Using 3D Seismic Imaging for Mine and Mineral Exploration

1
Using 3D Seismic Imaging for Mine and Mineral
Exploration
G. Schuster
University of Utah
2
Outline

• Reflection Imaging Principles

• Case History 3D Seismic Potash

• Case History 2D Tomography

• Case History Crosswell

• Summary

3
Seismic Section
Depth
Time
4
Seismic Section
DepthvelT
Depth
Time
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2-D Seismic Survey
30 m
6 km
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3-D Seismic Survey
30 m
6 km
7
2D vs 3D
Top View
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Outline

• Reflection Imaging Principles

• Case History 3D Seismic Potash

• Case History 2D Tomography

• Case History Crosswell

• Summary

9
Potash Geology(Pruegger Nemeth)

• Sakatchewn Province 12 km/12 km Potash mine 1
  km depth

• Geology

Potash
Salt
Karst
Limestone
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Potash Geology

• Sakatchewn Province 12 km/12 km Potash mine 1
  km depth

• Geology

Potash
Salt
Karst
Limestone
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3D Seismic Parameters

• Receiver line interval 180 m

• Source line interval 300 m

• 550 traces/shot

• 60 m shot/recievr interval

• 150 Hz Dynamite, 12-fold, 30 m

• 1/3 million dollars

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Top View of Mine (z1 km)
Disturbance
Seismic Anomaly?
10 km
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Collapsed Breccia
15 m
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Collapsed Breccia
10 m
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3D Seismic Sections
Summary
Boreholes
Top Salt
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3D Seismic Depth Slices and Cross Sections
1 km
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Integrated Lanigan Data
Mine
200 m
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Red Bed
200 m
Limestone
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Looking Down View
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200 m
Limestone
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Limestone
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Interpretation
Seismic
0.0 km
1.0 km
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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.

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WHATS 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 plan to keep adding data, minesite by
  minesite, and we want make GOCAD conveniently
  available at all our mine divisions.

25
Outline

• Reflection Imaging Principles

• Case History 3D Seismic Potash

• Case History 2D Tomography

• Case History Crosswell

• Summary

26
Mineral Geology

• Western State Large mining operation over many
  km, but at shallow depth (30 m)

• Geology

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Goal Cheaply Find Lateral Extent of Play
Soln 2-D Refraction Tomography

• Geology

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Seismic Imaging of Anomaly
Velocity Anomaly
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Seismic Imaging of Anomaly
Velocity Anomaly
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Seismic Imaging of Anomaly
Fast Time
Velocity Anomaly
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Seismic Imaging of Anomaly
Slow Time
Fast Time
Velocity Anomaly
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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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One Traveltime Equation for Each Measurement
T L /v L /v L /v
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Many Traveltime Equations for Each Shot
T L /v L /v L /v
T L /v L /v L /v
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Field Data

• Thirty-one shots and 120 traces
• total 3188 traveltimes picked.
• Shot interval 20 m
• geophone interval 5 m

• Source frequency 40 Hz.

• Record length 1 sec.
• sample interval 0.5 millisecond .

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Seismic Refraction Data
Common Shot Gather Line 2
West
East
Created with Vista
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Seismic Data Acquisition Parameters
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Seismic Refraction Data
Line 1 Final Model (3rd Schedule)
Ray Path Density
Velocity Profile
Ray Density Plot
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Summary
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Conclusions
gt 50K several sq. km
Depth Resolution 5-20 m
Offset Resolution 20-60 m
Requires Expert Processing
gt 5K several km
Always works, moderate expertise, 41
offset/depth ratio
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Crosswell Traveltime Tomography
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Fault Model
S
P
0
1150
2300
Depth (m)
3650 m/s
1825 m/s
210
0
0
90
90
Offset (m)
Offset (m)
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Fault Tomogram
S
P
0
A
A
1150
2300
C
C
D
Depth (m)
B
B
3650 m/s
1825 m/s
210
0
0
90
90
Offset (m)
Offset (m)
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W Texas Tomogram
P
S
0
A
7750
14000
B
Depth (ft)
D
12700 ft/s
22500 ft/s
C
500
0
0
184
184
Offset (ft)
Offset (ft)
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Crosswell McElroy Data
6505
820
201 shots from depths of 811 to 963 m
840
846m
6136
186 hydrophones from depths of 822 to 963 m
860
880
887m
5767
Shot, hydrophone interval 0.76m
Depth (m)
900
920m
920
Maximum source freq. 1400 Hz
5398
940
960
5029
0
50
40
30
20
10
(m/s)
Distance (m)
48
Crosswell McElroy Comparison
V
Receiver Well
V
Source Well
2700
Depth (ft)
3150
49
Poisson Ratio Comparison
Visco.
0
A
0.35
Depth (ft)
B
0.05
250
0
184
Offset (ft)
50
Summary