Laboratory apparatuses for measuring seismic attenuation in fluid-saturated rocks

Intrinsic wave attenuation at seismic frequencies is strongly dependent on rock permeability and on fluid properties and saturation. However, in order to extract this information from seismic, experimental studies on attenuation are needed for a better understanding of such dependency. Here, we briefly describe two apparatuses developed to measure seismic attenuation in fluid-saturated rocks: the SWAM and the BBAV. Additionally, we described some advantages and drawbacks of these two machines through comparisons with six other experimental setups that have been built in the last 25 years for similar studies. All these apparatuses measure frequency-dependent attenuation in dry and partially fluid-saturated rock samples under small strains which are comparable to strains caused by seismic waves in the Earth. Introduction From a geophysical perspective, understanding seismic wave attenuation in rocks is of great importance because most of the information available on the structure and composition of the subsurface is obtained from seismic data. In particular, since the presence of pore fluids cause significant increase of attenuation in rocks, information about their properties and saturation could potentially be obtained from seismic data. Attenuation is strongly related to the capacity of fluids to move (therefore, to permeability) as a consequence of transient stress perturbation, i.e., the propagating wave. Therefore, the resulting attenuation is typically frequency dependency. Experimental studies on low frequency attenuation of rocks have been reported in the last 30 years (e.g. Murphy 1982). They indicated the importance of the poresaturating fluids in inducing a frequency-dependent seismic response of the rock. Such behavior contrasts with the case of a dry rock for which a negligible dependency of the seismic response on frequency was observed (e.g., Tisato and Madonna, 2012). Batzle et al. (2006) explained the frequency-dependency of attenuation in fluid-saturate rocks in terms of fluid mobility defined as the ratio of rock permeability on fluid viscosity and pore pressure distribution within porous rocks. Experimental evidence to support this has been provided, for example, for sandstones (Batzle et al. 2006) and carbonates (Adam et al. 2006). The SWAM The Seismic Wave Attenuation Module (SWAM, Figure 1, Madonna and Tisato, 2013) has been developed to experimentally measure attenuation in extensional mode and the Young’s modulus fluid-saturated rock samples, 60 mm in length and 25.4 mm in diameter, in a gas medium high-pressure rig (Paterson rig, Figure 2). The sample is sealed with a copper jacket. Attenuation at low seismic frequencies (0.01 to 100 Hz) is obtained. The SWAM is suitable for natural rock samples and so far it has being tested using confining pressures up to 50 MPa and at room temperature. Figure 1. Sketch of the SWAM. After Madonna and Tisato (2013). To calculate attenuation, both the applied force and the shortening of the sample are measured, employing Linear Variable Differential Transformers (LVDTs). The LVDTs allows for measuring the bulk shortening of the sample