3d vsp: a useful tool for mining exploration. · 2015-08-07 · 3d vsp: a useful tool for mining...

13th SAGA Biennial Conference and Exhibition Short Paper

3D VSP: A useful tool for mining exploration.

Michel DENIS1, C. Naville², J.C Lecomte3, L. Nicoletis4, E. Suaudeau5, Q. Snyman6

1. CGGVeritas, France, [email protected] 2. IFPEN, France, [email protected]

3. IFPEN, France, [email protected] 4. IFPEN, France, [email protected]

5. CGGVeritas, France, [email protected] 6. Anglo American, RSA, [email protected]

INTRODUCTION Many papers have been written on the processing and acquisition of 3D VSPS but very few tackle the issue of the interpretation and even less address joint interpretation between surface and borehole data, in order to get a high confidence in a structural model. The case presented here is one of the first joint surface borehole seismic acquisition. The main driver behind this project was to de-risk a future shaft location by acquiring a high resolution data set to image potential small fractures or shear zones at the bottom of the shaft which could impact drastically the scheduled CAPEX of the shaft due to complication in the shaft bottom infrastructure cementation, shaft pillars set-up or mechanical instability of the walls. The VSP acquisition and processing has been addressed in Pretorius et al. (2011) and Humphries et al. (2011), but detailed

interpretation of both surface and borehole seismic was never published. METHOD AND RESULTS The survey design was based on the target depth (also the scheduled shaft depth) of 642m and borehole depth of 740m. The target is the platinum reef with different units used for structural interpretation UG1 and UG2 As the overburden except a low velocity layer below surface is nearly a constant velocity media, the geometry of the VSP is straightforward using basic geometrical rules. To image a 200m radius circle at the target level of 645m, a minimum acquisition circle of radius 850m would be enough. In practice we have used a 1200m radius circle filled with an even sampling of receiver points and source points on a 40m regular grid. Binning in either 20m or 5m bins was enabled.

ABSTRACT The 3DVSP technique becomes more popular with the emergence of multilevel 3 component borehole tools. The value of the information derived from VSP is not always well understood. In this paper we will present a case history of a joint surface and borehole seismic acquisition, with the goal to de-risk a shaft sinking location on a platinum mine. TD was about 650m and the VSP tool was 110m long. On surface a dense grid of receivers and vibrator source points were laid out in a 1.2 km radius circle centred on the well head. A12 level 3C VSP digital tool was lowered in the borehole, in three successive depth positions. The surface 3D cube was processed and interpreted independently from the 3D VSP data. On a near target reflector, the surface data structural interpretation showed mainly a clear E/W fault, and additional sub-seismic lineaments of differing azimuths, difficult to identify in terms of fault. The 3D VSP image limited to a short radius around the borehole confirmed the fault/dyke nature of these lineaments, separating monocline compartments. As a consequence, the surface data was carefully re-interpreted and on the second structural images derived from two surface seismic reflectors and the near surface fault footprint from 3DVSP residual statics, a series of subtle faults were clearly assessed. Last, the few faults intersecting the borehole can be clearly recognized on the logs and the borehole radar logs. This case study demonstrates the added value of a joint interpretation of surface and borehole data in a decision making process for shaft sinking. Key words: azimuthal anisotropy, VSP, interpretation, fault lineaments,

3D VSP: A useful tool for mining exploration Denis, Naville, Lecomte and all


Two boreholes were available, one with a 40 level dual hydrophone/geophone tool and the other borehole rig-up with a 12 level slim VSP tool, both tools with 10m depth interval sampling. We will focus on the latest tool and the very interesting results of the hydrophone tool will be published in a future article. The short VSP tool was successively clamped at three different positions in the borehole (Figure 1) for each of which about 2900 source positions were acquired, with a high frequency sweep of 30Hz to 250 Hz. The first 3D data set was first processed and interpreted in a straightforward and timely manner. The 3D VSP was processed be different contractors and finally by IFPEN as part of a R&D project on VSP pre-processing (old ideas in a new packaging). The surface data first interpretation was showing an expected EW fault but no other major faults or shear zones were detected with confidence. See Figure 2 for an example of such results and interpretation. The 3D VSP results from IFPEN evidenced additional faults not clearly seen in the first interpretation of surface seismic. The processing sequence was focused on time and amplitude picking of direct arrival (tricky with three different tool positions), improving the well tie with the sonic log, synthetic seismogram, deconvolution, wave field separation as summarized by Figure 3 below. As already mentioned all VSP displays, either stack through VSP-CDP stack, Shot domain stack or Bin migration showed a number of faults undetected by the first interpretation. These faults were organized along NW/SE trends and seen on different king of VSP images. (See figure 4 and 8) A new 3D surface interpretation was undertaken, auto-tracking seismic events only where the confidence was high. As the number of in-lines of this particular dataset is limited, the picking was sequentially applied to all the in-lines. A priori lateral variations on seismic traces that could possibly be related to any faint fault presence remained un-interpreted. At the end of this process, unpicked seismic events, or reflector interruptions, showing lateral coherency or line-up, could be attributed to a fault. Note that no interpolation was used during this procedure. A dip map display of the UG1 horizon indicates then a series of subtle faults / lineaments corresponding to some of the subtle faults clearly evidenced by the VSP image as illustrated on figure 5. A series of example of surface seismic with 3D-VSP data spliced on the surface data, clearly link the fault detection on the borehole dataset and the surface 3D data. A series of such images are shown in Figures 6 to 9. On figure 8, the subtle fault located near the borehole on the 3D-VSP image fits nicely with the subtle lineament surface migrated image. It was found that imperfect / non optimized 3D-VSP imaging does not prevent the detection of small faults and their strike.

A full image of the UG1 horizon generated by a triangulation of the interpreted horizon (set of points from time map) and different attributes such as the square root of the normal to each triangle, or the azimuth show a more “disturbed” structure than the first interpretation could have led to believe. Figures 10 and 11 represent such Gocad images of the UG1 reflector, with exaggeration of the vertical scale. The interpretation was further pursued by studying the picked times of direct arrivals and the derived “residuals” statics to detect potential structure effects on the VSP times. These residuals were obtained by subtraction of an axis-symmetric smooth model to the picked times of the direct arrivals. Then the residuals were averaged for level 6 to 12 of each tool position to build a surface consistent map of statics. This surface consistent term then has been subtracted to the residuals values obtained at different depths. This difference shows some organized features both for PP and SV times (figure 12). An identical approach was done on the direct arrival amplitudes, exhibiting the same structural alignments These P and S times and amplitude line-ups represent some structural features at weathered zone level, appearing the footprints of the sub-surface faults or dykes, identified on the deep target horizons (UG1). Some of these fault surfaces intersect the borehole, emerge close to the surface and create the observed anomalies in the residual seismic times. Moreover, we can check that at the expected fault position intersecting the borehole hole, the Omni-directional borehole radar image shows classic reflection responses linked to fault planes such as vanishing energy or “festoon type” features (figures.7, 9). On the following image (figure 13), we have superimposed the sketch maps of the faults at target level (UG1, blue), at intermediate horizon H-top (orange) and at the surface (black). The WE fault and NW-SE faults intersecting the borehole exhibit a geometry coherent with their steep dip (~65°) and respective strike. The NW-SE direction is coherent with the strike of nearby magnetic accident. The above results illustrate the successful integration of the effects detected on the direct arrivals of 3D-VSP (anomalies on residual times), of the fault confirmation from 3D-VSP reflections of the surface seismic reflection images (fault at target and intermediate level) and of the borehole data (core, logs, borehole radar). Remark: the surface data and borehole data have been mainly handled separately; innovative joint processing tests of the surface and borehole data sets have been undertaken, in order to investigate how to further improve the overall imaging results and methods.



CONCLUSIONS We have shown clearly in this paper the close link between the information from the surface seismic and the one form the borehole data. To alleviate the doubts from one domain it is important to have the same conclusion in terms of structural images from the two domains and even with the borehole radar to add another dimension with some particular logs. The integration of the interpretation of seismic data form theses different domains is a key point in the de-risking process and decision making on a future shaft location. ACKNOWLEDGMENTS Authors would like to thank Anglo Platinum and Anglo American for their constant support during the phases of this project and permission to publish the results. Geological exchanges with Jaco Vermeulen of Royal Bafokeng Platinum Ltd (RBP) are greatly appreciated as well. Additional geophysicists actively contributed to the pre-processing and processing of the presented VSP and

surface datasets, namely Patrice Ricarte, Vincent Clochard, Kazem Kazemi and Josette Bruneau of IFPEN, Eric Gillot and Fabienne Pradalie of CGGV. REFERENCES - Anstey, N.A., Seismic delineation of oil and gas reservoirs using borehole geophones, patent GB1569581, 1976, CA1114937, 1977 - Clochard V., Nicoletis L., Svay-Lucas J., Mendes M., Anjos L., 1997. Interest of Ray-Born modeling and imaging for 3D walk-aways, Extended Abstracts of the 59th EAGE Conference in Geneva. -Pretorius, C.C, Gibson, M.A, Snyman, Q, 2011, Development of high resolution 3D vertical seismic profiles, The journal of SAIMM. - Pretorius, C.C, Humphries, M, Trofimczyk, K, Gillot, E, 3D VSP in a mining context, 2011, Istanbul, EAGE borehole geophysics workshop, Abstracts,

Figure 1: Positions of the VSP tool string and the main logs

Figure 2: NS section of the surface 3D and dip-map of interpreted UG1 horizon. The EW fault is obvious. Other subtle lineaments are uncertain



Figure 3: VSP processing flowchart

Figure 4: Initial shot collection stack, NS line , 3D VSP run 2/415-525m with a clear fault about 120m south of borehole BH1.

Figure 5: Time map of UG1 on the right with dip map on the left showcasing the NW/SE lineaments

Figure 12: Evidence of line-ups at weathered zone level with residual statics



. Figure 13: Display of the interpreted faults and their line-ups derived from VSP statics on the dip maps (left) and residual times (right) Figure 6: S-N time section: 3D VSP data spliced on surface PSTM migration with EW red fault evident on both domains (surface and borehole), and blue SE-NW subtle lineament on surface image, fully confirmed as a fault on 3D VSP image.



Figure 7: S-N section: EW red fault seen by UG1, H-top horizons, the borehole radar image and emerging at surface (VSP)

Figure 8: SW-NE time section: 3D VSP data spliced on surface PSTM migration with SE-NW blue fault clearly evidenced by the 3D- VSP image, coinciding with a subtle lineament on the UG1 map derived from 3D surface dataset.



Figure 9: SW-NE section: SE-NW blue fault detected on UG1, H-top horizons, festoon on the borehole radar image where the fault intersects the hole, and by a track of residual times anomalies of the 3D VSP where the fault emerges at surface. Figure 10: UG1 triangulated surface from seismic times and the square root of normal attribute, evidencing accidents.



Figure 11: Gocad image of the structure map with the azimuth attribute spliced on a surface seismic cube 3D cubre.

3d vsp: a useful tool for mining exploration. · 2015-08-07 · 3d vsp: a useful tool for mining...

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