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Imaging over complex structures - Colombian Andes Case Hector A. Alfonso* Saúl E. Guevara, ECOPETROL S. A.

Summary

Subsurface imaging in overthrust areas poses a major challenge to exploration operations today. The topographic and velocity variations in these areas affect the quality of seismic sections and hinder the interpreters from defining both structure and lithology. This paper is focused on the image quality around a poorly defined target horizons on a 2d seismic section from a Colombian overthrust area We carried out a comprehensive approach of 2-D prestack depth migration over a seismic line acquired in Colombia over a complex area using Imaging technology from different contractors. In addition to depth migration, the work also includes a qualitative evaluation of the data and processing prior to the imaging regarding near surface effects and refraction statics approach .

Seismic data from a complex area in Colombia, which includes igneous rock outcrops, illustrates some shortcoming of the seismic method. Noise and near surface effects (which include rough topography) should be overcome before to search for an appropriate velocity field. Anyone of these steps is challenging by itself. Despite reasonable static correction and efforts on noise attenuation, the velocity analysis is difficult, since is hard to identify reliable reflections.

The final migrated image from this study showed dramatic differences with those founded over the traditional poststack migration approaches. It also showed the pitfalls that you can found interpreting only images migrated in time over zones that definitely required the use of prestack imaging technology.

Introduction

High mountains of the Andes are characterized by their rough topography and complex geology. These characteristics make them a challenge for the current state of seismic exploration. Acquisition and processing of seismic data over such areas has long been a challenge for the petroleum industry (e. g. Tilander et al. 1995). Most of the images acquired in these environments are of poor quality due to many problems, such as poor coupling of seismic sources and receivers, near surface anomalies, absence of a uniform weathering zone and nonhyperbolic moveout due to lateral velocity variations.

This data set was acquired in a geologically complex setting of the Andes at South West of Colombia, affected by a compressional tectonics whose dip is roughly oriented SW-NE. Besides of complex geological

structures, there are igneous rock outcrops, which could overlie on sedimentary formations. This case resembles someway the basalt flows exploration, which has been of increasing interest as shown by recent publications (e. g. Fliedner and White, 2001). However most of these publications deal on offshore data, whose near surface characteristics use to be quite different to the mountainous land settings. Weathering and topography effects are important issues to be considered here. This data corresponds to a high coverage seismic survey acquired by Ecopetrol S. A (Colombian oil state company) in 1999. A seismic line whose direction was approximately W-E was selected for this work. These data were acquired with 36 m distance between receivers, 4300 m maximum offset, and a 6300 g dynamite charge located at 11 m depth.

A simplified geological profile along the line, which illustrates the surface geology in the area, is Figure 1. In this setting igneous rocks outcrops at the side with to sedimentary rocks. Notice the volcanic rocks on the West and East sides.

Seismic data

Figure 2 illustrates a couple of typical shots, 2(a) to the west on the igneous rocks (see their location in Fig. 1) and 2b to the east on the sedimentary formations. As shown by Figure 2(a), there it can be identified an strong effect of coherent noise, which makes quite hard to identify reflections coming from potential depth targets. Other factors, such as wave attenuation, converted waves and dispersion related to near surface scatters, could be also present. Figure 2(a) also shows clearly the rough topography effect, comparing their first arrival time variations with the corresponding variations in the right hand side of Figure 2(b). It can be noticed also the horizontal variation of velocities, specially comparing the left (high velocity) and right (low velocity) sides in Figure 2(b). Most of these detrimental characteristics can be related to the igneous rock outcrop.

Figure 1. Geology along the seismic line location.

Imaging over complex structures in Colombia

Fig 3. Tomography velocity field

Figure 2. Shot Points illustrate variation in the data quality. (a) Source located to the West.(a in Fig. 1) (b) Source located to the East (b in Figure 1).

Method

This data set presents simultaneously a number of drawbacks, such as horizontal velocity variations due to the complex geological structure (possibly 3D), rough topography, noisy records, and possibly other additional factors, including attenuation, anisotropy, and so on. Most of the hypotheses of standard seismic processing are broken someway. To obtain a depth image of the data the two problems which should be overcome first are the effect of the near surface layer on the arrival times and the strong coherent noise shown by the data, specially noticeable on the West side of the line. Velocity of the near surface layer is a first issue to take into account. To obtain it, refraction statics method assumes a model of parallel layers with increasing velocity down in depth. Tomography allows much more flexible models. The tomography resulting velocity field is illustrated in Figure 3. Notice how it corresponds approximately to the geologic model of Figure 1, with low velocities to the East and higher velocities to the West. It can also be observed a near surface low velocity layer.Coherent noise can be identified in Figure 2(a). Reflections from the geologic target are hardly identified in this shot. Events generated at the weathering can

explain partially this coherent noise and the lack of reflections. The presence of this weathering on hard rocks can be related to the characteristics of the tropical weather. After statics correction and noise filtering using surface noise attenuation, radial dip filter and other methods, it was possible to try velocity analysis and after that Prestack depth Migration (PSDM) analysis, whose results are illustrated below.

Imaging

As mentioned before, the images obtained using only poststack time migration are misleading, since a nice anticline structure show up on the seismic section as long as you keep low velocities on your poststack time migration. Different migrations are depicted regarding a change in the velocity field. As we can see, (see figures 4 and 5) the images change dramatically as we increase the velocity field. The nice anticline is not present anymore as you even reach 70 % of the velocity field. If you use 100 percent of velocity field the events look over migrated.

Prestack depth migration was accomplished using velocity models derived from well information, tomography and migration velocity analysis checking for flattening of events.

From the preliminary results it was clear that the image changed dramatically in terms of position of events and shape regarding the lateral velocity variation due to overthrust feature over the area. The images were checked carefully from interpretation point of view and validated taking into account surface geology and well data available. Final PSDM image mapped to time is showed on fig 6.

(a)

(b)

Imaging over complex structures in Colombia

Fig. 4 Poststack time migration using only 50 % of RMS velocity field.

Fig 5 Poststack migration using 70 % of rms velocity field . The image start changing in shape and position dramatically

Fig. 6 Prestack depth migration mapped to time. Notice that a structure shows up toward east and main structural features has changed compared to poststack time migration images

Conclusions

For this data in particular it is quite clear that you should use the best technique available to image your data. As long as the data showed high sensibility to change in velocities you should go for prestack techniques to avoid misleading structures that can be interpreted as real. Using this imaging technology, also reduce the exploration risk involved on this environments where structural geology is quite complex.

According to the results and analysis, the quality of the seismic images over this area of Colombia relies on the following key aspects: noise attenuation in prestack data, near surface velocity estimation in the datuming process, and migration. Other issue to be considered is datumization, taking into account the rough topography. An option is wave equation datuming however it is related to good signal/noise ratio (Bevc, 1997). A correct velocity field is fundamental information to obtain image of complex areas. However, arrival times affected by near surface and strong noise, generated at the source, compromises reflected events, which make difficult to obtain a reliable approximation to this information.

The results, show that time processing cannot adequately image complex structure in the presence of strong lateral velocity variations. Reflections in such areas simply do not have hyperbolic behavior, hence stacking fails to improve signal to noise ratio. In contrast prestack depth migration allows to accurately handling non-hyperbolic moveout.

Acquisition parameters have a great influence on the final processing results. The success of modern processing techniques depends highly on the field parameters and sometimes restricts their use. From acquisition point of view one thing to do is to shot with longer offsets. This is difficult to accomplish because of the rough terrain and difficult access. Analyzing the final results and the assumptions of the method one can conclude that prestack imaging yield superior results over time migration.

In conclusion, the image in depth allows us to interpret the main structural features in a more reliable way since fewer incorrect assumptions have been made in the process. It is important to evaluate our results during the imaging process to any available outcrop and well control. The results must be reasonable and within the geologic constraints. A combination of seismic, and geology models will be the best approach to solve the interpretation.

References

Alfonso, H, 2000, Seismic Imaging and analysis over Colombian foothills. Master's thesis - University of Houston.

Brown, R.P., Comeaux, L.B., and Ward, R.W., 1998, Submitting an Expanded Abstract Using eSubmit, 68th

Ann. Internat. Mtg., Soc. Expl. Geophys., Expanded Abstracts, 00-00.

Bevc, D., Flooding the topography: wave-equation datuming of land data with rugged acquisition topography. Geophysics, 62, No. 5, 1997.

Fliedner, M. M. and White, R. S., 2001, Seismic structure of basalt flows from surface seismic data, borehole measurements, and synthetic seismogram modeling. Geophysics 66, 1925-1936.

Tieman, H., 1995, Migration velocity analysis: accounting for the effects of lateral velocity variations. Geophysics 60, 164-175.

Tilander, N. G., and Mitchel, R. G., 1995, “Processing of seismic data from overthrust areas in Latin America”: The Leading Edge, 14, 707-713.

Acknowledgments

The authors want to thank GX Technology Calgary office and Westerngeco Houston Division for their effort and work they did on imaging procedures over this area. Also thanks to Helen Isaac of FRP (University of Calgary) for her contribution to the analysis of this data.