Brian Steiner

Comparison of 2D and 3D time-reverse imaging-A numerical case study

Time-reverse imaging has become an efficient tool to detect the origin of passively acquired seismic tremor signals. Practical experience has mainly been developed for 2D applications. Three component signals are reduced to two components and reverse propagated on the plane vertically below a station line. The data used for time-reverse imaging are the vertical and the horizontal particle displacement parallel to the line. Dropping the horizontal component perpendicular to the line causes partial loss of information on particle motion and directivity of the recorded waves. We present a comparison of 2D and 3D time-reverse imaging for a specific site with small cross-line gradients and investigate how closely 2D imaging approximates 3D imaging. Our large-scale synthetic survey with different S/N-ratios demonstrates how a subsurface source of tremor-like signals is imaged in different vertical planes. An imaging condition based on the energy density gives best results. We observe higher sensitivity to noise and stronger out-of-plane focusing for 2D than for 3D imaging. We suggest normalized visualization of multiple planes from 2D imaging in one 3D display as an approach to reliably locate sources. Comparison with examples of full 3D time-reverse imaging shows that normalized visualization of multiple 2D planes with a proper imaging condition can adequately approximate the result from full 3D imaging for the particular model considered in this study.

Time‐reverse modeling of microtremors: A potential method for hydrocarbon reservoir localization

We investigate the applicability of time reverse modeling on revealing the location of sources of continuous lowfrequency signals like microtremors. In a first step we perform a numerical feasibility study. For this approach we generate different types of synthetic microtremor data by using a finite difference technique for seismic wave propagation. Time reverse modeling is able to localize all source types investigated. The main difference in accuracy depends on whether Por S-waves are dominant. Possible examples of microtremors from a specific source location are hydrocarbon reservoirs. Hydrocarbon reservoirs can be the origin of a continuous source of low-frequency seismic waves. In a first example the application of time reverse modeling on real data is shown. We observe a pattern similar to a synthetic case which is dominated by P-waves.

Comparison of 2D and 3D time reverse modeling for tremor source localization

Time reverse modeling has become an efficient tool to detect the origin of passively acquired low-frequency (<10 Hz) seismic tremor signals. Examples of such signals are observed before volcanic eruptions or above hydrocarbon reservoirs. Technical expertise has mainly been developed for 2D applications. We present an investigation of effects from reducing three-component signals for 2D reverse modeling. For 2D processing, signals are reverse propagated on a plane vertically below a station line. The vertical and the horizontal particle displacement parallel to the line are the data used for reverse modeling. The horizontal component perpendicular to the line is dropped. This causes partial loss of information on particle motion and directivity of the recorded waves. We perform a synthetic study with a tremor-like source to investigate how this source is imaged in different vertical planes. Planes within the distance of two wavelengths from the subsurface source render focusing spots. The spots appear at shallower depth than the true source location for the source plane and shift to deeper location for the other planes. Comparison with an example of full 3D reverse calculation shows that analysis of multiple 2D images adequately approximates the result from full 3D imaging.

Scientific Strategy to Explain Observed spectral Anomalies over Hydrocarbon Reservoirs Generated by Microtremors

A033 Scientific Strategy to Explain Observed spectral Anomalies over Hydrocarbon Reservoirs Generated by Microtremors E.H. Saenger* (ETH Zurich / Spectraseis) S.M. Schmalholz (ETH Zurich) Y. Y. Podladchikov (PGP Oslo) R. Holzner (Spectraseis) M. Lambert (ETH Zurich) B. Steiner (ETH Zurich) B. Quintal (ETH Zurich) & M. Frehner (ETH Zurich) SUMMARY Worldwide one has observed narrow-band low-frequency (1.5-4 Hz) tremor signals on the surface over hydrocarbon reservoirs (oil gas and water multiphase fluid systems in porous media). These ‘hydrocarbon tremors’ possess remarkably similar spectral and signal structure characteristics pointing to a common source mechanism even though the depth (some hundreds to

Considerations of observed spectral anomalies over hydrocarbon reservoirs generated by microtremors

Narrow-band, low-frequency (1.5-4 Hz) tremor signal s on the surface over hydrocarbon reservoirs (oil, ga s and water multiphase fluid systems in porous media) has been observed worldwide. These ‘hydrocarbon tremors’ possess remarkably similar spectral and signal structure characteristics, pointing to a common source mechanism, even though the depth (some hundreds to several thousands of meters), specific fluid content (oil, gas, gas condensate of different compositions and combinations) and reservoir rock type (such as sandstone, carbonates, etc.) for each of those sites are quite different. However, the physical mechanisms underlying these observations are presently not fully understood. We propose a scientific strategy for better understand ing those phenomena. Using well-known rock physical relationships we have identified on macro-, mesoand microscale different mechanisms which can induce anomalies in the low-frequency band. Using different numerical approaches we are able to compare these mechanisms with observations in the field.