First Break - Volume 29, Issue 8, 2011

To achieve an improved understanding of near surface seismic data, Claudio Strobbia, Alexander Zarkhidze, Roger May, John Quigley and Phil Bilsby of WesternGeco present a model-based approach to source-generated coherent noise attenuation.

Prof John M. Reynolds reviews where near surface geophysics and its various sub-disciplines are today and the challenges ahead, based upon work preparing a second edition of his wellknown textbook on environmental geophysics.

In this overview of research into geoelectrical monitoring at the British Geological Survey, Paul Wilkinson, Jonathan Chambers, Oliver Kuras, Philip Meldrum and David Gunn highlight the technology and algorithms that have been developed to investigate the dynamics of landslide processes, the physical integrity of vulnerable earth structures, and the thermal state of permafrost.

Nigel J. Cassidy, Rod Eddies, Peter Styles, Simon Brightwell and Sam Dods make the case that, in the testing of building structures, GPR and ultrasonic surveys can be used in combination across a much wider range of applications areas than previously envisaged.

The world’s largest oil discoveries in recent years have been pre-salt, in reservoirs located below or close to salt bodies. It is often assumed that the salt is homogeneous and that a near-constant isotropic velocity in the salt body can be used for seismic imaging. Here we show that this simplification may be too simple. Based on field observations from an old salt mine in Spain, we estimate anisotropic parameters describing the exploited salt diapir. In its outcrop we identify a regular pattern of alternating halite and clay layers, where the thickness of each layer is close to periodic. From this observation, we estimate corresponding anisotropic parameters for this salt outcrop and find that the degree of anisotropy is moderate, of the order of 5% difference between horizontal and vertical velocities. Furthermore, we identify potential shear zones within the salt outcrop that can be mapped over distances of several hundred metres. The thicknesses of these shear zones are of the order of metres. Based on these observations, it is a huge simplification to treat salt bodies as homogeneous and isotropic for seismic imaging purposes.

Geostatistical inversion of seismic data can be used to build highly detailed reservoir models with integration of information from diverse sources. A key to successful integration is the solution for facies during the inversion, provided that the facies definition is meaningful in the elastic, petrophysical, geological, and reservoir engineering domains. This case study from a field offshore Vietnam shows that the seismic data are only able to constrain the vertical distribution of facies when the beds are close to or above seismic resolution. Prior facies probability trends are required to ensure that the detailed vertical distributions of facies are consistent with geological and reservoir engineering expectations. However, even when below seismic resolution and when the probability trends are laterally invariant, the lateral variations of facies are driven by the seismic data.

We have run a 4D pre-stack inversion on seismic data acquired over a giant field located in deep water offshore Angola. The objective was to obtain dynamic information from 4D seismic data. The 4D inversion workflow started with a prestack 3D inversion of the baseline seismic survey. Using the relative P-wave velocity variations computed by warping, the initial impedance model of the baseline was updated in order to build the initial impedance model for the monitor survey. The update was done through a 4D mask which defines where impedance variations are allowed between the baseline and monitor impedance volumes. Due to the poor impedance discrimination between shales and water-bearing sands, where 4D effects may occur because of salinity differences between injected and aquifer water, reservoir model information was introduced in the mask in order to locate water-bearing sands. Ranges of relative impedance variations computed by the inversion were limited by 4D constraints derived from reservoir simulations before first oil and at the time of the monitor survey. 4D inversion brought sharper images compared to other 4D attributes. The high quality of the 4D inversion results evidenced by quantitative quality controls has opened the way to quantitative applications in reservoir management.