When the objectives of a 3-D survey are very shallow,source and receiver line spacings have to be reduced incomparison to surveys designed for deeper targets. Thisincreases the environmental impact and the cost per squarekilometer.

However, in 1996 Stammeijer et al. showed that con-verted wave recording could potentially provide betterquality shallow information than P-wave recordings alone.If this proved true for the Zuata area in Venezuela, linespacings could be increased and the surface access andacquisition costs could be reduced. As a result, before com-mencing data acquisition of a 200-km2 survey over theZuata Field, it was decided to record a small pilot survey(approximately 16 km2) with three-component geophones.This would be used to evaluate the value added (reservoircharacterization, high-resolution seismic) by convertedwaves in this area and to finalize the acquisition geome-try for the full survey.

When the survey was planned, only 1000 strings (withsix geophones per string) of 3-C geophones were available.This constrained the survey design.

A number of wells had been drilled and logged (sonic,density, gamma, etc.) in the area, but no information aboutthe shear velocities had been obtained.

Survey design. During preplanning of the survey, a 3-component noise study had been recorded. P-wave reflec-tion quality was fair, but although the existence of actualconverted signal was demonstrated on the PS-wave record-ing, no identifiable reflections were interpretable. Theresults of this study were therefore analyzed to determinethe parameters that would be applicable for P-wave record-ing, and these parameters were then adapted using

assumed Vp/Vs ratios to permit the computation of the sur-vey attributes for PS-wave common conversion point(CCP) binning. Elastic modeling was also used to evalu-ate the offset ranges over which precritical PS-wave reflec-tion amplitudes should be expected. This modelingassumed a Vp/Vs ratio of 2, which later proved to be muchless than that determined from the 3-D data.

The acquisition template was 8 receiver lines of 80receivers each, with a receiver line separation of 90 m.Group interval was 30 m. The source lines were orientatedat 45° to the receiver lines; source lines spacing was 240 m. This gave a nominal P-wave fold of 20 for the 15 × 15-m bins. At an offset equal to the depth of the deep-est sands of interest, the fold varies from 12 to 16. Becauseof the limited number of geophone strings and the size ofthe projected survey, a four-line roll was determined to pro-vide a reasonable compromise between geophysicalrequirements and operational efficiency.

For the determination of the converted wave binningattributes, Vp/Vs ratios from 1.5 to 2.5 were evaluated. Thefour-line roll causes a significant spatial variation of foldin the cross-line direction, which could be reduced with aone-line roll. It was decided that any amplitude anomaliescaused by this variation could be compensated for in dataprocessing.

Data acquisition. The terrain was gently rolling hills, withsome vertical drops caused by erosion (Figure 1). The hillswere predominantly gravel beds with a thin covering ofloose, sandy soil. The geophone cases were fitted withonly two spikes (one long and one short); this generatedconcern about the quality of the geophone coupling, par-ticularly for the horizontal elements. It was decided to

1762 THE LEADING EDGE DECEMBER 1998 DECEMBER 1998 THE LEADING EDGE 0000

Multicomponent 3-D seismic pilot study in the Orinoco heavy oil belt

R. MALCOLM LANSLEY, RICHARD R. VAN DOK, and MARK YALE, Western Geophysical, Houston, Texas, U.S.CLAUDE BORDENAVE, ABDELKADER CHAOUCH, and JEAN TACHET DES COMBES, Total,

Saint Remy-les-Chevreuse, FranceOLE NAESS, Statoil, Houston, Texas, U.S.JOHNNY PINTO, Pdvsa, Caracas, Venezuela

Regional focus: Venezuela

Figure 1. Eroding vertical drops show the type of unconsolidated materials that are characteristic of and a problemin the area.

verify the coupling of the geophones to the earth by det-onating caps at close range to the geophones and then look-ing for evidence of “ringing” after the first arrivals. Thisindicates inadequate coupling of the geophones to theground; it is usually most significant on the horizontal ele-ments but may also be seen to a lesser extent on the ver-tical component. Several tests of different planting methodsdemonstrated that planting the geophones in a shallowtrench and packing soil around the sides was optimum. Iteliminated the ringing while still permitting the azimuthalorientation and the leveling of the geophone to be easilyverified.

In order to ascertain the isotropic behavior of the hor-izontal components, one receiver line was acquired withtwo traces at each location. The second trace was consti-tuted of geophones oriented perpendicularly to the nor-mal orientation. The comparison of the shot gathers in the

0000 THE LEADING EDGE DECEMBER 1998 DECEMBER 1998 THE LEADING EDGE 1763

Figure 4. Shallow data from a line extracted from the PS-wave data volume.

Figure 3. CCP gathers and velocity analysis semblance plots for hyperbolic normal moveout (left) and normalmoveout with quartic correction term (right).

Figure 2. Shot-hole drilling.

in-line and cross-line directions showed a perfect match,validating the x,y data for further wavefield reorientation.

Many other start-up tests ensured the integrity of the 3-C geophones (e.g., polarity test, hookup of the individ-ual connectors for each component to the correct takeoutof the cable, geophone orientation, and leveling).

A major problem was actually planting the geophonesin the gravel layer, which varied in thickness from a fewcentimeters to more than 50 cm. Whenever possible, thecrew dug down to a more consolidated layer of sand wherethe geophones were planted. However, when the gravellayer was too thick, the spikes could not penetrate thegravel easily, and the geophone would become displacedfrom the vertical. Any attempt to reposition caused thegravel to become unconsolidated and severely degradedthe coupling. It was decided that as long as the bubble levelshowed that a geophone was level to 5° or less, no furtherattempts to level it should be attempted in the gravel areas.

Although some areas have relatively smooth changesin elevation, many have rapid changes. In these areas,aligning the axis of the geophones with the receiver linesbecame very difficult as one survey stake was frequentlynot visible from the adjacent stake. Because of this and alsoto assist in the accurate alignment of the geophones whenthe arrays were being contoured, a special geophone align-ment tool was manufactured. This tool had a special fit-ting at the base that would accept the geophone in onlyone direction, and a magnetic compass on the top. Oncecorrectly calibrated to allow for the magnetic declination,it was possible to accurately align the geophones.

At least seven steps were necessary to ensure the cor-

rect planting of each geophone: (1) mark the position ofthe geophone within the trace; (2) dig a trench; (3) plantthe geophone in an approximately vertical position; (4) orientate the geophone; (5) firmly plant the geophone andlevel it; (6) pack soil around the geophone; (7) check eachphone by an independent team.

The source was a small charge of biodegradable explo-sive (450 grams) at a depth of 10 m.

Despite the limited number of strings of geophones andproblems with cattle which chewed and disturbed thespread layout, 2200 shots were recorded in 28 days, an aver-age of close to 80 shots per day (Figure 2). Very rigorousfield quality control was essential for ensuring that the geo-phone layout was optimal. A number of additional testswere performed to evaluate possible changes in the acqui-sition parameters for future work.

Data processing. Processing began with the P-wave datarecorded on the vertical geophone; this included surface-consistent deconvolution, refraction and reflection statics,spatially dealiased (“fat”) DMO, and 3-D time migration.Source deconvolution operators, total source statics, andP-wave stacking velocities obtained during this flow wereall saved and utilized for the PS-wave sequence.

Initially, the PS-wave data were rotated to the radialand transverse directions. The radial component was thenused for further processing, assuming no azimuthalanisotropy, because very little energy was observed on thetransverse component. Source deconvolution operatorsand statics from the P-wave processing were applied tothe PS-wave data followed by a surface-consistent decon-volution using only the receiver and offset terms. Residualstatics, velocities, and common-conversion point binningwere iterated twice. The final pass of velocity analysisincluded a correction for the quartic, nonhyperbolic termin the normal moveout equation. Application of this cor-rection significantly improved the quality of the stack sec-tion. Figure 3 compares semblance plots from velocityanalyses performed with and without this correction.

Poststack time migration used a one-pass f-k algorithmand an average PS-wave velocity function. The poststackdata were then positioned properly for subsequent inter-pretation techniques, e.g., average and interval Vp/Vs analy-ses for lithology discrimination.

Conclusions. The pilot study demonstrated the feasibil-ity of acquiring a 3-C, 3-D survey within a reasonable timeframe, with minimal environmental impact, and with goodsafety, provided that all procedures are followed withmeticulous care. It also showed that it is possible to obtainshear wave data with a reasonable signal-to-noise ratio ina land environment with difficult operational problems.Figure 4 shows shallow data from one of the lines fromthe 3-D data volume.

Suggestions for further reading. “Combined processingof PP and PS data” by Stammeijer et al. (at the Workshopon Advances in Multi-component Technology: Acquisition,Processing and Interpretation, 1996 EAGE Convention).“Designing 3-component 3-D seismic surveys” by Cordsenand Lawton (SEG 1996 Expanded Abstracts). LE

Acknowledgments: The authors thank the SINCOR shareholders (NorskHydro, Statoil, Petróleos de Venezuela SA, and Total) for their supportand for permission to present this work.

Corresponding author: M. Lansley, malcolm.lansley@waii.com

1764 THE LEADING EDGE DECEMBER 1998 DECEMBER 1998 THE LEADING EDGE 0000

APPLICATIONS OF 3-D SEISMIC DATA TO

EXPLORATION AND PRODUCTIONGeophysical Developments Series, No.5/AAPG Studies in Geology, No. 42Edited by Paul Weimer and Thomas L. Davis

Applications of

ISBN 089181-50-1Catalog #455 1996 270 pages Paper List $133SEG Members $99

Seismic Data to Explorationand Production