrinsic absorptions along the transmission path. The intrinsic absorption depends on rock type (Assefa et al, 1999), material state (Del Pezzo et al, 1995), temperature (Roth et al, 2000), thermoelasticity (Aki, 1980;Frankel et al, 1990), effects of fluids (Hoang-Trong, 1983), and frictional heating (Fukuchi et al, 2005). On the other hand, scattering attenuation represents loss of energy of a direct wave caused by reflection, refraction, and conversion due to inhomogeneous medium and/or existence of cracks and fractures.…”

Velocities of compressional and shear waves in limestones

“…The rocks are composed mainly of calcite and dolomite with minor amounts of quartz and feldspars (Table 1). Assefa et al . (1999) have described the rocks in detail.…”

“…the ratio of the smallest to the largest pore diameter) were made on pores that were large enough (> 20 microns) to be measured under the optical microscope and were mainly associated with the grainstones and packstones (Table 2). This means that it was not possible to measure the aspect ratio of the small‐size (submicroscopic) pores, which may constitute up to 10% of the total porosity (Assefa et al . 1999).…”