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2 Acta Geophysica

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Wave Propagation Modeling and Amplitude-Variation-with-Offset Response in a Fractured Coalbed

Lin Q-X., Peng S-P., Du W-F., Shi S-Z., Gou J-W.

Acta Geophysica

2015

Vol. 63, no. 3

815--842

Coalbed methane can be detected employing the amplitudevariation- with-offset technique. However, there are two issues in applying this technique to a coalbed: strong azimuthal anisotropy resulting from high-density fractures, and the seismic response being composed of many or several individual reflections within the coalbed. To overcome these difficulties, we present an exact solution for reflections in extensive dilatancy anisotropy media. First, we build a three-layer model and simulate the wave propagation in this model. Then we derive an exact P- and converted S-wave reflection coefficient equation based on boundary conditions. Finally, substituting given model parameters into the exact equation, we obtain the variation in the reflection coefficient with incidence angle. The results show that the fracture factors, wavelet frequency and thickness of the coalbed have different effects on the reflection coefficient. Furthermore, we create a synthetic seismogram by forward calculation, and the result fits well with results of the exact equation.

Azimuthal anisotropy of the Pg-wave velocity in hypocentral volumes of NW Croatia

Herak M., Herak D., Stipčević J.

Acta Geophysica

2009

Vol. 57, no. 3

600-615

Based on a large set of arrival times of the Pg phase reported by local and regional stations, we estimate azimuthal anisotropy of the Pg-wave velocity in focal volumes of the upper crust in NW Croatia. The method is based on analyses of the azimuthal dependence of ratios of cumulative differences of arrival times and travel paths between foci of earthquake pairs, computed for rays propagating within narrow azimuthal windows. The results clearly indicate the presence of anisotropy of 3.3% with the direction of fast velocity (approximately NNE-SSW) coinciding with the direction of the maximum tectonic pressure as revealed by 23 available focal mechanisms and previous geological investigations. Although a large part of observed anisotropy can be explained assuming that focal volumes are pervaded by a system of vertical extensivedilatancy anisotropy (EDA) cracks aligned under the influence of local tectonic stress field, there is indication that - to a smaller extent - some role was also played by alignment of structural features in the region.