1. 3-D Karoo basin reconstruction from ambient passive seismic noise
Earth Stewardship Science
Research Institute
3-D Karoo basin reconstruction from ambient passive seismic noise
L. Bezuidenhout, V. Wagener, M. Doucouré
Department of Physics and Geoscience
Objectives
Apply the concept of passive seismic tomography using three-component geophones to construct 3-D image of the Karoo sub-surface.
Gain an improved understanding of the subsurface geology of the Karoo basin.
Construct a seismic (earthquake) survey of the Karoo prior to hydraulic fracturing.
WHY Karoo ?
Increased interest in the shale gas extraction within the Karoo basin is gaining momentum.
A scientifically thorough base-line of this region needs to be established to moderate any negative effects associated with fracking.
Study area: surrounds of Graaff Reinet, Aberdeen and Cradock (location of Soekor boreholes)
Karoo: Areas of Interest
Geological structure of the Karoo indicating the proposed study area
System and Data acquisition
Data
monitoring
Battery
Data Cube
Geophone
GPS
Acquisition
Processing
Interpretation
Static correction
Velocity analysis (Green’s function)
NMO/DMO
Stacking
Migration
Seismic data to subsurface geology
Seismic equipment Seismograph Methodology
This study
Deploy passive seismic network of 20 three-component (N-S, E-W, vertical) geophones for predetermined periods in various areas of interest in the Karoo.
Collect and monitor ambient seismic signals up to frequencies of 160 Hz.
Convert the data acquired from the geophones to a velocity signal.
[The highly non-linear inverse problem created from the retrieval of the velocity
variation of the travel times is numerically solved using Green’s cross correlation
function to construct the 3-D map.]
3-D mapping of the Karoo will possibly allow for identification of natural resources (i.e. shale layers) and localized faults planes (where fracking should not occur).
Increase resolution from previous seismic models of the Karoo.
The aim of this project is to establish a continuous monitoring microseismic acquisition network across the Karoo to possibly detect any varying subsurface perturbations / ambient noise and image the subsurface.
This will be a first of a kind type of work undertaken in South Africa with a view to create a forensic baseline of the Karoo region.
Previous seismic modeling of the Karoo and Cape fold belt
1993
~8 km
North
South
Escarpment
Cape Fold
Belt Front
2004
Extent of NVR profile ~100 km
Prince Albert
2006
2007/8 km 5 10
Halbich et al. (1993) model
Chevallier et al. (2004) model
Johnson et al. (2006) model
Acknowledgement
The authors would like to thank Inkaba yeAfrica, DST, NRF and GFZ.
Lindeque et al. (2007/8) model