A new method improves seismic reflection detail and continuity.

Satinder Chopra is manager, special projects; Emil Blias is research geophysicist; and Vasudhaven Sudhakar is president of the Reservoir Technologies Division of Core Laboratories, Canada.
Geophysicists are often frustrated at their inability to extract and understand the subtle stratigraphic detail contained in 3-D seismic volumes. Seismically, stratigraphic bodies with definitive shapes show up if they are encased in rocks with contrasting velocity. Low-porosity carbonate bodies associated with thin shales and encased in shaly carbonate rocks may not be seen on seismic data having a narrow frequency bandwidth. In such cases, the usual practice is to add a new version or vintage of the 3-D volume with target-oriented reprocessing in the hope of a better resolution and more accurate interpretation. In some cases this helps; in others, some questions still remain unresolved, as either the standard bandwidth of the data is inadequate to carry out an accurate stratigraphic interpretation or the available methodology falls short of the expected true amplitude preservation criteria.
Geoscientists tend to follow the old dogma in our industry: "Make the most of what you've got." So this problem can be addressed by having data of reasonable quality and augmenting it by some frequency restoration procedure. Different conventional procedures are adopted to compensate for frequency attenuation. A common practice has been to use multi-gate statistical deconvolution to correct for the dynamic loss of high frequencies. However, there are problems with this approach. The filters must be derived from smaller windows less likely meeting statistical assumptions, and these windowed zones often exhibit phase distortions at the points of overlap.
The other method is to use time variant spectral whitening (TVSW). The method involves passing the input data through a number of narrow band pass filters and determining the decay rates for each frequency band. The inverse of these decay functions for each frequency band are applied and the results summed. This way, the amplitude spectrum for the output data is whitened in a time-variant way. The number of filter bands, the width of each band and the overall bandwidth of application are the different parameters used and adjusted for an optimized application. This method usually has the high-frequency noise getting amplified, so a band pass filter needs to be run on the resulting data.
Being a trace-by-trace process, TVSW is not appropriate for amplitude versus offset (AVO) applications. If there were an analytic form for an attenuation function it would be easy to compensate for its effects. So in this method, first, attempts are made to estimate a Q model for the subsurface. Inverse Q filtering then removes the time-variant wavelet effects by absorption and broadens the effective seismic bandwidth by correcting the loss of high-frequency signal. These attempts have met with a varying degree of success depending on the assumptions used in the particular approach and how well they are met in practice. An element of uncertainty about the determined Q model usually persists. An important aspect that needs to be ascertained is how these conventional approaches affect the seismic amplitudes, which are diagnostic of stratigraphic anomalies. Conscientious interpreters view these approaches with suspicion.
New Method
A new method of enhancing the frequency bandwidth of seismic data has been developed. Referred to as High Frequency Restoration (HFR), this approach involves determining the amplitude decay experienced by different frequency components from vertical seismic profile (VSP) data and then compensating for that in surface seismic data. Figure 1 shows the downgoing VSP wavefield separated from the total VSP recorded wavefield. A close look at the wavelets in the corridor marked indicates the decrease in frequency levels from the shallow to the deeper levels. The two amplitude spectra seen as inset in Figure 1 show the decrease in amplitude of frequency components that have been attenuated. It is possible to determine the change in the trace amplitudes at successive VSP depth levels in the form of operators, which would describe the decay of the frequency components between these points. Once these are determined, the application of the inverse operators to seismic data would compensate the seismic data for physical processes such as absorption, transmission losses and scattering that result in attenuation of the seismic wave amplitudes. But before this is done, the window of application on the surface seismic section needs to be determined. For this purpose, the available log, VSP and seismic data are correlated as shown in Figure 2. Each formation top (in depth on the well) correlates with the aligned upgoing VSP wavefield (in depth and time) and the seismic (two-way time). This interpretation determines the window of application of the time-variant set of inverse operators. Application of these operators illustrates convincingly the advantages of adopting HFR methodology.
Examples
A 3-D VSP and a coincident 3-D surface seismic survey were recorded around Well 8-20 in the Hanna area of Central Alberta, targeting the Lower Mannville formation. The well encountered a gross Lower Mannville interval 67.3 ft (20.5 m) thick and thicker than the adjacent wells, 6-20 (37.7 ft, 11.5 m) and 8-21 (23 ft, 7m). The 3-D seismic programs (surface and VSP) were recorded to assist in defining sand presence and porosity development in the Lower Mannville interval found at a depth of between 4,265 ft and 4,430 ft (1,300 m and 1,350 m) at 950 to 1,000 ms (Figure 2). In fact, the objectives set for the 3-D VSP recording were to:
• tie the seismic reflections to lithology and stratigraphic boundaries,
• obtain a high-frequency image around the borehole (the fixed receiver array and its proximity to the reservoir would be expected to improve image quality); and
• obtain an improved subsurface velocity model.
Figure 3 shows an inline from a 3-D seismic volume from this area. A close look at the interval of interest (Figure 3a) indicates that there is no reflection detail to interpret. The bandwidth of the data is 12-50 Hz. Running the data through HFR restores the amplitudes of the attenuated frequencies and enhances the bandwidth as seen in Figure 3b. Notice the improvement in reflection detail (resolution) and continuity (indicated with arrows). At the level of interest it is now possible to interpret the sandstone and proceed with porosity determination. The sectional amplitude spectra before and after HFR (not shown) shows the extent of the frequency enhancement The bandwidth after HFR is 12-75 Hz, an increase of 25 Hz.
The high frequency restoration of surface seismic data also can be evaluated by running the Coherence Cube analysis on the seismic volumes before and after HFR. Higher frequency leads to better resolution and allows a visualization of different features on time or horizon slices with clarity. Subtle faults, edges, flow barriers or boundaries of reefs, which are not seen clearly on the coherence horizon or time slices before HFR, can be seen quite crisply and clearly on similar slices after HFR.
Impedance inversion carried out on data after HFR helps in a detailed interpretation of features of interest. Figure 4a shows a segment of an impedance section. A gas-producing well, W, is seen intersecting the highlighted portion corresponding to a gas sand. However, the green streak continues across the segment and does not distinguish the gas sand. Impedance inversion section after HFR (Figure 4b) shows the green streak (low impedance) clearly representing the gas sand within the highlighted zone.
An important element that lends strong support to the utility of any procedure is its robustness. In areas where the geology does not change fast laterally, HFR filters determined from VSPs in different wells are seen to be very close. Consequently, HFR is a robust methodology aimed at restoring the attenuated frequencies in seismic data.
Due to the absence of strong impedance contrasts above and below the zone of interest, the surface seismic reflection data may not show any significant reflection detail within the zone of interest. The HFR approach enables poor reflection zones in the seismic volume to show greater reflection detail and continuity, which match better with the corridor stack and vertical 3-D VSP sections. This helps meet the objectives set for seismic surveys and also helps define trends better, leading to more confident interpretations. Such applications could redefine prospects that may have been declared unsuccessful based on interpretation of seismic data with poor bandwidth.

Acknowledgements

Permission to publish data from Conoco Canada Resources Ltd. is gratefully acknowledged. Coherence Cube is a trademark of Core Laboratories.

Comments