Gary Hampson

Acquisition using simultaneous sources

We present a technique in which two or more shots are acquired during the time it normally takes to acquire one shot. The two (or more) shots are fired in a near simultaneous manner with small random time delays between the component sources. A variety of processing techniques are applied to produce the same seismic images which would have resulted from firing the simultaneous shots separately. These processing techniques rely on coherency of the wavefield in the common-shot domain and unpredictability in the common-receiver, common-offset, and common-midpoint domains. We present results of its application on synthetic 2D, real 2D, and real 3D data from the Gulf of Mexico. These results demonstrate that, in deep water with modest water-bottom reflectivity, no special processing is required, whereas in shallower water with stronger water-bottom reflectivity, the use of shot-separation techniques is necessary. We conclude that this technique can be used robustly to improve source sampling and, for example, ...

Simultaneous Source Separation By Sparse Radon Transform

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.

Simultaneous source separation: A prediction‐subtraction approach

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.

Over/under towed-streamer acquisition A method to extend seismic bandwidth to both higher and lower frequencies

Seismic resolution of towed marine-streamer data is affected by free-surface reflections. They strongly modulate the spectrum reducing energy at the so-called notch frequencies, fn = iv/2z {i = 0,1,…,∞}, in which v is the water velocity and z is the source or receiver depth. As a result there is a very strong loss of useful low-frequency energy and usually a similar loss at higher frequencies. The ghost effect occurs both at the source and at the receiver. At the source, the upgoing part of the source wavefield is reflected off the free surface with inverted polarity before its delayed journey into the Earth. At the receiver, the upgoing part of the receiver wavefield is reflected off the free surface with inverted polarity before it travels down to the receiver again. The over/under acquisition method allows separation of the up- and downgoing wavefields at the source (or receiver) using a vertical pair of sources (or receivers) to determine wave direction. In this article, we present the results of two ...

Decomposition into minimum and maximum phase components

In this paper we give a new solution to the problem of factoring p′(t) = a(t) * b(−t) * δ(t−τ) into its maximum‐phase, minimum‐phase, and pure‐delay components. Our method is based on the “winding number” and the solution of three Toeplitz systems of equations. First we use the winding number from complex analysis to estimate the pure‐delay component, which we subsequently remove using p(t)=p′(t)*δ(t+τ). Then we solve the symmetrical Toeplitz system of equations, considering only t ⩾ 0, p(t)*p(-t)*[a(t)*b(t)]-1=a(-t)*b(-t) to get [a(t)*b(t)]-1. Finally, we similarly solve the two asymmetrical Toeplitz systems of equations, p(-t)*[a(t)*b(t)]-1*a(t)=a(-t) for a(t) and p(t)*[a(t)*b(t)]-1*b(t)=b(-t) for b(t). The practical effectiveness of the method is confirmed by application to a synthetic waveform and to an air‐gun signature. When stabilization of the Toeplitz solution is required, we find that it is important to be frugal with the addition of white noise. The computation of the winding number is robust.

High Resolution Anisotropic Earth Model Building on Conventional Seismic Data Using Full Waveform Inversion: A Case Study Offshore

We present a case study from the North West Shelf of Australia where the complexity of the overburden consists of several thin multi-level channel systems filled with a combination of anomalously high or low velocity sediments. Not accounting for these strong velocity variations accurately, can lead to subtle image distortions affecting the underlying section down to and including the reservoir level. This can have significant impact on the volumetric estimates of reserves in place. To resolve these complexities in the overburden, full waveform inversion (FWI) was utilized to generate an updated earth model exploiting both early arrivals and reflection events. One caveat to using full waveform inversion is the need for low frequencies to be present in the seismic data, or, the initial starting velocity model must contain the correct low wavenumber components. However, conventional seismic data acquired at shallow tow depths are usually band limited particularly at the very low frequencies. Our case study will discuss these issues along with other limitations that this “conventional data” presented along with the workflows and quality control methods adapted to this data in order to converge to a plausible, high resolution earth model.

Overburden heterogeneities influence on time-lapse seismic repeatability: a finite difference modelling study

One of the aims of time-lapse seismic feasibility studies is to determine whether a desired time-lapse signal is distinguishable from incoherent noise. Acquisition parameters play a key role in repeatability, with source-receiver positioning errors commonly being regarded as the most important issue. A normalised root-mean-square (NRMS) variogram analysis measures the effect of source-receiver geometry differences on observed non-repeatability of traces. The overall trend of the variogram is strongly controlled by the heterogeneous characteristics of the overburden. We investigate the influence of heterogeneity on seismic repeatability using a NRMS variogram on synthetic data. We generate synthetic seismic data (with no incoherent noise) from velocity models with a variety of overburden characteristics and run finite-difference simulations over them. Variograms are generated from the synthetic data and show similar trends to those observed in real seismic data. We demonstrate that the length of the coherent signal of a target reflector (derived from the variogram) is directly related to the size and position in depth of the heterogeneity.

Analysis of a Coherency-constrained Inversion for the Separation of Blended Data: Discovering the Leakage Subspace

So-called blended or simultaneous source techniques are interesting because they offer the potential for better acquisition sampling, improved illumination and greatly enhanced computational efficiency. The procedure of retrieving data as if they were acquired in the conventional way is called deblending. Several inversion techniques have been proposed for solving this ill-posed problem with the iterative estimation and subtraction algorithm being one of them. This particular method uses a dedicated iteration that integrates a coherency-pass filter. However, fundamental questions regarding the convergence and limitations of this method have not previously been addressed. The notion of the leakage subspace is hereby introduced to aid in this quest.

Processing Low-frequency Data From the Faroe-Shetland Basin For Sub-basalt Imaging

Figure 1: Basalt covered areas of the North-East Atlantic Margin and associated igneous centres. Penetrating boreholes and hydrocarbon discoveries near the Shetlands are also shown. Following the findings that one has to use low frequency data to image beneath a multi-layered basalt sequence (Mack, 1997), Veritas DGC and Texaco UK re-shot a profile in August 2001 using a solid streamer and an additional low-frequency setting (Ziolkowski et al., 2001). Comparing the new low-frequency data with the old data shows not only a superior data quality due to the solid streamer used but also improved low-frequency response from beneath the basalt.

Reply by the authors to J. G. Saha, C. H. Mehta, and S. Benjamin

We appreciate the interest that Saha et al. have shown in our paper and thank them for their prompt response to our request for specific details of their numerical experiments. Our interpretation of these details is given in Appendix A.