Stack Amplitude Research Articles

Stacked

Analysis of prestack P‐wave seismic data yields information about both the P‐ and S wave properties of the earth. An anticipated advantage of having two measurements (P and S) is that they can be combined into a new measurement that is less sensitive to lithology variations and more sensitive to fluid effects. The amplitude‐variation‐with‐offset (AVO) gradient is one such measure that is often used qualitatively as a fluid indicator. The gradient always becomes softer (more negative) when hydrocarbon replaces brine in the pore spaces but the overall AVO response is dominated by the lithology. Fluid effects are expressed primarily by the normal‐incidence P‐wave response and only secondarily by the offset dependence. The gradient often does not function as an effective fluid indicator. This is partially due to the fact that the gradient is roughly twice as sensitive to S‐ than to P‐wave properties. More importantly, effective random noise in the CMP gathers introduces a strong correlation between the AVO intercept and gradient and, hence, between the measured P‐ and S‐wave properties. This correlation in the AVO attributes corresponds to a significant error in the estimation of the S‐wave properties and can dominate the measurements from many of the popular AVO techniques. A simple method to minimize the effect of this noise‐induced correlation is to stack the data. The stack corresponds to a coordinate rotation in elastic space with the stack amplitudes measured along one of the new axes and the other (unmeasured) axis naturally tending to line up with the noise and thus suppressing it. Fluid effects cause the data to move roughly perpendicular to this noise trend. The stack axis is then in the direction of the fluid effect. The stack thus combines both the P‐ and S‐wave (normal and oblique incidence) information into a single measurement which can be made to optimally suppress background noise and highlight fluid effects. A major consequence of this interpretation is the simplicity of both prospect identification and quantitative amplitude analysis.

Short‐period P wave constraints on D″ reflectivity

We present evidence of a P wave reflector in D″ beneath the central and eastern equatorial Pacific obtained from nearly 7000 short‐period seismic recordings of earthquakes in the Tonga‐Fiji and South America subduction complexes. Both regions overlap previous observations of an S wave triplication at similar depths. Double‐array waveform stacking places the reflector 190 ± 20 km above the core‐mantle boundary and allows for a velocity transition zone no thicker than 50 km. The data admit a wide range of compressional (vP) and shear wave velocities (vS); the 2–3% increase in vS suggested by earlier long‐period S wave studies, however, necessitates only a 0.5–0.6% increase in vP. The data stacks represent spatial averages over large areas of the lower mantle, such that reflector intermittence and topography decrease stack amplitude and lower the apparent vP jump. Our results constrain these processes to act over length scales of 10–100 km.

Thickness estimates of the upper-mantle transition zone from bootstrapped velocity spectrum stacks of receiver functions

SUMMARY We modify the receiver-functions stacking technique known as velocity spectrum stacking (VSS) so as to estimate combinations of velocity model (V P and V S ) and depth that stack the Ps conversion from upper-mantle discontinuities most coherently. We find that by estimating the diVerences in the depths to the 660 and 410 km discontinuities using velocities that maximize the stacked amplitudes of P410s and P660s phases we can estimate the thickness of the transition zone more accurately than the depths to either of these discontinuities. We present two examples indicating that the transition zone beneath Obninsk, Russia, is 252±6 km thick and that beneath Pasadena, California, is only 220±6 km thick.

Akaso Field, Nigeria: Use of Integrated 3-D Seismic, Fault Slicing, Clay Smearing, and RFT Pressure Data on Fault Trapping and Dynamic Leakage

The Akaso Field [STOIIP 78 × 106 m3 (401 Million STB)] of The Eastern Niger Delta is separated from the large Cawthorne Channel field [STOIIP 208 × 106 m3 (1307 million STB)] by a major antithetic (counterregional) normal fault. In 1989, three-dimensional (3-D) seismic data acquired and interpreted over the field revealed a subtle splinter or backsplit fault off this major antithetic fault west of the main Akaso field. Behind this backsplit fault we observed stacked amplitude anomalies. These anomalies extended to the structural spillpoint defined against the Akaso backsplit fault. Fault slicing and clay smear studies of the backsplit and main antithetic fault indicated strong seal potential with possible leak windows along the fault planes. In 1989, the sealing potentials of the western end of the Akaso boundary fault were fully confirmed by a deviated appraisal well, which found a STOIIP of 29 × 106 m3 (183 million STB) in a series of stacked footwall closures. The initial trapping potential of this major antithetic normal fault can be contrasted with its subsequent behavior during depletion by using RFT data acquired in a series of development wells drilled in 1990 in the main Akaso structure at the eastern end of the antithetic fault. Here, major stacked columns of oil and gas are trapped and sealed by the antithetic fault, but we identified evidence of subsequent leakage due to production effects from the adjacent Cawthorne Channel field. Depletion of the unproduced Akaso G sands was found to be due to production from the juxtaposed E sands of the adjacent Cawthorne Channel field, thus we inferred dynamic fault leakage. We observed a dynamic pressure drop of 400-2144 kPa (58-311 psi) between the juxtaposed Akaso and Cawthorne Channel sand due to active crossflow. The Akaso field, therefore, represents a prime example of the trapping potential of a clay-smeared fault and its subsequent leakage during differential depletion. Our observations, methodology, and implications described herein may have an important bearing on reservoir management philosophies in similar structural settings.

The Akaso Field, Nigeria: Use of Integrated 3-D Seismic/Fault Slicing/Clay Shearing on Fault Trapping and Dynamic Leakage

The Akaso field on the Eastern Niger delta (STOIIP 401 MMstb) is separated from the large Cawthorne Channel field (STOIIP 1307 MMstb) by a major antithetic fault. In 1989 3-D seismic data revealed a subtle back-split off this antithetic to the west of the main Akaso field, behind which stacked amplitude anomalies occurred extending to the tip line of the back-split. Fault slicing and clay smear studies of the back-split and main antithetic indicated strong seal potential with occasional leak windows. These results were fully confirmed by a deviated appraisal well in late 1989 that found 183 MMstb STOIIP in a series of stacked footwall closures. The trapping potential of this fault can be contrasted with its behavior during depletion based upon RFT data acquired in a series of Akaso development wells in 1990. Depletion of the Akaso E and G sands was observed due to production from the juxtaposed C-D and E sands of Cawthorne Channel and subsequent fault breakdown. Dynamic pressure drops of 120 psi and 311 psi observed in the Akaso field therefore represent a prime example of the trapping potential of clay-smeared faults and their subsequent leakage during differential depletion.

VELOCITY‐STACK PROCESSING1

AbstractA conventional velocity‐stack gather consists of constant‐velocity CMP‐stacked traces. It emphasizes the energy associated with the events that follow hyperbolic traveltime trajectories in the CMP gather. Amplitudes along a hyperbola on a CMP gather ideally map onto a point on a velocity‐stack gather. Because a CMP gather only includes a cable‐length portion of a hyperbolic traveltime trajectory, this mapping is not exact. The finite cable length, discrete sampling along the offset axis and the closeness of hyperbolic summation paths at near‐offsets cause smearing of the stacked amplitudes along the velocity axis. Unless this smearing is removed, inverse mapping from velocity space (the plane of stacking velocity versus two‐way zero‐offset time) back to offset space (the plane of offset versus two‐way traveltime) does not reproduce the amplitudes in the original CMP gather. The gather resulting from the inverse mapping can be considered as the model CMP gather that contains only the hyperbolic events from the actual CMP gather. A least‐squares minimization of the energy contained in the difference between the actual CMP gather and the model CMP gather removes smearing of amplitudes on the velocity‐stack gather and increases velocity resolution. A practical application of this procedure is in separation of multiples from primaries.A method is described to obtain proper velocity‐stack gathers with reduced amplitude smearing. The method involves a t2‐stretching in the offset space. This stretching maps reflection amplitudes along hyperbolic moveout curves to those along parabolic moveout curves. The CMP gather is Fourier transformed along the stretched axis. Each Fourier component is then used in the least‐squares minimization to compute the corresponding Fourier component of the proper velocity‐stack gather. Finally, inverse transforming and undoing the stretching yield the proper velocity‐stack gather, which can then be inverse mapped back to the offset space. During this inverse mapping, multiples, primaries or all of the hyperbolic events can be modelled. An application of velocity‐stack processing to multiple suppression is demonstrated with a field data example.

Experimental investigation of interference from other seismic crews

In a study of the contamination of reflection seismic data by interfering noise from other seismic crews, controlled experiments were performed in the Gulf of Mexico and the North Sea. In each experiment, a survey ship traversed a line several times collecting both data free of and data contaminated by interfering crew noise. In the Gulf of Mexico experiment, the “noise” ship followed a prescribed course about 11 km from the survey ship. In the North Sea experiment, the noise ship was positioned at stationary locations 10 and 40 km broadside to the survey line. Recorded interference noise in both experiments had peak amplitudes well above the 0.5 to 1.5 Pa (5 to 15 μbar) limit beyond which crews typically must agree on time‐sharing. Despite recorded crew noise that was three to eight times higher than levels typically considered acceptable, the conventionally processed common‐midpoint stack of the contaminated Gulf of Mexico data shows only slight evidence of the interference noise; in contrast, the North Sea stack is severely contaminated by crew noise as early as 1 s. However, when each unstacked trace is scaled by time‐varying weights that vary inversely with the local power in the trace, the crew noise is no longer visible in the contaminated stack of either data set. Trace‐weight normalization in this process is designed to ensure that stacked signal amplitudes are generally preserved. A simulated line wherein the actual Gulf of Mexico data are contaminated by crew noise five times stronger than that recorded in the field [yielding effective peak noise values of 7.5 to 20 Pa (75 to 200 μbar)] also shows no evidence of crew noise after inverse‐power weighted stacking. When data processing includes conventional stacking, we recommend that the specified tolerable amount of crew noise be based upon the root‐mean‐square amplitude of the crew noise computed over an entire record. With burst suppression techniques, such as inverse power‐weighted stacking, we recommend that the specified level be based upon the duration of the strong‐amplitude burst as well. With both criteria, field specifications can be chosen that remain conservative while tolerating considerably more crew‐interference noise than in the past. Issues of the influence of crew noise on the analysis of prestack data remain for future study.

Radon transform wave field separation for vertical seismic profiling data

The data obtained by the vertical seismic profiling (VSP) method require unique processing before interpretation. An important step is the separation of upgoing and downgoing signals by application of an f-k velocity filter and muting of the direct wave. Similarly, if the VSP section is transformed to the p-τ plane, the upgoing and downgoing wave fields become separable because they map to different p-τ quadrants according to their dips. This allows either wave field to be reconstructed by application of the inverse Radon transform after windowing in the p-τ plane. Results obtained by the Radon transform wave field separation (RTWS) method are similar to those obtained by f-k velocity filtering. However, by implementing an amplitude‐ratio testing filter in the forward slant stack, the quality of the reconstructed wave field is much improved. For a specific range of p-stack values in the forward transform, this filter sorts and removes the unwanted events from the desired ones by testing their consistency in stack amplitudes. Synthetic examples of separating the upgoing wave field from a full VSP wave field by both methods are presented.