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Seismic and Tomographic Imaging of Bedrock Topography for Newmont Mining in Nevada 2008/5/1 Naoshi Aoki Outlines • Survey objective and collected data overviews • Tomographic imaging • Seismic imaging • Interpretation of tomographic and seismic images • Conclusion Objective • Delineate the bedrock topography by 2D tomographic and reflection seismic imaging. Buffalo Valley North central Nevada Buffalo SeismicValley Line Mine Pershing County Bedrock Depths from the Wells here! Humboldt County Lander County The bedrock is expected to be at depth of several hundred meters from ground surface Pershing County Buffalo Valley Mine #124 680ft(200m) #125 320ft(97m) 3.36km Survey Site Seismic Recording Unit Seismic Source Data Acquisition Geometry #124 #125 #125 #124 X:Z=1:10 #124 #125 X:Z=1:10 Velocity of Refraction Waves: Line A SP28 W SP58 E TWT (S) 0 0.6 Unit: m/s 0 600 0 600 Velocity Variation : Line B SP35 W SP63 E TWT (S) 0 0.6 Unit: m/s 0 600 0 600 Velocity Variation : Line C SP1 W SP15 E TWT (S) 0 0.6 Unit: m/s 0 600 0 600 Dominant Period 0.05 TWT (S) T0/4=0.008sec 0.1 9 Ch # 21 Observed Parameters • Apparent refraction wave velocity: Dominant: 600～3500 m/s Anomalous: > 6000 m/s • T0/4: 0.008 sec • Wavelength: 20 - 120 m Outlines • Survey objective and collected data overviews • Tomographic imaging • Seismic imaging • Interpretation of tomographic and seismic images • Conclusion First Break Pick • Data for traveltime tomography are first arrival times. • The quality of our data is that the first break is easy to pick from traces within an offset of 350 m. • Far offset traces are noisy but the 1st trough or peak are identifiable. • First Break pick is conducted in the following way: – Pick only reliable first breaks on the raw CSGs. – Pick in the noisy part on the bandpass filtered CSGs. Raw Data TWT (S) 0 0.6 1 Ch # 120 Bandpass Filtered CSG TWT (S) 0 5-10-50-100 of frequency band is passed. 0.6 1 Ch # 120 Traveltime Data Statistics • • • • Number of traveltime picks: 22973 Number of source locations: 115 Number of receiver locations: 360 1469 bad picks, which has an error of 4 msec, are eliminated by reciprocity check. Starting Velocity Model This starting model is create from the min and max refraction wave velocity from the data. Tomography Results Tomographic Image Z (m) 0 m/s Final smoothing operator: 50m in X , 25m in Z 400 Ray Path Image Z (m) 0 # of Ray 400 0 X (m) 2500 Outlines • Survey objective and collected data overviews • Tomographic imaging • Seismic imaging • Interpretation of tomographic and seismic images • Conclusion Reflection Seismic Imaging • Seismic data processing is conducted to obtain the bed rock image. • Since coherent noises generated by the source mask reflection waves, linear noise attenuation, NMO stretching factor and stacking offset range are carefully tested. Noise Attenuation Raw CSG SWA Filter TAR + FK Filter TWT(S) 0 1.5 0 Offset (m) 575 0 Offset (m) 575 0 Offset (m) 575 Comparison of NMO Stretch Factor CMP Gather NMO Stretch 30% NMO Stretch 350% TWT(S) 0 1.5 -1460 0 0 0 900 -1460 900 -1460 Offset (m) Offset (m) Offset (m) 900 Stacked Section with NMO Stretch 30 % TWT(S) 0 1.5 280 Offset (m) 2500 Stacked Section with NMO Stretch 350 % TWT(S) 0 1.5 280 Offset (m) 2500 Range-limited Stack No Far Offset Traces Offset Range : 200 -600 m TWT(S) 0 1.5 280 Offset (m) 2500 Outlines • Survey objective and collected data overviews • Tomographic imaging • Seismic imaging • Interpretation of tomographic and seismic images • Conclusion Comparison of Tomographic and Seismic Image Tomographic Image m/s Z (m) 0 300 Seismic Refractor Image A C Amplitude Z (m) 0 B 300 0 X (m) 2500 Interpretation of Tomogram and Seismic Refractors A B #125 0 #124 320ft (97m) Z (m) 300 0 C 680ft (200m) 900 ft (273m) X (m) 2000 m (ch48 of Line C) 845ft (256m) 2500 150 m off from the end m/s Conclusions • Seismic and tomographic imaging of bedrock are conducted for Newmont mining in Nevada. • The both results clearly show a faulted bedrock topography 50 – 200 m below surface.