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

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

Seismic Section Depth Time

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

2-D Seismic Survey 6 km 30 m

3-D Seismic Survey 6 km 30 m

2D vs 3D TopView

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

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

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?

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

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

Collapsed Breccia 15 m Breccia Limestone

Collapsed Breccia 10 m

Summary 3D Seismic Sections Boreholes Top Salt

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

Boreholes+Logs Winnipegosis Limestone Mine Integrated Lanigan Data 200 m

Red Bed Limestone Gamma Log Potash 200 m

“Looking Down” View

Limestone 200 m Mine Roof Mine Room

Mine near potash above limestone Limestone Subtle Sags = Collapse Zones

1.0 km 0.0 km SeismicInterpretation

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

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.

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

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

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

Seismic ImagingofAnomaly Seismic Imaging of Anomaly Velocity Anomaly

Seismic ImagingofAnomaly Seismic Imaging of Anomaly Velocity Anomaly

Seismic ImagingofAnomaly Seismic Imaging of Anomaly Fast Time Velocity Anomaly

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

Seismic ImagingofAnomaly Seismic Imaging of Anomaly 3.0 m

Discretize Earth Model into Grid of Unknown Velocities

One Traveltime Equation for Each Measurement

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

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

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.

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

Seismic Data Acquisition Parameters

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 Depth & Distance in Meters

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

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

Crosswell Traveltime Tomography Time

0 90 Offset (m) Depth (m) P 0 90 Offset (m) S m/s m/s Fault Model

0 90 Offset (m) Depth (m) P 0 90 Offset (m) S m/s m/s A B C A B C D Fault Tomogram

B A C D Offset (ft) Depth (ft) P Offset (ft) S ft/s ft/s W Texas Tomogram

Crosswell McElroy Data (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 Hz 846m 920m 887m Distance (m)

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

Poisson Ratio Comparison 0 Offset (ft) Depth (ft) Visco. 184 A B

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