Examination of Dr Palmer s GRM Interpretations on Line 00 (or 1000N) at Mt Bulga. Class tutorial Prof. Bob Whiteley, Feb. 2012

Transcription

1 Examination of Dr Palmer s GRM Interpretations on Line 00 (or 1000N) at Mt Bulga Class tutorial Prof. Bob Whiteley, Feb. 2012

2 References & bibliography Harley, B. F A geophysical case history of the Mt. Bulga Prospect: Extended Abstracts, 3 rd Biennial ASEG Conf.,Brisbane, 31 Oct.-3 Nov. 1983, Leung, T.M., 2003, Controls of traveltimedata and problems with the generalized reciprocal method: Geophysics, 68, Palmer, D., 2006a, Refraction traveltimeand amplitude corrections for very near surface inhomogeneities. Geophysical Prospecting,54, Palmer, D. 2006b, Integrating amplitudes and traveltimeswith high resolution refraction methods. SAGEEP2006, Palmer, D., 2009 Maximising the resolution of near-surface seismic refraction methods, Exploration Geophysics, 40, Palmer, D Characterizing the near surface with detailed refraction attributes in Advances in nearsurface seismology and ground penetrating radar: SEG Development Series, 15, Ch. 14, Palmer, D Uncertainty in Near-Surface Refraction. Ext. Abs. 22 nd ASEG Int. Geophysical Conference, Brisbane, Australia, Feb. 2012, 1-4. Palmer D Response to comments by Robert J. Whiteley. Exploration Geophysics, 42, Whiteley, R.J., Hawkins, L.V. & Govett, G.J.S : The Seismic, Electrical and Electrogeochemical Character of the Mount Bulga Orebody, NSW Australia, Soc. Explor. Geophys. SEG Expanded Abs, 3, 310, Doi: / Whiteley, R. J., 2004 Shallow seismic refraction interpretation with Visual interactive ray trace (VIRT) modelling. Exploration Geophysics, 35, Whiteley, R. J. & Eccleston, P. J Comparison of seismic refraction interpretation methods for regolith mapping. Exploration Geophysics, 37, Whiteley, R. J Comments on Palmer, D Is visual interactive ray trace an efficiousstrategy for refraction inversion, Exploration Geophysics, 41, Exploration Geophysics, 42,

3 Palmer (2009, p87 & p89) has stated that the GRM, a ray-based refraction interpretation method introduced 40 years ago, is a subjective and empirical method. It relies on supposed improved performance based on simplified seismic models. The GRM contains all the model assumptions of the Reciprocal (Plus-Minus) Method (~ 50 years old) with the additional assumption that an optimum XY can be reliably extracted from processed first arrival data. In application, the GRM requires a simplified overburden velocity model. The theoretical basis that an optimum XY actually exists in variable earth models and meets Palmer s definition has not been established. Using other more complex seismic models representing laterally variable earths Leung (2003) has shown that extraction of an optimum XY using Palmer s criteria is unreliable. Interpreted seismic sections produced with the GRM in variable conditions do not produce synthetic TX data whose times agree with measured TX data from inner and near end shots where overburden velocities are represented. This is in contrast with tomography i.e. how do you check GRM interpretations?

4 Location of the Mt. Bulga Massive Sulphide Deposit Mt. Bulga Modified from Agnew et. al. (2005). Lewis Ponds, a hybrid carbonate and volcanic-hosted polymetallicmassive sulphide deposit, New South Wales, Australia. MineraliumDeposita, 39,

5 Approximate location of Palmer s (2006) & Whiteley s(1984) refraction lines at Mt Bulga with local geology Whiteley Stn. 100E Line 00N Palmer Stn. 49 From Harley (1983)

6 1984 site photos Looking east Mt Bulga deposit is at crest of ridge Whiteley Stn. 100E Line 00N Palmer Stn. 49 Looking south 2:Old collapsed workings, with the highly porous Mt Bulga gossan extending to the ground surface, note the sharp contact with the volcanics. Top of arch is at ~8m depth. 3: Close up of Mt Bulga gossan showing extensive voids and wallrock collapse breccia (dime gives approx. scale) 1: DDH 9 outcropping weathered volcanics

7 SP1 SP97 Palmer s (2006a, Figure 2) TX data. Data from the offset shots SP1 and SP 97 were used for the GRM interpretations but not for the various tomographic models unless Palmer created artifical data to connect the offset shots with their respective shot points (unlikely).

8 The TX data from the offset shots (white) are not used by Dr Palmer for the tomographic models. The far offset shot data is used for his GRM models SP1 SP97

9 Palmer s data: segmented, 3 or 4 layers? SP 1 SP 97 SP 1 and SP 97 data were used for Palmer s GRM interpretations

10 SP1 SP97 Parallelism shows that offset shot data from SP1 and S97 are inconsistent with critical refraction from the same refractor. (Data from SP13 and SP85 have been overlain). Therefore, none of Dr Palmer s GRM interpretations can be correct.

11 Original refraction data & Reciprocal Method (RM) Interpretation Whiteleyet al. (1984 ) Approx. location of overlap with Palmer s data

12 Diffractions from the westernmargin of the ore zone Diffractions from the easternmargin of the ore zone

13 Palmer s (2006a, Figure 17 ) various GRM interpretations using data from SP 1 and SP 97 Palmer s (2006b, Fig. 18) GRM interpretation Are any of these interpretation correct? How do we check and which one do we use?

14 Comparison of Whiteley s (1984) RM with Palmer s (2006a, Fig. 17) GRM interpretations for XY = 2.5m Ground surface differs due to rehabilitation works after Whiteley s survey

15 Comparison of Whiteley s (1984) RM with Palmer s (2006b, Fig. 18) GRM interpretations for XY = 2.5m Ground surface differs due to mine rehabilitation works after Whiteley s survey

16 RM AND GRM interpretations (Palmer 2006a, Fig. 17) with XY = 2.5m Tomogram from Palmer (2012, Fig. 3). No offset data used

17 Initial VIRT (Whiteley, 2004) interpretation of Palmer s data with raypathsfrom SP 1 and SP 97 by Tak-Ming Leung (completed in 2009) Note close fit with inner and near offset source data, deepest refractor represents approx. base of weathering and top of the fresh sulphides. The fit to the offset data could be improved a little with some more VIRT modelling but this would not change things much.

18 Whiteley s Geological Interpretation of VIRT Model obtained from Palmer s data Soils & EW Volcanics Soils MW Volcanics EW Volcanics Porous Gossan EW Meta seds. MW-SW Volcanics Significant diffraction sources SW-F Volcanics Supergene ore with mine voids Fractured/altered massive sulphides MW-SW Meta seds. SW - F Meta seds. Weathering codes used: EW - Extremely MW Moderately SW Slightly F- Fresh

19 Outcropping weathered volcanics Soils & deeply weathered siltstones Mt Bulga gossan Comparison of tomogram with VIRT model Tomogram from Palmer (2012, Fig. 3) Interface in this region not well defined

20 Comparison of GRM interpretation (Palmer 2006b, Fig. 18) with VIRT model Interface in this region not well defined by the seismic data

21 Relative first arrival amplitudes from Palmer (2006, Figure 9) Forward offset shot ray paths (dashed lines) from SP1 at -60m A B C D E Forward ray groups, note rapidly decreasing amplitudes over low velocity gossan (Zone C)

22 Reverse offset shot ray paths (solid lines) from SP97 at 180m K J I H G F Reversed ray groups, again note rapidly decreasing amplitudes over low velocity gossan (Zone H)

23 Comments: Dr Palmer s GRM interpretations on Line 00 at Mt. Bulga are inconsistent with the assumptions contained within the GRM Comparison between tomographic images from various GRM starting models in Palmer (2012) is technically questionable as the inversion process has not used the offset TX data used for the GRM models. So what is being compared? First arrival amplitude behaviour is inconsistent with Palmer s GRM interpretations Prof. Bob Whiteley, Feb. 2012