Ed Kragh

Seismic repeatability, normalized rms, and predictability

Time-lapse data are increasingly used to study production-induced changes in the seismic response of a reservoir as part of a reservoir management program. However, residual differences in the repeated time-lapse data that are independent of changes in the subsurface geology impact the effectiveness of the method. These differences depend on many factors such as signature control, streamer positioning, and recording fidelity differences between the two surveys. Such factors may be regarded as contributing to the time-lapse noise and any effort designed to improve the time-lapse signal-to-noise ratio must address the quantifiable repeatability of the seismic survey. Although there are counter-examples (for example, Johnston et al., 2000), minimization of the acquisition footprint and repeatability of the geometry to equalize residual footprints in both surveys are considered important. This has been a key objective in the development of point receiver acquisition systems. In this study, which develops the analysis from Kragh and Christie (2001), we examine the use of two repeatability metrics in assessing the similarity of two sets of repeat 2D lines acquired with a marine point receiver system. In one repeat set, no streamer positioning control was in use; in the other repeat set, positioning differences were minimized using the streamer positioning control. There does not appear to be a standard measure of repeatability, defined as a metric, to quantify the likeness of two traces. One commonly used metric is the normalized rms difference of the two traces, at and bt within a given window t1-t2: the rms of the difference divided by the average rms of the inputs, and expressed as a percentage: \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \[NRMS\ =\ \frac{200\ {\times}\ RMS(a\_{t}\ {-}\ b\_{t})}{RMS(a\_{t})\ {+}\ RMS(b\_{t})}\] \end{document} where the rms operator is defined as: \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \[RMS(x\_{t})\ =\ \sqrt{\frac{{\Sigma}^{t\_{2}}\_{t\_{1}}(x_{t})^{2}}{N}}\] \end{document} and N is the number of samples in the interval t1-t2. The values of nrms are not intuitive and …

Rough-sea deghosting using a single streamer and a pressure gradient approximation

We present a method for receiver ghost correction of towed streamer data that accounts for the rough sea surface. The method explicitly uses the fact that the pressure is zero at the free (sea) surface to estimate the vertical pressure gradient. Continuous elevation measurements of the wave height directly above the hydrophones are required—a measurement which is currently unavailable. The new deghosting method is fundamentally limited to frequencies below the first ghost notch. The lowest‐order implementation requires that the streamer is towed no deeper than approximately 6 m and a receiver spatial sampling interval of about 3 m or less.Using the lowest‐order and simplest implementation of the new method, the rough‐sea error is reduced from 1.5–2.5 dB to about 1–1.5 dB in amplitude and from 20° to 10° in phase, at 50 Hz in a 4‐m significant wave height sea. Higher‐order terms in the approximation promise to further reduce the error.

Optimized deghosting of over/under towed-streamer data in the presence of noise

Seismic exploration is widely used to locate geologic formations for hydrocarbon accumulations. In a typical marine seismic survey, one or more marine seismic streamers are towed behind a survey vessel. As the streamers are towed, acoustic signals, commonly referred to as “shots,” are produced by the seismic source. These travel down through the water column into strata beneath the water bottom surface, where they are reflected from the various geologic formations and travel back to the sea surface. One well-known marine seismic problem is that these upgoing waves are then reflected with inverted polarity at the sea surface because of the air/water interface. Hence, the sensors in the seismic streamer cable record not only the desired wavefield (i.e., the upgoing wave-reflected signal from various subterranean geologic formations), but also their reflections from the sea surface (the downgoing wave). The undesired downgoing reflected signal is known as the “receiver ghost.” Depending on the incidence angl...

Rough-sea deghosting of streamer seismic data using pressure gradient approximations

A new method is presented for seismic deghosting of towed streamer data acquired in rough seas. The deghosting scheme combines pressure recordings along one or several cables with an estimate of the vertical pressure gradient (or the vertical component of the particle velocity). The estimation of the vertical pressure gradient requires continuous elevation measurements of the wave height directly above the receivers. The vertical pressure gradient estimate is obtained by spatially weighting the pressure field. Each spatial weight generally is the product of two weight functions. The first is a function of partial derivatives acting solely along the horizontal Cartesian coordinates. It can be implemented by finite-difference or Fourier derivative operations. The second is a function of the vertical Cartesian coordinate and accounts for the varying sea state. This weight can be changed from one receiver to the next, making the deghosting a local process. Integrated with the measured pressure field, the esti...

Acquisition of marine point receiver seismic data with a towed streamer.

Summary A new concept for the acquisition of point receiver towed streamer data is introduced. Recording of point receiver data, rather than the analogue outputs of hardwired arrays, enables both dynamic group forming and enhanced attenuation of noise. Dynamic group forming allows better signal preservation and enhanced frequency recovery, particularly at far offsets. Data adaptive noise attenuation methods allows improved attenuation of the low frequency noise induced by water flow around the streamer, enabling the enhancement of the low frequency signal-to-noise ratio through avoidance of low cut filters. Point receiver marine seismic data will improve the dynamic range of the seismic signal and enhance the viability of time-lapse monitoring of reservoirs with towed streamer data.

Scattered ground‐roll attenuation using model‐driven interferometry

Scattered ground roll is a type of noise observed in land seismic data that can be particularly difficult to suppress. Typically, this type of noise cannot be removed using conventional velocity-based filters. In this paper, we discuss a model-driven form of seismic interferometry that allows suppression of scattered ground-roll noise in land seismic data. The conventional cross-correlate and stack interferometry approach results in scattered noise estimates between two receiver locations (i.e. as if one of the receivers had been replaced by a source). For noise suppression, this requires that each source we wish to attenuate the noise from is co-located with a receiver. The model-driven form differs, as the use of a simple model in place of one of the inputs for interferometry allows the scattered noise estimate to be made between a source and a receiver. This allows the method to be more flexible, as co-location of sources and receivers is not required, and the method can be applied to data sets with a variety of different acquisition geometries. A simple plane-wave model is used, allowing the method to remain relatively data driven, with weighting factors for the plane waves determined using a least-squares solution. Using a number of both synthetic and real two-dimensional (2D) and three-dimensional (3D) land seismic data sets, we show that this model-driven approach provides effective results, allowing suppression of scattered ground-roll noise without having an adverse effect on the underlying signal.

Efficient broadband marine acquisition and processing for improved resolution and deep imaging

Summary We present a new method for broadband marine acquisition and processing. A 3D shallow towed-streamer spread is deployed, designed to optimize the mid- and high-frequency parts of the bandwidth. In addition, data are simultaneously acquired from a small number of deeper towed streamers. The depth of these deeper streamers is optimized for the low frequencies such that the combined overall bandwidth is enhanced. Because the deep streamers will only provide the low-frequency part of the bandwidth, we can more sparsely sample these data enabling efficient acquisition scenarios as fewer streamers are required. The data are combined in processing, optimizing the signal-to-noise ratio over the entire bandwidth. The resulting data exhibit both high resolution and deep penetration, for subsalt and sub-basalt imaging, for example. In addition, inversion for acoustic impedance, imaging, and velocity model building, also benefit from the broadband result. Data acquired in this way are also more robust to poor weather conditions than conventionally acquired data. Data for a 3D case study using this new acquisition method were acquired off the NW Shelf of Australia. The streamer spread consisted of six shallow streamers towed at a depth of 6m and two deeper streamers (below shallow streamers 2 and 5) towed at a depth of 20m.

Increasing Spatial and Temporal Bandwidth with Multi-component Streamer Data

Increasing bandwidth is not only about temporal frequencies but also about spatial wavenumbers, in particular those which are poorly sampled in the cross-line direction with streamer separations of 16 to 24 times the inline sampling interval. In this talk, we present results from a test with a mini-3D array of prototype 4C marine streamers in which we use, in addition to the pressure, the vertical and crossline gradients of the pressure wavefield in order to reconstruct and 3D deghost the wavefield at arbitrary points within the aperture. From the experimental 3D survey, we show examples of spatial and temporal enhancement of wavefields reconstructed using a generalised matching pursuit algorithm, comparing pressure-only and multi-component reconstructions. We find that multicomponent reconstruction is able to de-alias high wavenumber diffractions, that are completely missed by a pressure-only matching pursuit algorithm with priors, and generate broad-band unmigrated timeslices with excellent resolution.

The optimal deghosting algorithm for broadband data combination

Data acquisition with concurrently towed shallow and deep streamers leads to deeper penetration and increased resolution. These benefits are achieved by using data processing algorithms that combine data acquired at different depths into a single dataset. However, the conventional algorithms used for this purpose do not consider the presence of noise. We present the optimal deghosting (ODG) algorithm which increases the bandwidth by optimizing the signal-to-noise ratio (SNR) of the combined data. The ODG algorithm estimates the statistics of the noise on shallow and deep streamers and minimizes the residual noise on the deghosted data in a least-squares sense. Hence, by optimally combining the data from streamers at different depths, the ODG method results in a broad bandwidth with enhanced low-frequency response and optimal SNR.

Acquisition Geometries For Model-driven Interferometric Ground-roll Removal

David Halliday WesternGeco Summary Model approach that takes advantage of a modified seismic interferometry t scattered ground roll from land was data and a novel 2D acquisition geometry. In this abstract we review the existing applications shortcomings of the application to 3D data. We then consider better application Introduction Interferometric on the cross observed at a pair of receiver locations due to a number of appropriately placed sources (Halliday et al., 2010). The result of this correlation and summation process is an estimate of the wavefield at one receiver as if a source had been placed at the ot process can be configured such that the result is dominated by ground roll. Provided located close to a receiver, the resulting estimates can be adaptively subtracted from sourc receiver records. In this abstract, we consider a modification to interferometric input to interferometry by a model of the direct ground roll (Halliday, 2010