J. Sheiman

Improving the virtual source method by wavefield separation

The virtual source method has recently been proposed to image and monitor below complex and time-varying overburden. The method requires surface shooting recorded at downhole receivers placed below the distorting or changing part of the overburden. Redatuming with the measured Green’s function allows the reconstruction of a complete downhole survey as if the sources were also buried at the receiver locations. There are still some challenges that need to be addressed in the virtual source method, such as limited acquisition aperture and energy coming from the overburden. We demonstrate that up-down wavefield separation can substantially improve the quality of virtual source data. First, it allows us to eliminate artifacts associated with the limited acquisition aperture typically used in practice. Second, it allows us to reconstruct a new optimized response in the absence of downgoing reflections and multiples from the overburden. These improvements are illustrated on a synthetic data set of a complex laye...

Acquisition geometry requirements for generating virtual-source data

Using model and field data, this article reviews the virtual-source method and its acquisition geometry requirements. Before we go into the details of the acquisition geometry requirements, let us briefly review the basic concept and the advantages of the virtual-source method. A typical surface seismic experiment has sources on the surface to excite waves that propagate through the subsurface. Surface receivers record the reflected waves. In order to image the subsurface, we migrate the reflected wavefield recorded by the receivers, using an estimate of the subsurface velocity model. However, the near surface is usually complex, and the velocity is difficult to estimate. These near-surface inhomogeneities, if not represented in the migration velocity model, defocus the deeper image. In order to avoid the estimation of the near-surface velocity model, Bakulin and Calvert (2006) proposed the virtual-source method, a technique that uses cross-correlation of the wavefield recorded by a given pair of receiver...

Synthetic aperture controlled source electromagnetics

Controlled?source electromagnetics (CSEM) has been used as a de?risking tool in the hydrocarbon exploration industry. Although there have been successful applications of CSEM, this technique is still not widely used in the industry because the limited types of hydrocarbon reservoirs CSEM can detect. In this paper, we apply the concept of synthetic aperture to CSEM data. Synthetic aperture allows us to design sources with specific radiation patterns for different purposes. The ability to detect reservoirs is dramatically increased after forming an appropriate synthetic aperture antenna. Consequently, the types of hydrocarbon reservoirs that CSEM can detect are significantly extended. Because synthetic apertures are constructed as a data processing step, there is no additional cost for the CSEM acquisition. Synthetic aperture has potential for simplifying and reducing the cost of CSEM acquisition. We show a data example that illustrates the increased sensitivity obtained by applying synthetic aperture CSEM source.

Virtual source gathers and attenuation of free‐surface multiples using OBC data:implementation issues and a case study

Virtual source imaging is a technique based on extracting the Green’s function that characterizes wave propagation between two receivers by cross-correlating the wave-fields recorded by these receivers. We focus on implementation issues in generating a virtual source gather from a multi-component OBC data recorded at the Mars field. The implementation issues include choice of the receiver that acts as the virtual source and the number of sources over which the cross-correlated data is stacked. The pre-stack correlated data (correlation gather) is a useful diagnostic for quality control and for assessing the source locations that give a stationary phase contribution. By stacking over specific source locations, we restrict the direction of the incoming energy and generate virtual source gathers containing arrivals within a specified horizontal slowness interval. We compare the virtual source gather generated by using a small number of sources to the virtual source gather generated by using a larger source aperture for stacking. Artifacts due to the traces at the edges of the source aperture can be suppressed by applying a taper before stacking the correlation gather. Another artifact observed in virtual source gathers is due to side-lobes of the auto-correlation of the source-time function. We show the use of dualsensor summation to separate the upand the down-going energy in the raw data and using that to generate virtual source gathers containing only the upgoing energy, hence attenuating the free-surface multiples.

The virtual‐source method applied to Mars field OBC data for time‐lapse monitoring

The virtual source method has recently been proposed to image and monitor below a complex and time-varying overburden. The method requires surface shooting recorded by subsurface receivers placed below the distorting or changing part of the overburden. Redatuming the recorded response to the receiver locations allows the reconstruction of a complete downhole survey as if the sources were also buried at the receiver locations. The ability to redatum the data independent of the knowledge of time-varying overburden velocities makes the virtual source method a valuable tool for time-lapse monitoring. We apply the virtual source method to the Mars field OBC data acquired in the deepwater Gulf of Mexico with 120 multi-component sensors permanently placed on the seafloor. Applying to the virtual source method, a combination of up-down wavefield separation and deconvolution of the correlation gather by the source power spectrum suppresses the influences of changes in the overburden (sea water), thus strengthening the virtual source method for time-lapse monitoring.

Further thoughts on the stacking response in seismic data processing

In a recent technical article in First Break, Gausland (2004) made the case that the result of stacking is not limited to the often quoted factor of reduction in noise where n is the fold of a CMP-gather. Through his figures and illustrations, Gausland showed that stacking also acts as a frequency and wavenumber filter. Although the intentions of the article were not to, as Gausland put it, ‘give methods or formulae’ and that a ‘simplified analysis can be made using simulation’ instead of mathematics, we could not help but see a connection between his main points and the method of stationary phase (Born & Wolf, 1980). In this comment, we give our interpretation of Gausland’s results within the language of stationary phase. Gausland’s conclusions are supported by the stationary phase analysis, save for some details concerning the time-delay induced by mis-stacking. In addition, we find other factors affecting the stacking response that were not pointed out by Gausland. The issues on fold versus spreadlength brought up by Gausland cannot be addressed explicitly within the stationary phase analysis; we return to them at the end of this comment. Arguments based on stationary phase have been used extensively in the literature on imaging and migration (Bleistein et al., 2001). Hence, a result of this mathematical excursion is a clear connection between stacking and migration, which Gausland alluded to briefly when he stated that ‘a further analysis of the similarities between . . . stacking and migration is necessary for a full understanding of these important aspects of seismic data processing . . .’. Suppose that in a CMP-gather there is a single event, with a zero-offset waveform ƒ(t) at zero-offset two-way-time T0, that has hyperbolic moveout with NMO-velocity v and a wavelet that does not change with offset (see Fig. 1). The event is stacked with a hyperbola whose apex is at T0 using a stacking velocity vst not necessarily equal to v. When vst does not equal v, a time shift ∆tk occurs at the k-th offset trace before stacking. Hence, the normalized stacking response, neglecting NMO-stretch, is

Increasing the sensitivity of controlled source electromagnetics by using synthetic aperture

Controlled-source electromagnetics (CSEM) has been used as a de-risking tool in the hydrocarbon exploration industry. Although there have been successful applications of CSEM, this technique is still not widely used in the industry because the limited types of hydrocarbon reservoirs CSEM can detect. In this paper, we apply the concept of synthetic aperture to CSEM data. Synthetic aperture allows us to design sources with specific radiation patterns for different purposes. The ability to detect reservoirs is dramatically increased after forming an appropriate synthetic aperture antenna. Consequently, the types of hydrocarbon reservoirs that CSEM can detect are significantly extended. In this paper, we mainly show one type of synthetic aperture antenna whose field can be steered into a designed angle. Consequently, the field concentrates on the target reservoir and the airwave is reduced. We show a synthetic example and a data example to illustrate the increased sensitivity obtained by applying synthetic aperture CSEM source. Because synthetic apertures are constructed as a data processing step, there is no additional cost for the CSEM acquisition. Aside from the applications to marine CSEM, synthetic aperture can be widely applied to other electromagnetic methods such as on land electromagnetics and bore hole electromagnetics.

Improving the Virtual Source Method By Wavefield Separation

The virtual source method has recently been proposed to image and monitor below complex and time-varying overburden. The method requires surface shooting recorded at downhole receivers placed below the distorting or changing part of the overburden. Redatuming with the measured Green’s function allows the reconstruction of a complete downhole survey as if the sources were also buried at the receiver locations. There are still some challenges that need to be addressed in the virtual source method, such as limited acquisition aperture and energy coming from the overburden. We demonstrate that up-down wavefield separation can substantially improve the quality of virtual source data. First, it allows us to eliminate artifacts associated with the limited acquisition aperture typically used in practice. Second, it allows us to reconstruct a new optimized response in the absence of downgoing reflections and multiples from the overburden. These improvements are illustrated on a synthetic data set of a complex layered model modeled after the Fahud field in Oman, and on ocean-bottom seismic data acquired in the Mars field in the deepwater Gulf of Mexico.

Fault-Plane Reflections as a Diagnostic of Pressure Differences in Reservoirs - South Eugene Island, Offshore Louisiana

Seismic data taken at Blocks 314, 315, 330, and 331 of the South Eugene Island field contain reflections from a major growth fault. Out of a list of possible causes, we find that differences in pore pressure across the fault give rise to the fault-plane reflections over a large portion of the fault. The pressure differences are detectable since pore pressures that exceed the hydrostatic pressure, or overpressures, lower the seismic velocity. Thus, the presence of the fault-plane reflections point to the fault providing a significant lateral seal. We develop a processing scheme to highlight the fault-plane reflections while simultaneously removing the reflections from the layered structure. Using this processed data set, we extract the amplitude of the fault-plane reflections on the fault-plane. The areas of strong reflection amplitude correlate well with the geology and known zones of overpressure. Over a limited area on the fault, we observe reflectivity originating fiom elevated pore pressures in the fault zone itself.