Resolution of airborne VLF data
Introduction
Airborne very low frequency (VLF) surveys which make use of single frequency signals (14–30 kHz) from remote VLF transmitters are useful and fast techniques for reconnaissance geological mapping, especially in countries like Sweden and Canada, where the sedimentary cover is thin. The VLF method (referred to as VLF-EM or VLF-Z elsewhere) has proved to be an effective exploration tool for massive sulphides, graphites, carbonaceous shales, sheared contacts, and fracture zones. Therefore, during the last decades, it has been employed world-wide to identify conducting features in mineral exploration, geological, engineering and environmental problems (Barbour and Thrulow, 1982, Fischer et al., 1983, Poddar and Rathor, 1983, Sinha and Hayles, 1988, Sinha, 1989, McNeill and Labson, 1991, Ogilvy et al., 1991, Tabbagh et al., 1991, Chouteau et al., 1996, Gharibi and Pedersen, 1999, Oskooi and Pedersen, 2001, Smith et al., 2001, Becken and Pedersen, 2003). The principle of VLF surveying has been described by Poikonen and Suppala (1989), Spies (1989), Vallee et al. (1992), and Kaikkonen and Sharma (2001). This method relies on wave-field interaction with two-dimensional (2D) and three-dimensional (3D) resistivity structures (Beamish, 2000) which was first developed as an inductive profiling technique measuring the amplitude (and subsequently phase) relationship between the vertical (secondary) magnetic field, Hz, relative to the horizontal (primary + secondary) field, Hy. Interpretation of the VLF data represents an important aspect of geophysical mapping which has recently seen considerable qualitative and quantitative improvements.
VLF anomalies have been computed for various models by Olsson (1980). He introduced a simple way to determine the depth to the top of the conductor and possibly the dip angle. Since then a considerable amount of theoretical and numerical studies were made to calculate the VLF response of subsurface conductors. Serious efforts towards the inversion of VLF data to obtain quantitative information about the subsurface were made in the 1990s. Beamish (1994) showed that regularized 2D inversion of single frequency VLF data could deliver quite detailed information about the subsurface electrical conductivity structures. Quantitative 2D VLF data interpretation by Beamish, 1998, Beamish, 2000 demonstrated that when single frequency VLF data are collected with a high lateral density, the measurements can be used to infer the main elements of the subsurface resistivity distribution. The tools required are regularized, smooth model inversion schemes that have been developed for multi-frequency, magnetotelluric data sets. The use of Radiomagnetotelluric (RMT) data, when available, will invariably add to the resolution capabilities of the VLF method. Synthetic modeling and inversion have been done by the authors (Pedersen and Oskooi, 2004), for both VLF (14–30 kHz) and LW (Long Wave Band, 30–300 kHz). Also Persson (2001, PhD thesis) examined the resolving power of 2D inversion on some synthetic models as well as applying it to field VLF and RMT data.
Previous attempts to correlate ground and airborne data, (Hoekstra et al., 1975, Arcone, 1979) have indicated that much detail and amplitude of ground resistivity variations can be lost at airborne levels, even at minimum flight altitudes of 75 m. Therefore, to interpret airborne data and correlate them with similar measurements made on the ground there is a need to understand better the relationship between ground and airborne measurements. To aid in this understanding, we have made an analysis of airborne VLF responses for some typical resistivity structures, namely a long thin conductor (10 m thick, a resistivity of 10 Ω m and 1 km long) and a conductive dike (a resistivity of 100 Ω m and 500 m wide) both buried in a host of 10,000 Ω m. Then airborne VLF data from an area southeast of Stockholm (Fig. 1) were processed and interpreted. For this purpose the anomalies from selected flight-lines are modeled by using an inversion technique developed by Siripunvaraporn and Egbert (2000). Ground RMT data along a short profile, measured by the Enviro-MT system designed by Uppsala University (Bastani, 2001), are also presented. The aim is to test the prediction of the airborne measurements and to get independent estimates of resistivities of the most important lithological units. Finally, a geological interpretation of the conductive structures in terms of conductive metasedimentary rocks in valleys is made.
Section snippets
VLF measurements, the scalar tipper data
In standard VLF measurements only one horizontal, say Hy and one vertical component, Hz are measured. For a 2D structure with a strike direction along the x-axis the derived scalar transfer function Bsca = Hz / Hy will be identical to the unique tipper component B and the tipper component A is identically equal to zero (Pedersen and Oskooi, 2004). Only in this case it is straightforward to interpret scalar VLF measurements with 2D models. If the strike direction differs from that of the x-axis it
2D synthetic forward modeling and inversion of two simple structures
The ability to interpret EM field data depends on having a good understanding of the anomalies to be expected from different subsurface structures. In order to study the resolution of airborne VLF data, synthetic tipper data at a frequency of 16 kHz were produced for two generic models in a halfspace of 10,000 Ω m corresponding to a skin depth of 400 m. Model 1 is a thin conductive layer (10 m thickness, 1.2 km wide and a resistivity of 10 Ω m) (Fig. 2a and b). The top surface is placed at
3D synthetic forward modeling
3D modeling has been performed using the XPLATS code (Avdeev et al., 2000) with regional currents flowing in the x-direction, parallel to ‘strike’. Our aim is to study the responses of the thin conductors (10 m thickness, 100 m wide, and a resistivity of 10 Ω m) with varying length at airborne conditions. In Fig. 9a the conductors are shown in a 10,000 Ω m halfspace with lengths of 100 (A), 200 (B), 300 (C) and 400 (D) m, respectively. A simulation of disconnected surface conductors (100 by 100
Scalar tipper data along flight Lines 1644800, 1645000 and 1645200
In the Stockholm area, using signals from the Rugby transmitter located in England (GBR, operating at16 kHz), VLF data were measured along flight lines with 200 m separation, at an altitude of about 70 m and with a sampling distance of approximately 17 m. The flight lines were in the N–S direction which is fairly well adapted to the strike of the main geological structures in the area.
The real and imaginary parts of the scalar tipper vary typically between − 0.5 and 0.5. Notice that the real and
RMT ground measurements and comparison with airborne VLF results
RMT data were collected at 10 m interval along a 600 m long profile coinciding with a part of the airborne profile 1645000. This S–N profile crosses a wide valley, and is shown in the middle of Fig. 1 as a short thick line marked Number 1. The aim is to make a comparison with the airborne measurements and to get independent estimates of resistivities of the most important lithological units. The determinant apparent resistivity and phase data (Berdichevsky and Dmitriev, 1976, Pedersen and
Discussion and conclusions
Single frequency transfer functions (tippers) calculated from airborne VLF data were modeled quantitatively using a regularized 2D inversion scheme developed originally for deep electromagnetic MT soundings. There are two main conclusions to be made from this study.
The apparent discrepancy between airborne and land measurements in the Stockholm area can be largely explained as due to the small lateral scale of surface conductors. These small scale conductors give rise to small scale VLF
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