Seismic facies discrimination incorporating relative rock physics

Seismic facies discrimination is usually performed based on a rock-physics-driven quantitative interpretation approach. The accuracy of the study of rock physics largely impacts the reservoir and fluid recognition. However, the study is commonly conducted with absolute well logs without removing the trend effect. Such an approach may introduce inappropriate low-frequency information and bias further analysis of seismic data (crossplotting, facies probability density function generation, and projection angle determination). By contrast, relative rock physics with the trend decomposed reflects the rock-property variation of the overburden and underlying formation. The relative portions are more consistent with the seismic reflectivity, providing an alternative tool to facies interpretation through a seismic inversion scheme. A workflow for seismic facies discrimination has been investigated that incorporates relative rock physics, long short-term memory-based nonlinear seismic inversion, and Bayesian classification. This workflow is employed in a case study from Songliao Basin in northeast China, through which the results of relative and absolute approaches in key steps are analyzed and compared. The consistency of facies, determined through relative and absolute methods with petrophysical interpretation, is calculated. The relative analysis exhibits improved agreement with petrophysical interpretation in overall facies and reservoir sand discrimination of the blind wells. This indicates the potential to minimize the trend bias by integrating relative rock physics in quantitative interpretation.

Seismic anisotropy in metamorphic rocks from the COSC-1 borehole, Sweden: A cross-scale investigation from thin section analysis to seismic scales

<p>Metamorphic and deformed rocks in thrust zones show particularly high seismic anisotropy causing challenges for seismic imaging and interpretation. A good example is the Seve Nappe Complex in Jämtland, Sweden, an exhumed orogenic thrust zone characterized by a strong but incoherent seismic reflectivity and considerable seismic anisotropy. However, only little is known about the origin of the anisotropy in relation to composition, structural influences, and implications for measurements at different seismic scales. We present an integrative study of the seismic anisotropy at different scales combining mineralogical composition, microstructural analyses and seismic laboratory experiments from samples of the 2.5 km-deep COSC-1 borehole. While there is a pronounced crystallographic preferred orientation in most of the core samples, variations in anisotropy correlate strongly with bulk mineral composition and dominant core lithology. Based on three major lithologic different facies (felsic gneiss, amphibole-rich rocks, and mica schists), we propose an anisotropy model for the full length of the borehole, which indicates two prevailing anisotropic units. Comparison of laboratory seismic measurements and electron-backscatter diffraction (EBSD) data reveals a strong scale-dependence, which is more pronounced in the highly deformed, heterogeneous samples. This highlights the need for comprehensive cross-validation of microscale anisotropy analyses with additional lithological data when integrating seismic anisotropy through seismic scales.</p>

Low-frequency earthquakes reveal punctuated slow slip on the deep extent of the Alpine Fault, New Zealand

We present the first evidence of low-frequency earthquakes (LFEs) associated with the deep extension of the transpressional Alpine Fault beneath the central Southern Alps of New Zealand. Our database comprises a temporally continuous 36 month-long catalog of 8760 LFEs within 14 families. To generate this catalog, we first identify 14 primary template LFEs within known periods of seismic tremor and use these templates to detect similar events in an iterative stacking and cross-correlation routine. The hypocentres of 12 of the 14 LFE families lie within 10 km of the inferred location of the Alpine Fault at depths of approximately 20-30 km, in a zone of high P-wave attenuation, low P-wave speeds, and high seismic reflectivity. The LFE catalog consists of persistent, discrete events punctuated by swarm-like bursts of activity associated with previously and newly identified tremor periods. The magnitudes of the LFEs range between ML - 0.8 and ML 1.8, with an average of M L 0.5. We find that the frequency-magnitude distribution of the LFE catalog both as a whole and within individual families is not consistent with a power law, but that individual families' frequency-amplitude distributions approximate an exponential relationship, suggestive of a characteristic length-scale of failure. We interpret this LFE activity to represent quasi-continuous slip on the deep extent of the Alpine Fault, with LFEs highlighting asperities within an otherwise steadily creeping region of the fault. © 2014. American Geophysical Union. All Rights Reserved.

Low-frequency earthquakes reveal punctuated slow slip on the deep extent of the Alpine Fault, New Zealand

We present the first evidence of low-frequency earthquakes (LFEs) associated with the deep extension of the transpressional Alpine Fault beneath the central Southern Alps of New Zealand. Our database comprises a temporally continuous 36 month-long catalog of 8760 LFEs within 14 families. To generate this catalog, we first identify 14 primary template LFEs within known periods of seismic tremor and use these templates to detect similar events in an iterative stacking and cross-correlation routine. The hypocentres of 12 of the 14 LFE families lie within 10 km of the inferred location of the Alpine Fault at depths of approximately 20-30 km, in a zone of high P-wave attenuation, low P-wave speeds, and high seismic reflectivity. The LFE catalog consists of persistent, discrete events punctuated by swarm-like bursts of activity associated with previously and newly identified tremor periods. The magnitudes of the LFEs range between ML - 0.8 and ML 1.8, with an average of M L 0.5. We find that the frequency-magnitude distribution of the LFE catalog both as a whole and within individual families is not consistent with a power law, but that individual families' frequency-amplitude distributions approximate an exponential relationship, suggestive of a characteristic length-scale of failure. We interpret this LFE activity to represent quasi-continuous slip on the deep extent of the Alpine Fault, with LFEs highlighting asperities within an otherwise steadily creeping region of the fault. © 2014. American Geophysical Union. All Rights Reserved.