Towards a Systematic Understanding of Reflection Energy on Long Offset Sub Basalt Data
Martin Widmaier1, Jan Langhammer1, and Karl Kravik2

1PGS, Lysaker, Norway

2TotalFinaElf, Stavanger, Norway

Abstract

Several long offset seismic 2D lines were acquired and analyzed over the last years to test new imaging strategies in areas covered by basalt layers. E.g., Widmaier et al (2000) performed long offset multi-component imaging in the West of Shetland/Faeroe area. FD modeling has been used to drive the imaging experiment. Besides a PP depth image, a long offset PSSP image of the base basalt has been presented. The long offset PSSP imaging of deeper targets did not work due to the interference with other wave modes (e.g., multiples). Keggin et al. (2000) demonstrated the benefit of long offset PP acquisition in the Vøring area for intermediate depths. However, basalt has not been present in this particular example.

Further kinematic and dynamic modeling has been done in order to study the impact of basalt layers on long offset target reflections compared to the basalt-free case. In addition to PP reflections, also the utilization of multi-mode-converted waves (e.g., S propagation in and beneath the basalt layer) has been evaluated. The main goal of the modeling study has been to guide the long offset processing.

One of the conclusions of the model-based analysis is that the presence of high velocity basalt layers limits the energy bearing offset range for sub basalt PP reflections significantly. Another observations is that PS converted reflections from top basalt, which are converted back from S to P wave mode in the overburden while propagating upwards (“hidden PS reflections”), seem to be one of the most dominant long offsets reflection events arriving after the top basalt PP reflection (Figure 1).

Figure 1: Amplitude comparison of interbed multiples, hidden PS reflections, and multi-mode converted target reflections for a model with a high-velocity basalt layer.

References

Keggin, J., Larssen, B., and Bernard, D., 2000: Long Offset Seismic in the Vøring Area, NPF Seminar “Geophysical Interpretation in the new Millennium”, Expanded Abstracts.

Widmaier, M., Söllner, W., and King, D., 2000: Multi Component Sub Basalt Modeling and Imaging, in 62nd Mtg. EAGE, Extended Abstracts.

FORCE workshop: Imaging of sub-(ba)salt areas using long offset seismic data — Stavanger, 28 Feb 2001