An analysis of seismic information obtained from crooked line seismic surveys in crystalline rocks, Australia

Transcription

1 An analysis of seismic information obtained from crooked line seismic surveys in crystalline rocks, Australia Milovan Urosevic* and Christopher Juhlin *Curtin University of Technology, Perth, Australia Uppsala University, Uppsala, Sweden

2 Introduction Hard rock seismic exploration is HARD Geological reasons Regolith and crystalline rock complexities Other reasons Inaccessibility and mine site restrictions Environmental and native title issues in Australia Restricted (often) to existing roads and bush tracks

3 HR seismic data acquisition Crooked line seismic surveys are exploration reality in HR environments Difficulties (processing issues - variable crookedness) Potential benefits (crooked lines are, in segments, identical to swath surveys)

4 Processing issues Travel time delays through regolith (surface cover) Regolith can be up to 150 m thick: low velocity, highly heterogeneous cover Large static shifts (total 150 ms) can completely destroy seismic image Accurate refraction statics are crucial (but not sufficient) Additional residual surface consistent refraction statics on LMO corrected gathers with variable velocity of the fresh rock are often necessary Typical piece-wise geology (structures) presents difficulty for application of residual surface consistent reflection statics Static problems can be amplified by the line crookedness, hence additional application of residual reflection statics may need to be attempted In general, crooked/swath lines 3D statics are needed

5 Processing issues Imaging problems (crooked line surveys) In HR environments we have poor conditions for imaging Intrinsic low signal-to-noise ratio Irregular spatial distribution of both common mid-point and underground reflection points Irregular fold and offset-azimuth population in bins Irregular illumination (lack of cross-line aperture) Numerous interfering reflections Variable crookedness of the line

6 HR exploration with crooked lines Often successful! (Favourable geology, velocity nearly constant) Predicted target depth error 10 m! (at 1Km)

7 More common - complex geology Complex 3D structures probed by 2D surveys (usually a mix of 2D/pseudo-swath surveys along the same seismic traverse) Out-of-the-plane events (cross-dip events) affect velocity analysis, deteriorate stack quality, affect performance of DMO, pre and poststack migrations What to do with them? Where do they originate from? Possibilities Correct for the cross-dip (Nedimovic and West, 2003) Detect cross-dip events: - use in interpretation - if significant, use to build initial velocity model for imaging (3D) - or verify whether a true 3D survey is necessary

8 North WA Example I meta sediments/hard rocks Line track plan view E Raw shot records source MV 6000 lb W

9 Stacks Stack DMO stack DMO has mixed performance (degrades events with cross dip)

10 Post and pre-stack migrations DMAS PSDM PSDM better resolution but some events degraded

11 CV stacks 3500 m/s 4000 m/s CV stacks initial indication for off-the-plane events 4500 m/s 5000 m/s Set of events expressed in all CVS panels and start with too low velocity, qualifies for crossdip analysis

12 Crossdip time correction - simplified Nedimovic and West, 2003 For small angle between the processing line and s-r azimuth and small offsets we have t = s-r traveltime for CMP displaced from the line track; slowness terms p an p y are functions of s-r azimuth and reflector s dips x and y; velocity=const. in HR; and t 0 p ZOT ; h offset; y y timecorrec tion y crossline or CDMO offset Finally for small dip component x P y 2sin / V Time correction y y P Easy to implement in a commercial package y

13 Crossdip analysis Line track Cross dip( 0 ) -78 =90- y Due to approximation can be apparent, but still indicates out-off-plane events

14 Example II all out hard rocks Gold Mine sites Low order shear Target Seismic line Mine site Mag-map Mine site Mine site Mine site Mine site Target Mine site Out of plane events expected

15 First three shots along the line; good S/N ratio for HR Raw shots Raw shot records source V lb Changes in reflection character and shape over 160 m distance

16 Raw shots First, middle and last shots along the line Line direction changes abruptly Potential out of plane event

17 Line track Stacks CMP Stack DMO Stack Mixed performance of DMO

18 Line track Time migration of DMO stack V=real Migrated stacks Time migration of DMO stack V=85% of real Steep dips Vreal appears overmigrated in parts Vslow appears correct, dip errors

19 Crossdip analysis Cross dip( 0 ) CMP Crossdip appears to change with depth

20 Example II crossdip analysis Cross dip( 0 ) CMP Crossdip appears to change with depth

21 Swath or pseudo 3D migration Data re-organised into 3D grid Selected cross-sections animation Some events better imaged away from the line track Sparse line migration

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23 Conclusions Crooked seismic lines result in dispersion of underground reflection points which can be utilised for an improved structural interpretation In favourable cases, when the line track is severely deviated, crossdip analysis can assist in building the 3D velocity model Subsequent (pseudo) 3D PSDM is then likely to further improve our understanding of the 3D geology, or at least show whether a true 3D survey is necessary to resolve geological complexities at the site Problems (many) relate to variable line crookedness (often only patchy analysis is possible), poor S/N ratio and cross-line illumination, irregular fold, offsetazimuth distribution, etc. Hence only true 3D surveys can fully resolve complex 3D structures in HR environments

24 Acknowledgement Centre of Excellence for High Definition Geophysics (Curtin University of Technology) LANDMARK Data courtesy of Gold Fields, Newmont and Rio Tinto dgb Earth Sciences We also thank Hesam Kazemeini of Uppsala University for help with 3D graphics