1. Multidisciplinary shallow crustal imagery at Boliden Tara Mines
Senad Subašić, Christopher J. Bean, Nicola Piana Agostinetti
Geophysics section, School of Cosmic Physics, Dublin Institute for Advanced Studies
Objectives
We have analysed data from the LaBarge Array (Leahy et al, 2012):
receiver functions used at increasingly higher frequencies (1, 2, 4 and 8 Hz in the images to the left);
features sharpen, and single peaks turn into two or three as we move to higher frequencies;
candidate Ps conversion arriving at approximately 6 seconds (Moho);
direct P arrival (first blue peak), basement reverberation (second main peak, at around 2 s).
Receiver functions use station-local seismic wave conversions from distant earthquakes to enable the determination of sharp boundaries beneath seismic stations.
They are a widely used method for studying the structure of the crust and upper mantle, e.g. the Mohorovičić discontinuity (Moho), or the lithosphere-astenosphere boundary (LAB). Studies have shown that the same method can be applied at a smaller scale, and can be useful in exploration geophysics as well.
The project aims to obtain geophysical images of the shallow crustal structure in the vicinity of the Boliden Tara Mines orebody at Navan, using a multi-disciplinary approach:
broadband surface-wave seismic;
receiver function passive seismic;
electromagnetic data sets;
gravity data sets.
The area has good geological constraints and other geophysical data available. The idea is to constrain the images over areas with good borehole and/or 2D seismic coverage, and to move away from these constrained areas in 3D using passive methods.
The underlying structure can be inferred from the amplitudes and delay times of these conversions. The amplitude depends on the impedance contrast at a specific interface (mostly differences in velocities), while the delay time contains information about seismic velocities and the depth of the interface.
Receiver function analysis is generally a deconvolution problem. There are several approaches to solving it:
Receiver functions
In general, positive peaks correspond to seismic interfaces where the S-velocity increases with depth.
Systematic changes in amplitudes or delay times can indicate anisotropy or dipping of the interface. It is therefore important to have a good back-azimuthal coverage, which requires a longer deployment.
Fig. 2. Receiver functions for station L40 in the LaBarge Array, sorted by backazimuth, filtered at 1, 2, 4 and 8 Hz.
Deployment
21 stations in total, arranged in a square grid, 5 by 5 km.
Instruments will be recording for at least six months, in order to ensure adequate coverage.
17 stations (red markers) cover a wider area and will be used by other projects as well.
4 extra stations (white markers) along an existing seismic profile, 200 m apart, will be used to test the resolving power of the method.
The selected profile is perpendicular to a known structure, and is far enough from the mine to limit the effect of noise.
Fig. 1. Top: A schematic receiver function; bottom: ray paths. Adapted from Hu et al. (2015)
Time domain
Iterative deconvolution (Ligorria and Ammon, 1999)
numerically stable, good results
Frequency domain
Water-level deconvolution (Langston, 1979)
unstable, struggles with weaker earthquakes and noise
Multiple-taper correlation (Park and Levin, 2000)
robust, minimizes spectral leakage
Frequency domain deconvolution (Di Bona, 1998)
provides estimates of RF variance
Fig. 3. Study area, inset shows planned instrument locations.
References:
Di Bona, M. (1998), Variance estimate in frequency-domain deconvolution for teleseismic receiver function computation. Geophysical Journal International, 134: 634–646. doi: /j X.1998.tb07128.x
Langston, C. A., Structure under Mount Rainier, Washington, inferred from teleseismic body waves. J. Geophys. Res., 84, , 1979.
Leahy, G. M., Saltzer, R. L. and Schmedes, J. (2012), Imaging the shallow crust with teleseismic receiver functions. Geophysical Journal International, 191: 627–636. doi: /j X x
Ligorría, J.P., and Ammon, C.J. (1999), Iterative deconvolution and receiver-function estimation. Bulletin of the Seismological Society of America, v. 89, p. 1395–1400
Park J. Levin V. (2000), Receiver functions from multiple-taper spectral correlation estimates. Bull. seism. Soc. Am, 90, 1507–1520.
This publication has emanated from research supported in part by a research grant from Science Foundation Ireland (SFI) under Grant Number 13/RC/2092 and co-funded under the European Regional Development Fund..