Guy Purnell

Effects of salt‐related mode conversions on subsalt prospecting

Mode conversion of waves during seismic reflection surveys has generally been considered a small phenomenon that could be neglected in data processing and interpretation. However, in subsalt prospecting, the contrast in material properties at the salt/sediment interface is often great enough that significant P-to-S and/or S-to-P conversion occurs. The resulting converted waves can be both a help and a hinderance for subsalt prospecting. A case history from the Mississippi Canyon area of the Gulf of Mexico demonstrates strong converted-wave reflections from the base-of-salt that complicate the evaluation of a subsalt prospect using 3-D seismic data. Before and after stack, the converted-wave reflections are evident in 2-D and 3-D surveys across the prospect. Ray-tracing synthetic common midpoint (CMP) gathers provides some useful insights about the occurrence of these waves, but elastic-wave-equation modeling is even more useful. While the latter is more time-consuming, even in 2-D, it also provides a more realistic simulated seismic survey across the prospect, which helps to reveal how some converted waves survive the processes of CMP stack and migration, and thereby present possible pitfalls to an unwary interpreter. The insights gained from the synthetic-data study suggest some simple techniques that can assist an interpreter in the 3-D interpretation of subsalt events.

Imaging beneath a high‐velocity layer using converted waves

High‐velocity layers (HVLs) often hinder seismic imaging of deeper reflectors using conventional techniques. A major factor is often the unusual energy partitioning of waves incident at an HVL boundary from lower‐velocity material. Using elastic physical modeling, I demonstrate that one effect of this factor is to limit the range of dips beneath an HVL that can be imaged using unconverted P‐wave arrivals. At the same time, however, partitioning may also result in P‐waves outside the HVL coupling efficiently with S‐waves inside. By exploiting some of the waves that convert upon transmission into and/or out of the physical‐model HVL, I am able to image a much broader range of underlying dips. This is accomplished by acoustic migration tailored (via the migration velocities used) for selected families of converted‐wave arrivals.

Observations of wave velocity and attenuation in two-phase media

The velocity and attenuation of a wave transmitted through a two‐phase material are functions of the material’s composition. In physical model experiments, I used suspensions of grains in a silicone rubber matrix to reduce or avoid uncertainties about framework elastic constants, porosity, and permeability that result from using fluid‐saturated grain frameworks. I varied the composition to produce materials that are useful in physical seismic modeling. In the tested suspensions, ultrasonic P-wave velocity, velocity dispersion, and attenuation all increase with grain concentration and frequency. I compared seven published mathematical models for wave propagation in two‐phase media. One given by Mehta most closely agrees with the P-wave velocities I observed. The agreement is sufficiently close to merit use of Mehta’s model in the design of physical model materials. The observed P-wave attenuation generally increases approximately linearly with frequency. This approximate linearity leads to reliable constan...

Analysis of multicomponent seismic data from offshore Gulf of Mexico

SUMMARY 3D 4C surveys are a natural extension of 3D streamer surveys for time lapse reservoir monitoring. Until the use of seafloorrecorded converted waves reaches a level of maturity comparable to that of P waves, extra care will need to be taken to assure that events interpreted as converted-wave arrivals add value to the interpretation process. Careful analysis of the velocities associated with interpreted converted waves can indicate whether the interpretation is plausible. Using horizontal-component seismic data collected at Teal South, we generated velocity profiles for P-wave and S-wave velocities that agree with earlier published work (Hamilton, 1979). Modeling based on velocity fields generated by focusing analysis (with extrapolations for the shallow S-wave velocities from Hamilton) yields a synthetic common-receiver gather that closely resembles the field data in terms of changes in arrival time and amplitude with offset. This agreement between modeled data and field data supports our interpretation of the high amplitude events as waves that convert from P to S at the reflector.

Migrating sparse-receiver data for AVO analysis at Teal South Field

Summary In processing a 3-D ocean-bottom-cable (OBC) survey from Teal South Fi eld in the Gulf of Mexico, we had to address unusual challenges to imaging and AVO analysis. Sources were located on a dense grid, but the receiver grid consisted of only 24 widely spaced locations. In processing the survey, we obtained the best results by applying conventional AVO preprocessing followed by prestack migration (of common-receiver gathers). In comparing prestack and poststack migrations of the survey, we found that for bright-spot analysis and structural interpretation, migration after stack is sufficient. However, comparison of unmigrated and migrated CMP stacks makes it clear that AVO analysis should be preceded by migration. The common practice of comparing limited-range stacks (with migration either before or after stack) conveys some useful information, but is also complicated by acquisition footprint effects, especially in the near-range data. The common-receiver migration provided imaged anomalies that are stronger, more sharply defined, and more consistent spatially with fault-bounded reservoir compartments. It also provided 24-fold image gathers that we input to linearized AVO inversion. A resulting fluid-factor volume highlights AVO anomalies that are consistent with well control and with structure and bright spots known from a previous 3-D streamer survey.