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Correcting for Non-Repeatable Source Signatures in 4D Seismic with Buried Receivers: Virtual Source Case Study from Saudi Arabia
Alexandrov, Dmitry (Saint Petersburg State University) | Bakulin, Andrey (Saudi Aramco) | van der Neut, Joost (Delft University of Technology) | Kashtan, Boris (Saint Petersburg State University)
Summary Virtual source redatuming is an effective way for improving repeatability of onshore seismic data acquired with buried receivers that can suffer from near-surface variations and acquisition geometry changes. However, redatuming is less effective in correcting for amplitude variations of the downgoing wavefield caused by variable source signatures, coupling or other factors. We present an improved redatuming workflow, which has the benefits of the virtual source approach and corrects for additional non-repeatability of the downgoing wavefield between surveys. The method involves construction of the virtual source gather for each survey, deconvolution with the corresponding point spread-function (PSF) and convolving with a reference PSF. Here we employ a reference PSF computed for a homogeneous replacement near surface. This reduces imaging artifacts and provides additional control over the dominant frequency of the output data. We demonstrate a significant repeatability improvement using a field 4D multi-survey onshore dataset from Saudi Arabia. Introduction Time-lapse seismic reservoir monitoring on land is a challenging task. The repeatability of seismic data can suffer from various factors such as near-surface changes, variability of source/receiver geometry and coupling, and surface topography variations over time. Recently an experiment was reported, which involved 11 repeated land seismic surveys in a desert environment over an onshore reservoir (Bakulin et al., 2012). The data were acquired using shallow buried receivers and surface sources. This acquisition design has great potential to improve repeatability as well as enable virtual source redatuming (Bakulin and Calvert, 2006; Bakulin et al., 2007) in order to address source positioning errors, source coupling changes, and diurnal/seasonal temperature variations. A synthetic case study in a realistic synthetic model (Alexandrov et al., 2012) confirmed that virtual source redatuming could reduce non-repeatability caused by the factors listed above. In particular, source-coupling variations, modeled as random phase perturbations of the source wavelet, were completely removed. All these improvements are expected if the amplitude spectra of the source signatures remain unchanged between 4D surveys. Field data has clearly showed that such an assumption is not met in desert environments of Saudi Arabia over time periods of months to years (Bakulin et al., 2014). We present an improved redatuming technique based on multi-dimensional deconvolution (MDD) that can correct for variable source amplitude spectra between surveys or more generally correct for variable downgoing wavefield illuminating in each 4D survey. The method involves construction of virtual source gathers for all surveys, deconvolving the gathers with the corresponding PSF from the same survey and re-convolving with a common reference PSF, computed assuming a homogeneous replacement layer.
Acoustic Wavefield Separation Using Horizontal Receiver Arrays Deployed at Multiple Depths on Land
van der Neut, Joost (Delft University of Technology) | Bakulin, Andrey (Saudi Aramco) | Alexandrov, Dmitry (St. Petersburg State University)
Summary We present a novel inversion scheme for decomposing upgoing and downgoing wavefields from vertical particle velocity recordings in downhole arrays at multiple depth levels. Our method requires no knowledge of the subsurface medium parameters as the required operators are obtained directly from the data by direct-wave interferometry. As we demonstrate, the method can be applied with as few as two receiver arrays, as long as their vertical spacing is sufficiently small. Additional depth levels can be used to improve the stability of the inversion.
- Geophysics > Seismic Surveying > Surface Seismic Acquisition (0.48)
- Geophysics > Seismic Surveying > Seismic Processing (0.47)
The Effects of Time-gating And Radiation Correction On Virtual Source Data
van der Neut, Joost (Delft University of Technology) | Bakulin, Andrey (Shell International Exploration and Production)
Introduction Summary In the Virtual Source (VS) method we cross-correlate seismic recordings at two receiver locations to create a new data set as if one of these receivers is a Virtual Source and the other is a receiver. We evaluate the amplitude and phase spectra of VS data, generated in a laterally invariant medium, in the FK-domain. It is shown that phase information is accurately retrieved, whereas amplitudes are subject to an imprint of the overburden. This imprint, which can be interpreted as the VS amplitude radiation pattern, can be estimated by auto-correlation of the downgoing wave field that was used for VS creation. We can optimize the radiation characteristics by spatial deconvolution with the estimated radiation pattern. Another strategy is to apply time-gating to the downgoing wave field, as is commonly applied in the current best practice of the VS method. We offer an explanation why time-gating indeed enhances the amplitude radiation characteristics of VS data, while having minor influences on the phase spectrum. It can be argued that the phase is best retrieved if the correlated signals have long recording times, as multiple scattering will be most effectively exploited. We show how phase information from total field correlations can be combined with amplitude information from timegated fields to improve the phase spectrum without deteriorating the amplitudes. Finally, we show how timegating and amplitude radiation correction through spatial deconvolution can be combined. The Virtual Source (VS) method is an innovative technique to image and monitor the subsurface in cases where complex overburden prevents seismics and VSP to deliver good results (Bakulin and Calvert, 2004, 2006). Placing receivers below complex overburden allows measuring the propagation response directly and apply time-reversal logic to redatum surface shots into downhole receiver locations without any additional information about the medium between sources and receivers. Redatumed shots are called Virtual Sources. Theory suggests that a correct response can be recovered when sources are located on a closed surface surrounding the receivers, however practical applications typically involve one-sided illumination with limited aperture (Bakulin and Calvert, 2006). This creates artifacts and it generally distorts VS amplitudes. An important improvement of the VS method was suggested by Mehta et al. (2007), who reasoned that separation of the up- and downgoing wave fields before cross-correlation would eliminate spurious events, caused by the lack of illumination from below the receiver array. Time-lapse imaging of VS data produces highly repeatable data even in the presence of changing overburden (Bakulin & Calvert, 2004; Bakulin et al., 2007a). However, quantitative interpretation of 4D signals requires confidence in the recovered VS amplitudes. We show that these amplitudes are subject to the radiation characteristics of the generated VS. We make a clear distinction between the amplitude and phase spectra of VS data. For simplicity we restrict ourselves to Virtual Sources in laterally invariant media, although part of the theory can be extended to general inhomogeneous media. We show how the amplitude radiation pattern of a Virtual Source can be estimated and how VS radiation can be manipulated by either a radiation correction in the FK-domain, or by time-gating, as currently applied in the best practice of the VS method.