Marc-André Lambert

Soft Soil Amplification of Ambient Seismic Noise – Field Measurements and Numerical Modeling of H/V Ratios

Modifications of the seismic background noise spectrum were observed above hydrocarbon reservoirs (‘hydrocarbon tremors’). Other effects, such as for example the presence of soft soil layers, can also amplify the background noise at certain frequencies. The effect of soft soil amplification is widely used to derive resonance frequencies of the site as well as the S-wave velocity and thickness of the soil layer. One of the most popular techniques to determine those values is the so called Nakamura’s H/V spectral ratio technique. We observe soil related modifications in our measured data and we are able to reproduce those effects with a 2D finite difference wave propagation model. The synthetic results match very well the analytical predictions. Such simulations help to clean measured data from soft soil related effects and thus to improve the identification of hydrocarbon related information in the microtremor spectra.

Detection of a viscoelastic inclusion using spectral attributes of the quasi-stationary seismic surface response

This study investigates the feasibility of detecting a viscoelastic inclusion in the subsurface by analyzing the seismic wavefield at the mediums surface. A synthetic quasi-stationary wavefield is generated by random body wave sources at depth. The seismic response at the surface is then analyzed in the frequency domain for lateral variations of two specific spectral attributes. The results show that an inclusion with anomalous attenuation properties can create spectral attribute anomalies in the quasi-stationary background wavefield at the surface. This is even the case in the presence of a shallow low velocity layer. The study provides physical theoretical support for the contention that spectral anomalies in the ambient background wavefield can occur due to anomalously high attenuation properties of hydrocarbon reservoirs.

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

Using spectral attributes to detect seismic tremor sources — a synthetic study

Summary A method is presented to detect subsurface seismic tremor sources by analyzing surface data. Spectral attributes of the recorded seismic wave-field at low frequencies are used to map the surface projection of the sources. We illustrate the concept on a synthetic data-set generated with a homogenous forward model and show how spectral attributes can be used for detecting locations of seismic tremor sources. In a second part we apply the method to an example of hydrocarbon reservoir related tremors. The results show that increased complexity of the subsurface seismic properties and/or the presence of several tremor sources can strongly complicate the interpretation. In addition, the presence of dominant surface noise may mask the signals emitted by the subsurface tremor sources and make it impossible to detect them at the surface without additional processing. F-K filtering is successfully applied to noise-contaminated data and retrieves masked anomalies. Care has to be taken for using a proper data-set and proper processing parameters in order to avoid artifacts introduced by the F-K filter. Although we discuss an application for possible hydrocarbon reservoir related tremors, we believe that the methods can also be applied to any other type of seismic tremor signal.

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.