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Using 3D Seismic Imaging for Mine and Mineral Exploration G. Schuster University of Utah

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Outline Reflection Imaging Principles Reflection Imaging Principles Case History: 3D Seismic Potash Case History: 3D Seismic Potash Case History: 2D Tomography Case History: 2D Tomography Case History: Crosswell Case History: Crosswell Summary Summary

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Seismic Section Depth Time

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Depth Time Depth=vel*T Depth Resolution = Horiz. Resolution > = 10 m = 60 m

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2-D Seismic Survey 6 km 30 m

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3-D Seismic Survey 6 km 30 m

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2D vs 3D TopView

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Outline Reflection Imaging Principles Reflection Imaging Principles Case History: 3D Seismic Potash Case History: 3D Seismic Potash Case History: 2D Tomography Case History: 2D Tomography Case History: Crosswell Case History: Crosswell Summary Summary

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Potash Geology (Pruegger & Nemeth) Sakatchewn Province: 12 km/12 km Potash mine 1 km depth Sakatchewn Province: 12 km/12 km Potash mine 1 km depth Geology Geology Potash Salt Limestone KarstReefs 12 km 200 m

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Potash Geology Sakatchewn Province: 12 km/12 km Potash mine 1 km depth Sakatchewn Province: 12 km/12 km Potash mine 1 km depth Geology Geology Potash Salt Limestone KarstReefs 12 km 200 m Can Seismic Predict Disturbances?

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3D Seismic Parameters Receiver line interval = 180 m Receiver line interval = 180 m Survey Area 15 km Survey Area 15 km2 Source line interval = 300 m Source line interval = 300 m 550 traces/shot 550 traces/shot 60 m shot/recievr interval 60 m shot/recievr interval 150 Hz Dynamite, 12-fold, 30 m 150 Hz Dynamite, 12-fold, 30 m 1/3 million dollars 1/3 million dollars

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Top View of Mine (z=1 km) Disturbance 10 km 10 km 15 m 5 m = Seismic Anomaly?

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Collapsed Breccia 15 m Breccia Limestone

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Collapsed Breccia 10 m

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Summary 3D Seismic Sections Boreholes Top Salt

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3D Seismic Depth Slices and Cross Sections Mine Entry 1 km Cross section Cross Sections Depth Slices

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Boreholes+Logs Winnipegosis Limestone Mine Integrated Lanigan Data 200 m

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Red Bed Limestone Gamma Log Potash 200 m

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“Looking Down” View

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Limestone 200 m Mine Roof Mine Room

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Mine near potash above limestone Limestone Subtle Sags = Collapse Zones

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1.0 km 0.0 km SeismicInterpretation

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Prugger and Nemeth Summary We found a number of inconsistencies with our data (eg.- seismic depth conversion, borehole orientation, coordinate conversion) once everything was put together in GOCAD. We found a number of inconsistencies with our data (eg.- seismic depth conversion, borehole orientation, coordinate conversion) once everything was put together in GOCAD. We use GOCAD (almost every day) to view various types of data simultaneously Data formats are well described, so we’ve generally found adding data to be easy. $333K, Seismic-> Rooms, anoamlies

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WHAT’S NEXT FOR POTASHCORP ? We are building a “MODEL3D” earth model from 3D seismic & well-logs, which results in a velocity VOXET. This is then used to depth- migrate the seismic data, resulting in an improved seismic depth-image.We are building a “MODEL3D” earth model from 3D seismic & well-logs, which results in a velocity VOXET. This is then used to depth- migrate the seismic data, resulting in an improved seismic depth-image. We plan to keep adding data, minesite by minesite, and we want make GOCAD conveniently available at all our mine divisions.We plan to keep adding data, minesite by minesite, and we want make GOCAD conveniently available at all our mine divisions.

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Outline Reflection Imaging Principles Reflection Imaging Principles Case History: 3D Seismic Potash Case History: 3D Seismic Potash Case History: 2D Tomography Case History: 2D Tomography Case History: Crosswell Case History: Crosswell Summary Summary

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Mineral Geology Western State: Large mining operation over many km, but at shallow depth (30 m) Western State: Large mining operation over many km, but at shallow depth (30 m) Geology Geology ?????? km 20 m

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Goal: Cheaply Find Lateral Extent of Play Geology Geology ?????? km 20 m Soln: 2-D Refraction Tomography

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Seismic ImagingofAnomaly Seismic Imaging of Anomaly Velocity Anomaly

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Seismic ImagingofAnomaly Seismic Imaging of Anomaly Velocity Anomaly

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Seismic ImagingofAnomaly Seismic Imaging of Anomaly Fast Time Velocity Anomaly

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Seismic ImagingofAnomaly Seismic Imaging of Anomaly Velocity Anomaly Fast Time Slow Time

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Seismic ImagingofAnomaly Seismic Imaging of Anomaly 3.0 m

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Discretize Earth Model into Grid of Unknown Velocities

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One Traveltime Equation for Each Measurement

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T = L /v + L /v + L /v

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Many Traveltime Equations for Each Shot T = L /v + L /v + L /v

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Field Data Thirty-one shots and 120 traces Thirty-one shots and 120 traces total 3188 traveltimes picked. total 3188 traveltimes picked. Shot interval: 20 m Shot interval: 20 m geophone interval 5 m geophone interval 5 m Source frequency 40 Hz. Source frequency 40 Hz. Record length 1 sec. Record length 1 sec. sample interval 0.5 millisecond. sample interval 0.5 millisecond.

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Seismic Refraction Data Common Shot Gather – Line 2 WestEast Created with Vista

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Seismic Data Acquisition Parameters

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Seismic Refraction Data Ray Path Density Velocity Profile Ray Density Plot Line 1 – Final Model (3 rd Schedule) 55 m No Salt 46 m No Salt 20 m Hit Salt Well Tunnel Surface 300400500 60 40 20 Depth & Distance in Meters

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Summary $5K, Seismic-> Mapped Lateral extent of Play extent of Play 4:1 Offset-Depth ratio Less Resolution Than Reflection 4 More surveys

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Conclusions 3D Seismic Reflection Mine Survey: > $50K several sq. km Depth Resolution = 5-20 m Offset Resolution = 20-60 m Requires Expert Processing Refraction Tomography: > $5K several km Always works, moderate expertise, 4:1 offset/depth ratio

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Crosswell Traveltime Tomography Time

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0 90 Offset (m) Depth (m) 0 210 P 0 90 Offset (m) S 2300 3650m/s 1150 1825m/s Fault Model

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0 90 Offset (m) Depth (m) 0 210 P 0 90 Offset (m) S 2300 3650m/s 1150 1825m/s A B C A B C D Fault Tomogram

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B A C D 0 184 Offset (ft) Depth (ft) 0 500 P 0 184 Offset (ft) S 14000 22500ft/s 7750 12700ft/s W Texas Tomogram

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Crosswell McElroy Data 820 840 860 880 900 920 940 960 05040302010 6505 5767 5398 5029 6136 (m/s) Depth (m) 201 shots from depths of 811 to 963 m 186 hydrophones from depths of 822 to 963 m Shot, hydrophone interval 0.76m Maximum source freq. 1400 Hz 846m 920m 887m Distance (m)

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270 0 V Depth (ft) 3150 V Receiver Well Source Well Crosswell McElroy Comparison

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Poisson Ratio Comparison 0 Offset (ft) Depth (ft) 0 250 Visco. 184 A B 0.35 0.0 5

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Summary $25K, Seismic-> Mapped Lateral extent of Play extent of Play 3 m Resolution!

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