APPLIED GEOPHYSICS Amplitude changes with offset evident in subsalt data

Richard O. Lindsay
Diamond Geoscience Research Corp.
Tulsa

Davis Ratcliff
Diamond Geoscience Research Corp.
Houston

A positive answer would greatly extend the range of a tool that explorationists have used for more than a decade to directly detect hydrocarbons in hospitable velocity regimes. Earth volumes below salt, of course, do not fit that characterization.

It is too soon to claim that 3D AVO technology can be used to detect subsalt hydrocarbon accumulations. What can be said is that modern seismic methods are resolving the velocity problems created by the presence of salt to such an extent that amplitude changes related to offset are apparent in subsalt reflections. What is not yet known is whether the changes result from the presence of hydrocarbons, changes in rock properties, ray-path distortion, or something else.

An important lesson is becoming clear, however: Discrete parts of the seismic cable contain vital information that can be lost when interpretation is confined to the full-volume stack.

AVO effects

AVO is a change in amplitude of the reflection signal related to the angle of reflection. This angle is a function of offset-the distance between the energy source and receiver.

Fig. 1 [149412 bytes] illustrates the effect on a series of common depth point (CDP) gathers corrected for normal moveout, which aligns the reflections in time and shows how amplitudes, in this case, increase with offset (from left to right). The change in this example is associated with the presence of hydrocarbons.

The amount and direction of amplitude change depend on rock properties on either side of the reflecting horizon. These properties depend on density (r) and elastic characteristics of the rock itself, which are measured as functions of compressional-wave velocities (Vp) and shear-wave velocities (Vs).

It is possible to predict AVO effects from rock properties measured in wells. Fig. 2 [87840 bytes] shows such a prediction based on well data for a shale overlying a gas sand. The predicted amplitude is expressed as reflection coefficient, a ratio of the amplitude of the incident (downward traveling) wavelet to that of the reflected wavelet. Because velocities associated with the shallower layer exceed those of the lower layer, contrary to the norm, amplitude values are negative.

This prediction implies a seismic signature that may indicate the presence of hydrocarbons in similar horizons elsewhere: a trough (first swing of the reflection wavelet in the negative direction) with amplitudes increasing with offset. The signature can help the interpreter identify drilling leads in the seismic data.

Fig. 3 [19785 bytes] shows how the energy of a compressional wave (P-wave) changes at a reflecting horizon. Paths of resulting P-waves and shear (S) waves differ because in a given rock layer their velocities differ (P-waves travel faster).

Similar effects can be observed in seismic reflections from below salt. Fig. 4 [127799 bytes] shows CDP gathers, depth-migrated before stack, from reflection points below a 4,000 ft layer of salt. In the subsalt reflections, amplitudes clearly grow as offsets increase. Before the changes can be said to result from the presence of hydrocarbons, much more must be learned about the ray path pattern, rock properties, and patterns of illumination below salt.

Near offsets, far offsets

A close look at amplitude changes suggests that interpreters pay attention to offset ranges and not rely entirely on the full stack volume.

Fig. 5 [144567 bytes] compares a near-offset stack with a full-offset stack for a seismic line across the Phillips Petroleum Co., et al., Mahogany subsalt discovery in the Gulf of Mexico. Amplitudes are stronger in the full-offset section for the reflection produced by the productive P sand cut by the Mahogany 1 well. This means the far offset reflections contain data absent in the near-offset data. If the change is a true AVO effect, hydrocarbons seem to be present north of the well, although this hasn't been tested yet.

In contrast to the P sand pattern, amplitudes in the reflection from the base of salt are stronger in the near-offset section than they are in the full-offset section. Analysis of rock properties-densities and P and S-wave velocities-explains part of the phenomenon, predicting a wavelet trough on near offsets and strong decay in amplitudes as offset increases.

In addition, the top of salt refracts energy toward the horizontal because the energy accelerates as it enters the salt. At a certain angle of incidence-the critical angle-the direction of energy travel-the ray path-becomes lateral within the salt. This degrades reflections of far-offset energy from the base of salt.

The implications are that the interpreter can extract much more information about the subsalt section from the middle and far offsets than from near offsets. Yet better information about the top and base of salt comes from near-offset data.

Fig. 6 [146049 bytes] shows similar results in a comparison of near and far-offset stack sections from a line across the Emerald salt sill in the South Timbalier South area of the Gulf of Mexico. A strong reflection appears below the base of salt in the far-offset section. Again, this may not be due to the presence of hydrocarbons but rather to ray-path distortion or rock properties.

The map view

The effects evident on 2D sections become dramatic in map view of many lines across a 3D prestack depth migration data volume.

Fig. 7 [157330 bytes] gives two very distinct pictures of amplitudes, based on offset range, along the subsalt reflection from the South Timbalier South area. "Hot" colors, such as red and orange, represent high amplitudes. If the change results from the presence of hydrocarbons, it is very significant.

Fig. 8 [89301 bytes] provides a similar comparison from another location. Fig. 8a maps amplitudes from the entire offset range being stacked-200-6,000 m. The apparent hot spot in the center would be the likely drilling target.

In Fig. 8b, which maps amplitudes of the near range of offsets, the feature looks more like a channel. The deepwater, highly stratigraphic nature of the location supports this interpretation, which certainly leads to a drilling recommendation different from that implied in Fig. 8a. The apparent AVO response was smeared out in stacking of the full data volume.

Obviously, each section of the cable is saying something different about the subsurface. Further experience and research will be required before geologists and geophysicists can say with certainty what those differences mean.

Salt and wave fronts

Crucial to interpretation of subsalt seismic events is an idea about energy penetrating the salt body. The interpreter must ask: In three dimensions, what does this salt body do to the wave front?

Ray-tracing helps provide the answer. Fig. 9 [54792 bytes] shows ray paths described by a simulated series of 3D seismic shots through the Mahogany salt sill. The subsurface model is a velocity cross section generated from full-volume, 3D prestack depth migration velocity analysis.

For a reflecting horizon at slightly deeper than 4 km, sampling is fairly uniform and complete. Below the edge of salt, however, sampling is incomplete. Directly beneath salt, sampling quality varies with changes in the top or base of the salt body.

This picture of what is and is not illuminated beneath salt helps with the interpretation of AVO effects. In Fig. 10 [131486 bytes], the fold-counter map from ray-tracing is superimposed on a horizon map. The rust color is 50-fold, blues are 30-fold. The subsalt fold variation is caused by ray path distortion of rays as they travel through the salt body.

These subsalt fold coverage maps generated from 3D ray trace modeling are integrated with the 3D subsalt AVO maps to enhance understanding of amplitude variation with offset observed in the subsalt data.

Pore pressure prediction

The ability to ray-trace with accuracy sufficient to guide AVO interpretation is a side-benefit of the core, computationally intensive technique for imaging salt and subsalt features: 3D prestack depth migration. Ray-tracing is nothing new. What is new is the availability of an accurate velocity model in depth on which to use it.

Applying another standard technique to the newly available velocity model, geophysicists can produce pressure volumes useful in predicting high and low pressures above and below salt. Methods for transforming velocities in time to pressures have been available since the late 1960s. Adapting them to 3D depth data, the geophysicist can turn the velocity models from 3D prestack depth migration into pressure models of unprecedented detail and accuracy.

Fig. 11 [138905 bytes] is a depth slice at 3 km from the South Timbalier South (Emerald) area showing pore pressure gradients. The pressure data were derived from a velocity field produced by seven iterations of a 3D prestack depth migration. The white area is salt.

Fig. 12 [107633 bytes] is an east-west cross section through the South Timbalier South pressure volume. This 2D profile shows great pressure variation related to the salt body.

The pressure volume derived from a 3D velocity model improves the standard pressure data used in planning drilling programs. It also extends what has been an engineering tool into exploration by helping seismic interpreters predict seal integrity in three dimensions. In addition, pressure information is important to seismic interpreters trying to match seismic signatures in AVO analysis.

Frequency analysis

Another important consideration in AVO analysis is frequency content of the data.

When geophysicists interpret seismic data in the time domain, they usually decrease the frequency filter as time increases because the frequency content of the seismic signal diminishes with depth. Different assumptions must be applied when interpretation focuses on salt in the depth domain.

Fig. 13 [127454 bytes] is a frequency analysis of the Mahogany well location, with signal (frequencies carrying information) in blue. Frequencies are in cycles per kilometer, rather than the usual cycles per second, or hertz, because the data are in depth.

The abrupt decline in frequency content at about 2.5 km results from salt, which stretches the wavelet and lowers the frequency. At the bottom of salt, frequency rises because velocity of the wave front declines, compressing the wavelet.

A conventional frequency filter that decreased with depth in the pattern of normal time data filters would remove signal in this case.

More work needed

Work remains to be done on AVO analysis below salt. The amplitude changes now observable are very interesting and warrant further investigation.

The changes do show, however, that seismic interpretation should not confine itself to the full stack volume of 3D data. Offset ranges vary in their abilities to resolve different sections of the subsurface.

With vastly improved velocity information now at hand, geophysicists have a rapidly growing set of tools with which to test what amplitudes preserved beneath salt do and do not mean.

The Authors