Transcript Seismic Prospecting for Concealed Carlin

Hardrock Seismic Attribute Analysis
and AVO modeling for Carlin-type
Deposit Exploration
Kyle T. Gray1
Jared Townsend2
John Louie1
Great Basin and Western Cordillera Mining Geophysics Symposium
11/23/2013
1 University
of Nevada, Reno Seismological Laboratory
2 Barrick Gold Corporation
Overview
• Both petroleum and mineral exploration
attempt to locate zones of increased porosity
• The CTD environment shares similarities with
carbonate oil and gas reservoirs, which allows
for adoption of oil and gas methodologies
• We cannot get all of the benefits of seismic
data without abundant wire-line logs
Seismic Interpretation Methods
1. Conventional interpretation
– Manual picking of horizons and faults
2. Seismic attributes
– Analogous to filtering of potential fields
data
3. Forward modeling
– Synthetic data from wire-line logs
4. Inversion
– Incorporation of wire-line logs to invert
seismic data into elastic properties
Petrophysical
studies
important in
every step
Hardrock Seismic
• Structural complexity
• Crystalline environment
– or close spatial association with magmatism
• Heterogeneous regolith zone
• High ambient noise
– Haul trucks, mining operations
• Variable source- receiver couplings
• Ground relief
PREVIOUS HARDROCK
SEISMIC STUDIES
Seismic in Mining
Bushveld, South Africa
Witwatersrand, South Africa
Flin Flon, Quebec
Mount Morgan, Australia
• Previous seismic data for mining exploration is
largely collected in the Proterozoic setting
– No carbonates
– No extant porosity
– No direct analogies to petroleum industry
Bell Allard, Quebec
Manitouwadge, Ontario
St. Ives, Australia
Mt. Isa, Australia
Kristienberg, Sweden
Pyhasalmi, Finland
Half Mile Lake, Canada
Voisey’s Bay, Labrador, Canada
(Harrison and Urosevic, 2012)
CTD Formation
• Ore genesis relates to
structural and magmatic
overprint on the platformslope environment
(Cook and Corboy, 2004)
Carlin Type Deposit paleo-stratigraphic
setting: carbonate platform-slope
http://www.beg.utexas.edu/lmod/_IOL-CM03/cm03-step01.htm
Seismic response in carbonate
) platform-slope environment
(Jansen et. al, 2011
Alteration and Petrophysics
• Decarbonatization = decreased seismic
parameters and increased porosity.
• Sample bias in drill-core petrophysics
towards less altered rock.
Acoustic Impedance Vs. Porosity
(Wenban Subset)
20
Decarbonitized
Porosity (%)
15
No Decarbonitization
No DECAL, ARGIL, SILIC
R² = 0.4867
10
5
R² = 0.0026
0
0
-5
2000
4000
6000
8000
10000
12000
14000
Acoustic Impedance
16000
18000
20000
Wire-line Logs
Tertiary
Limestone
Decarb.
Intensity
from
geolog
3700 m/s
5700 m/s
Pseudo wire-line log
• Drill core sampled at
high resolution
• Cannot sample
heavily altered rock
• Stresses importance
of wire-line logging
150-250 ft
Case Study
1300 ft
Seismic Attributes - potential field
analogy
RTP Magnetics
Total Horizontal Gradient
Average frequency
Instantaneous amplitude
Intro to Pre-stack Data
Stacked
trace
Increasing offset
Sum all
offsets
http://subsurfwiki.org/wiki/Gather
Amplitude Versus Offset (AVO) Model
• Far offset anomaly at
base of
decarbonatization
Gradient plot for base of
decarbonatization
Amplitude
Offset
Offset
Pre-stack Migrated Gathers
offset
offset
offset
…
Inversion
• Pre-stack inversion using ONE wire-line log
Tuff in geolog
2.6 g/cc
2.15 g/cc
Conclusions
• Decarbonatization produces distinct rock
property contrasts but more wire-line data is
needed to fully understand the system.
• Seismic attributes calculated in CTD
environment can help reveal alteration
features
• Inversion shows promise given that the case
study only used one wire-line log.
References
• Salisbury, M., Harvey, C., Matthews, L., The acoustic properties of
ores and host rocks in hardrock terranes In: D. Eaton, B. Milkereit &
M. Salisbury, eds. Hardrock Seismic Exploration. Tulsa: Society of
Exploration Geophyiscs, p. 141.
• Harrison, C. B. & Urosevic, M., 2012. Seismic processing, inversion,
and AVO for gold exploration - Case study from Western Australia.
Geophysics, 77(5), p. 235-243.
• Cook, H. & Corboy, J., 2004. Great Basin Paleozoic Carbonate
Platform: Facies, Facies Transitions, Depositional Models, Platform
Architecture, Sequence Stratigraphy, And Predictive Mineral Host
Models, Field Trip Guidebook—Metallogeny of the Great Basin
Project, August 17—22, 2003 :U.S. Geological Survey Open-File
Report 2004-1078.
• Janson, X., Kerans, C., Loucks, R. Marhx, M., Reyes, C., Murguia, F.,
2001, AAPG Bulletin, v. 95, No. 1 (January 2001) p. 105-146