Seismic Based Rheological Parameters Provide Detailed Lithological and
Structural Subsurface Images In Mining Exploration
Zoltan Hajnal1, Erno Takacs1, and Bhaskar Pandit1
zoltan.hajnal@usask.ca
1Department
of Geological Sciences, University of Saskatchewan
114 Science Place, Saskatoon, SK, S7N 5E2, Canada
The Meso-Proterozoic Athabasca Basin is recognized for its numerous high grade uranium
mineralizations. Only the shallow margins of this 100,000 km 2 depression have been effectively
explored up to this date. With intent to extend the activities into the deeper part of the basin, seismic
reflection surveys were introduced in the early 2000s. Seismic images effectively recognize deep
basement faults/shear zones, the unconformity (UC) between the sandstone basin fill and the
crystalline basement. It is also well recognized (Goodway, 2001; Ikelle and Amundsen, 2005; Russell
& Hampson, 2006) that the seismic shear waves (S) and compressional waves (P) are controlled by
different rheologic parameters, thus extraction of these elastic parameters from the seismic signal
may be utilizable to detect lithologies, their variations, characterization of permeable zones (fractures),
and mineralization-related clay alteration halos. This study is our first attempt in the basin to extend
this combined seismic and borehole data analysis with intention to determine the effectiveness of
these intrinsic signal properties as indicators of favourable mineralization target zones. Five Full Wave
Sonic logs and two Optical Televiewer observations were utilized for this investigation.
Density-depth values were established through known compressional wave velocity (VP) and
density (ρ) relationship. Specific lithology/alteration related parameters, acoustic impedance (Vρ),
VP/VS ratio, Poisson’s ratio, Lambda-Rho (λρ) and Mu-Rho (μρ) were computed for every depth point
of the boreholes. Recent investigations (Goodway, 2001) demonstrated that interconnected
relationships of these rheology parameters provide more demonstrative indications of distinct
lithologies, alteration zones or anomalous fracture zones than their single values alone. Cross-plots of
these combined parameters help to recognize several characteristic hierarchical clusters associated
with distinct fracture zones within specific lithologic entities and prominent alteration intervals. High
velocity related clusters are a consequence of silicification of the sandstone; low velocity intervals are
direct indications of clay alteration (Figure 1).
References:
Goodway, B. 2001: AVO and Lame constants for rock parameterization and fluid detection. CSEG Recorder,
June 2001, 39-60.
Ikelle, L.T. and Amundsen, L. 2005: Introduction to petroleum seismology. Investigations in geophysics No.12,
Society of Exploration Geophysicist (SEG), pp. 46.
Russell, B. and Hampson, D. 2006: The old and the new in seismic inversion. CSEG Recorder, December 2006,
5-11.
Figure 1: Parameter logs derived from Full Wave Sonic (FWS) survey showing the elastic properties of clay
filled fracture zones in the cristalline basement of the Athabasca Basin. For the investigated (clay) zones
compressional wave velocity (VP) is low, Poisson’s ratio is high, Lambda*Rho is high, and Mu*Rho is low (Rho is
the density of the rocks). This separation of the elastic parameters provides a useful tool to detect clay alteration
zones from 1-D FWS measurements and also from 2-D or 3-D surface seismic observations.
Keywords: Full Wave Sonic logging, elastic parameters, Poisson’s ratio, Lambda, Mu, cross-plot
analysis, Athabasca Basin, cristalline basement, clay, alteration, uranium mineralization.
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