Numerical simulations of seismic wave attenuation in fractured rocks

RUBINO GERMÁN; MÜLLER TOBIAS; MILANI MARCO; GUARRACINO LUIS; CASTROMÁN, GABRIEL ALEJANDRO; MONACHESI LEONARDO; QUINTAL BEATRIZ; ZYSERMAN, FABIO IVÁN; HOLLIGER KLAUS

Ensenada

Workshop; 10th Workshop on Applications of Physics of Porous Media; 2014

The presence of fractures is very common in the upper part of the Earth?s crust. Since these objectstend to control the mechanical and hydraulic properties of the embedding material, there is currentlygreat interest in improving geophysical monitoring techniques to detect and characterize fracturenetworks. In this sense, seismic attenuation has been recognized as an important parameter for thispurpose, as laboratory studies and field measurements indicate that seismic attenuation levels are veryhigh in fractured rock masses and tend to increase with increasing fracture density. In particular,fracture connectivity controls the flow and transport properties of fractured formations and, hence,corresponding relevant information extracted from seismic observations would be of great importance.The elevated seismic attenuation levels typically observed in fractured rocks can be due to wave-induced fluid flow (WIFF) between the fractures and the embedding matrix. That is, due to the veryhigh compressibility contrast between the fractures and the porous background, seismic waves inducestrong fluid pressure gradients followed by local fluid flow between such regions, which in turnproduces significant attenuation and velocity dispersion. In this talk, we will show how numericaloscillatory compressibility simulations based on the quasi-static poroelastic equations can be exployedto explore WIFF effects on seismic attenuation and dispersion in fractured media [1]. We will firstdemonstrate that relaxation experiments are more suitable than creep tests for this purpose [2]. Next,we will explore the characteristics of seismic attenuation and phase velocity curves in rock samplescontaining fracture networks having varying degree of connectivity. In particular, we will show that anadditional manifestation of WIFF and a modification of the attenuation peak related to WIFF betweenfractures and embedding matrix arise in presence of fracture connectivity [1, 3]. Moreover, theseeffects become stronger as the connectivity degree increases. Finally, we will show how some of theseporoelastic effects can be included in the framework of the linear slip theory, a computationallyconvenient and common theoretical framework that allows to perform seismic wave propagation ingeological models containing fractures.[1] Rubino, J.G., Müller, T.M., Guarracino, L., Milani, M. and Holliger, 2014, K., ?Seismoacousticsignatures of fracture connectivity?. Journal of Geophysical Research, 119, 2252-2271.[2] Milani M., Rubino J. G., Müller T. M., Quintal B. and Holliger K., 2014, ?Velocity and attenuationcharacteristics of P-waves in periodically fractured media as inferred from numerical creep andrelaxation tests?. SEG Expanded Abstracts, 2882-2887.[3] Rubino, J.G., Guarracino, L., Müller, T.M. and Holliger, K., 2013, ?Do seismic waves sensefracture connectivity??. Geophysical Research Letters, 40, 692-696.