Sergio Grion

Mirror imaging of OBS data

The world’s demand for energy is accelerating, while its hydrocarbon reserves are diminishing. Producers are compelled to explore and produce oil and gas in more challenging environments and to maximize recovery in existing reservoirs. New technology has always been a key to success. One such new technology is seismic acquisition using ocean bottom station (OBS) nodes (Berg et al., 1994; Ronen et al., 2003; Amal et al., 2005; Docherty et al., 2005; Granger et al., 2005).

Wide-area imaging from OBS multiples

The subseafloor structure offshore western Canada was imaged using first-order water-layer multiples from oceanbottom seismometer OBS data and the results were compared to conventional imaging using primary reflections. This multiple-migration mirror-imaging method uses the downgoingpressurewavefieldjustabovetheseafloor,which is devoid of any primary reflections but consists of receiverside ghosts of these primary reflections. The mirror-imaging method employs a primaries-only Kirchhoff prestack depth migration algorithm to image the receiver ghosts. The additional travel path of the multiples through the water layer is accountedforbyasimplemanipulationofthevelocitymodel and processing datum: the receivers lie not on the seabed but on a sea surface twice as high as the true water column. Migration results show that the multiple-migrated image provides a much broader illumination of the subsurface than is possible for conventional imaging using the primaries, especially for the very shallow reflections and sparse OBS spacing. The resulting image from mirror imaging has illumination comparable to the vertical incidence surface streamer single-channelreflectiondata.

The ups and downs of ocean-bottom seismic processing: Applications of wavefield separation and up-down deconvolution

A key advantage of acquiring multicomponent data using ocean-bottom sensors, whether using cables or nodes, is the ability to separate the wavefield into up- and downgoing parts. This opens up a host of attractive possibilities such as mirror imaging using the downgoing wave, attenuating receiver-side multiples using the upgoing wave only, or combining both up- and downgoing waves to completely remove the free-surface effect using up-down deconvolution. We focus here on the latter.

Converted-wave controlled-beam migration for vector-offset volumes

Kirchhoff migration is routinely used for converted-wave (PS) imaging, partly because the suite of high-fidelity imaging algorithms available for PP seismic reflection data is not yet widely available for PS waves. This paper discusses the implementation issues of strong and weak TTI PS equations and presents application results for a new PS controlled-beam migration (CBM) algorithm working in the common-offset vector (COV) domain. CBM is a specialized version of Gaussian-beam migration aimed at signal-to-noise ratio enhancement.

Up‐down deconvolution and subsurface structure: theory, limitations and examples

In this paper we investigate the use of up-down deconvolution to simultaneously eliminate both source-side and receiver-side multiples in ocean-bottom data processing. In theory, this method is strictly valid for a horizontally layered earth only, but we demonstrate analytically that it is robust even in the presence of structure, provided that at least the sea floor is flat or gently dipping. We successfully apply the method on synthetic and real data examples.

Azimuthal velocity uncertainty: estimation and application

Azimuthal velocity models for HTI (Horizontal Transverse Isotropy) media are extensively and widely used for land seismic exploration in North America, North Africa and the Middle East. A surface fitting technique honouring all azimuths can invert for an HTI velocity model, which can then be used to perform azimuthally dependent NMO to flatten the CMP gathers. When performing the velocity inversion it is important to estimate the degree of confidence in the estimated velocity model. The main subject of this paper is velocity uncertainty estimation. Furthermore, we investigate the estimated errors in the model parameters with varying acquisition direction for various offset-azimuth distributions including azimuthal sectors and Common Offset Vector (COV) classes. The application of the technique to WAZ land data from Algeria illustrates the strength of the proposed technology.

Azimuthal AVO Analysis: Stabilizing the Model Parameters

AVO is an important tool in the interpretation of seismic data. Azimuthal AVO models (AVAZ) have been used to characterize fracture distributions and directions in HTI (Horizontal Transverse Isotropy) media. The main subject of this paper is a method for the stabilization of AVAZ parameters. We present an extension of the technique of Whitcombe, Dyce, McKenzie and Hoeber (2004) for gradient stabilization in the standard 2-term AVO model given by the Shuey equation to the case of Ruger and Tsvankin’s (1997) azimuthal AVO analysis. We also investigate the estimated errors in the AVAZ model parameters with varying HTI isotropy plane direction for a selection of offset-azimuth distributions, including azimuthal sectors and Common Offset Vector (COV) classes. The application of the technique to WAZ land data from Algeria illustrates the use of the techniques.

Practical Aspects of Non Local Means Filtering of Seismic Data

Non Local Means (NLM) filtering is a well-known image-processing algorithm for random noise attenuation. It is based on the assumption that coherent and non-coherent features can be identified and separated using a measure of similarity between adjacent samples. In this paper we review and extend previous work on the use of NLM in seismic data processing. Our objective is to improve the computational efficiency of the method and investigate practical aspects of its implementation and application. Synthetic and real data examples demonstrate the achieved improvements.

A new approach to remove the water column effect from 4‐D ocean bottom data

Repeated ocean bottom seismic (OBS) surveys are an effective tool for time-lapse reservoir monitoring. It is desirable to acquire high-fidelity and repeatable 4-D OBS data in order to reveal small reservoir changes. However, compensating for water-column variations occurring between repeated surveys is a challenging task. In this paper, we propose using the up-down deconvolution (U/D) method to remove changes in water velocity and water depth from 4-D ocean bottom seismic data. We successfully apply the U/D method to time-lapse synthetic examples.

Tomographic and AVO analyses of Bottom Simulating Reflectors (BSR) on real data

In this paper we describe the application of different tools, as AVO, reflection tomography and pre-stack depth migration, in order to analyze the physical properties of BSR observed in two different real data sets. The tint dataset pertains to a marine line acquired in the South Shetland margin and the second one is relative to offshore South Chile. In both cases the results of the seismic study are consistent with the presence of a consolidated gashydrate layer which overlies a porous level saturated with free gas.