Resolution of airborne VLF data

Abstract

The interpretation of airborne VLF data represents an important aspect of geophysical mapping of the upper few hundred meters of the Earth's crust, especially in areas with crystalline rocks. We have examined the ability of the single frequency VLF method to provide quantitative subsurface resistivity information using two generic models and standard airborne parameters with a flight altitude of 70 m and a frequency of 16 kHz. The models are long thin conductor (10 m thick, 10 Ω m resistivity and 1 km long) and a wider buried conductive dike (100 Ω m resistivity and 500 m wide). Using standard regularized inversion it turned out that for both models the conductivity of the conductors are underestimated and the vertical resolution is rather poor. The lateral positions of the minimum of the resistivity distributions coincide well with the true positions of the shallow conductors. For deeper conductors the position of the minimum resistivity moves from the edges of the conductor into the conductor. The depth to the minimum of the resistivity anomalies correlates well with the true depth to the top of the conductors although the latter is always smaller than the former.

Interpretation of field airborne data collected at 70 m flight height resolved both small scale and large scale near surface conductors (conductance ∼1 S). Deeper conductors show up in the VLF data as very long wavelength anomalies that are particularly powerful in delineating the lateral boundaries of the conductors. Many of the VLF anomalies in the Stockholm area are dominated by these deep conductor responses with some near surface conductors superimposed. The deep conductors often follow topographic lows coinciding with metasediments. We interpret the frequent absence of near surface responses at 70 m flight height as a result of weak coupling between the primary VLF wave and the small scale (in all three dimensions) near-surface conductors.

Radio magnetotelluric (RMT) ground measurements were carried out along a short profile coinciding with part of an airborne profile. Using data at 9 frequencies (14–250 kHz) small scale conductors in the upper few tens of meters, not identified from the airborne data, could be well resolved. Large scale deeper conductors could be identified by both methods at nearly the same positions.

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, H_z, relative to the horizontal (primary + secondary) field, H_y. 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.