Results

SUMMARY

The term "simultaneous source" refers to the idea of firing several seismic sources so that their combined energy is recorded into the same set of receivers during a single conventional shotpoint timing cycle. The idea is to collect the equivalent of two or more shots worth of data in the same time as it takes to collect one. The potential advantages include cost or time savings in field acquisition, which is of renewed interest due to the popularity and expense of WATS data. We were motivated to the work presented here by observations made on a 3D dataset acquired over the Petronius field in the Gulf of Mexico with two source vessels. The second source was fired with a random delay compared to the first, so that the energy from secondary source is similar to asynchronous noise. While the random nature of the crosstalk in combination with the two known geometries had been enough to successfully apply relatively standard processing techniques for other studied datasets, we found that this one required an improvement on those techniques. This paper describes a high-resolution (sparse) Radonbased separation technique with that aim. We find that while the technique does not by itself do all the required separation, it sufficiently separates the data to allow subsequent standard noise attenuation techniques to complete the task.

INTRODUCTION

A more detailed motivation for simultaneous source recording and several investigations of this idea can be found in (Beasley et al., 1998; de Kok and Gillespie, 2002; Stefani et al., 2007) and references therein. This work is based on the 3D data described by Stefani et al. (2007), which was collected over the Petronius field in the Gulf of Mexico with two sources. The second source was fired with a random delay compared to the first such that the arriving energy from both sources are recorded within a conventional shotpoint timing cycle. Due to the randomness of the delay the cross-talk between primary and secondary source energy is similar to asynchronous coherent noise, such as seismic interference, which can be expected to be fairly effectively attenuated by conventional stacking and migration procedures (Krey, 1987; Stefani et al., 2007).

As Beasley et al. (1998) describe, in order to reduce crosstalk between two sources in the final product beyond what is achieved by treating it as random noise, one can make explicit use of the fact that both sources are fired at known locations and known timing. The resulting distinct geometries of the two wavefields can be used to define "geometry-related filters". Such filtering relies on coherency along predicable trajectories, and is included in the technique described here. There are at least two important features of the Petronius dataset which makes it more of a challenge than the other datasets presented by Stefani et al. (2007). Firstly, this 3D dataset was collected to investigate the simultaneous rich-azimuth recording so the acquisition geometry was different. In particular, the gun-arrays were positioned such that the primary and secondary events are less orthogonal than in previously examined datasets.

Summary

A marine streamer parameter test was planned and acquired for a very large (approximately 10,000 km2) project in the Arabian Gulf. The test was designed to efficientlydetermine the optimal source and streamer depths for acquiring seismic data over different geologic objectives in a single vessel pass across the survey area. Field processing of the acquired data through stack was conducted to provide a quick answer without lengthy and expensive stand-by time. The field recommendations were verified by in-house processing.

Summary

Spectral decomposition has proven to be a powerful means to identify strong amplitude anomalies at specific frequencies that are otherwise buried in broad-band response. We compute Teager-Kaiser Energy for each component of a joint time-frequency representation to generated from a 3D survey acquired over a Brazilian deep water carbonate reservoir. This nonlinear energy tracking algorithm allows us to differentiate between high amplitude reservoir and other high amplitude reflections. We calibrate our algorithm against synthetic seismic traces generated from the well logs and and then apply to the real seismic data to reveal important geological features.

Summary

This paper presents a target oriented methodology to perform illumination studies considering several different acquisition geometries. The Green’s function, used to evaluate the illumination amplitude for specific subsurface points, was estimated using the complete two-way wave equation solved by Finite Difference Method.

The proposed methodology requires considerably less resources, in terms of processing time and disk space, than the traditional procedure using wave equation. The traditional procedure compares depth images obtained by a migration scheme, after modeling the complete dataset for a certain acquisition geometry. Both methodologies are more accurate than the results based on ray tracing modeling in complex areas. Numerical results are presented and analyzed on a modified 2D version of the SEG/EAGE Salt Dome. This model was especially selected because in order to get good depth image results - underneath salt bodies with complex geometries - the first necessary condition is to have an appropriately acquired dataset, and the illumination analysis is the first step in this direction.

Summary

At low frequencies the energy produced from a seismic vibrator is constrained by several mechanical and hydraulic limitations: the reaction mass stroke, the hydraulic pump flow, the pump response time, the servo valve stroke, the accumulator size, the engine horsepower, the peakdecoupling force, the harmonic distortion, and the vehicle chassis isolation. Among these factors the reaction-mass stroke and the peak-decoupling force are key parameters for setting the target fundamental force that can be achieved at low frequencies. The peak-decoupling force defined as the smaller value of either the maximum peak force or the hold-down weight. Formulas for estimating fundamental force, peak force and reaction-mass stroke are derived as a guideline for sweep designers who plan to design low frequency sweeps with considerable dwell time there. Test data show that formulas can be used to profile the vibrator envelope at low frequencies.

Summary

The energy angle-distribution in the local image matrix (LIM) for a planar reflector and for a discontinuous point are different with the former exhibiting a linear energy concentration along certain dip direction while the latter showing a scattered energy distribution. Therefore the cross-correlation value of the local image matrix between adjacent image points can be used to distinguish these two situations. The seismic images of these diffraction points may provide important information about geological discontinuities.

Summary

Acquisition footprint shown on seismic time and depth slice by periodic amplitude artifacts after seismic image is caused by infrequent sample determined by the roll array geometry, the source-receiver interval and line to line interval. The footprint is one of noises on the 3D seismic data volume from the surface geometry. It can cause false interpretation by making the stratal configuration regular variation change in seismic image. So, it is one of the worst factors affecting seismic acquisition accuracy. In the paper, the distribution rule of the source energy traveling to the interface and the reflection energy received by geophones at different locations is analyzed based on the Huygens- Fresnel theory. Then, the footprint produced in offshoreseismic towing streamer acquisition is simulated, and the effect of interface depth variation on the footprint is calculated.

Summary

This paper describes the theoretical basis for polarization filters with strong spatial constraints. In this case both the amplitude and polarization properties of the ground roll at any receiver location are estimated using data from that station as well as some of its neighboring stations. This greatly improves the separation of signal and noise, resulting in better preservation of reflected P-wave and Swave energy in the filtered data. The filter is successfully applied to a 2D 3C dataset form South America.

Summary

The acquisition of n-shots, more or less simultaneously, increases acquisition efficiency and collects a wider range of information for imaging and reservoir characterisation. Its success relies critically on the ability to separate n-shots from one recording. Stefani et al (2007) showed that while some datasets may be easily separated, others are more difficult. Using the more difficult data example from Stefani et al (loc.cit.), we show that a PEF-based adaptive subtraction (Spitz, 2007) of the estimated wavefield due to a secondary source provides an effective separation of the sources.

Summary

We present a theory for multiply-scattered waves in layered media which takes into account wave interference. The inclusion of interference in the theory leads to a new description of the phenomenon of wave localization and its impact on the apparent attenuation of seismic waves. We use the theory to estimate the localization length at a CO2 sequestration site in New Mexico at sonic frequencies (2 kHz) by performing numerical simulations with a model taken from well logs. Near this frequency, we find a localization length of roughly 180 m, leading to a localization-induced quality factor Q of 360. Introduction

Intrinsic seismic attenuation bears the direct imprint of rheological properties and fluid conditions in the subsurface and is thus a valuable parameter to measure in the field. Such a measurement is complicated by the fact that subsurface conditions not necessarily related to rheology or fluids, for example heterogeneity, also attenuate seismic waves. As a result, heterogeneity causes field measurements of attenuation to reflect an apparent instead of an intrinsic attenuation (Gorich and Muller, 1987). In layered media, the apparent attenuation is a weighted combination of intrinsic attenuation and scattering attenuation due to reflection and transmission at interfaces. As famously shown by O.Doherty and Anstey (1971), the multiple scattering of waves must be taken into account to properly gauge the attenuation due to scattering. White et al. (1990) and Shapiro and Zien (1993) have further shown that a particularly strong type of multiple scattering, known as wave localization, is key to the understanding of scattering attenuation in layered media.

We adapt a recently published theory (Haney and van Wijk, 2007) for multiply-scattered waves to describe scattering attenuation in a general layered subsurface model. An example of such a subsurface model is one constructed from well logs. The modifications are needed since the original theory shown in Haney and van Wijk (2007) used a model of identical thin layers randomly located within a homogeneous background medium. Here, this restriction is relaxed and a model consisting of layers of random density, P-wave velocity, and thickness is assumed.

The theory takes into account wave interference and is therefore able to represent wave localization. We find that scattering attenuation is in fact a combination of two distinct scattering mechanisms: one due to scattering out of the main direction of wave propagation which would exist in the absence of interference and the other due to wave localization. The length scale over which the latter mechanism acts is called the localization length and is critical to assessing the amount of scattering attenuation in a particular model. We show an application of the theory to the estimation of the localization length in a 1D model taken from well logs at the West Pearl Queen Field, a CO2 sequestration site in New Mexico.

Theory

We consider a simple model of plane waves propagating at normal incidence to planar interfaces. The model is specified in Figure 1 by the density ? and P-wave velocity ? in each layer, as well as the layer thickness L.