Estimating fracture orientation from elastic‐wave propagation: An ultrasonic experimental approach

Abstract

Elastic‐wave propagation in fractured and cracked media depends on the dominant spatial orientation of the discontinuities. Consequently, compressional and shear‐wave velocities can give valuable information about the orientation of the cracks. The main goal of this work is to estimate the preferential fracture orientation based on an analysis of cross‐correlated S‐wave seismograms and Thomsen parameters. For this purpose, we analyzed ultrasonic measurements of elastic (P and S) waves in a physical‐modeling experiment with an artificially anisotropic cracked model. The solid matrix of the model consisted of epoxy‐resin; small rubber strips simulate cracks with a compliant fill. The anisotropic cracked model consists of three regions, each with a different fracture orientation. We used the rotation of the S‐wave polarizations for a cross‐correlation analysis of the orientations, and P‐ and S‐wave measurements to evaluate the weak anisotropic parametersγ and ε.The shear and compressional wave sources had dominant frequencies of 90 kHz and 120 kHz. These frequencies correspond to long wavelengths compared to the spacing between layers, indicating a nearly effective‐media behavior. Integrating the results from cross‐correlation with anisotropic parameter analysis, we were able to estimate the fracture orientation in our anisotropic cracked physical model. Theγparameter showed good agreement with the cross‐correlation analysis and, beyond that, provided additional information about the crack orientation that cross‐correlation alone did not fully resolve. Moreover, our results show that the shear waves are much more strongly influenced by, and can thus contain more information about, crack orientation than compressional waves.

Stress‐dependent permeability and wave dispersion in tight cracked rocks: Experimental validation of simple effective medium models

“…A convenient way to conduct experiments under well‐controlled conditions involves the use of synthetic materials for which the microstructure is well characterized and reproducible. Several attempts have been reported in the literature for manufacturing synthetic rocks with controlled microstructures that are simpler to analyze than typical microstructures found in natural rocks [e.g., Ass’ad et al ., , ; Rathore et al ., ; Tillotson et al ., , ; Stewart et al ., ; De Figueiredo et al ., , ; Amalokwu et al ., ; Ding et al ., ]. The manufacturing process of these synthetic rocks allows for a relatively accurate control of the final microstructure.…”

Effect of fracture orientation on ultrasonic wave signatures

“…S2-wave is polarized in the direction orthogonal to the fracture plane, i.e. fracture orientation is 90° with respect to the plane of wave propagation [5,15,16] (red wave in Fig. 2(b)).…”

“…[2,15]). Cross-correlation between S1 and S2 wave can be used to estimate the fracture orientation [16].…”

Experimental verification of effective anisotropic crack theories in variable crack aspect ratio medium

“…(), De Figueiredo et al . () and Santos et al . () constructed anisotropic samples with elongated cracks positioned in such a way to create different regions where within each region, the cracks shown the same orientation, but the latter changed from region to region.…”

“…In this work, it was experimentally verified that the linear relation between the crack density and the Thomsen's parameter (γ ), predicted by Hudson's (1981) effective cracked model, is valid for fractured media with crack density lower than 7%. Based on similar technique developed by Assad et al (1992), De Figueiredo et al (2012) and Santos et al (2015) constructed anisotropic samples with elongated cracks positioned in such a way to create different regions where within each region, the cracks shown the same orientation, but the latter changed from region to region. They showed the importance of S-wave birefringence to find the fracture preferential direction in fractured media with different fracture orientations.…”