Methods

The two main methods are in situ electrical resistivity measurements with VLF-R technique along with fracture frequency measurements on outcrops.

Electric Resistivity Measurements with the VLF-R Technique

Electric resistivity is the inverse of conductivity, and the unit used is the ohmmeter (Qm). The aggregate rock resistivity is, in a first approximation, the weighted sum of the resistivities of the rock components. One of the components is water (the electrolyte) residing in the pores of sedimentary rocks or in the fractures in crystalline rocks. The fracturing of rocks changes the rock physical properties by an increase in porosity and thus the electric conductivity. This effect has previously been studied for fracture zones (e.g., Eriksson 1980; Henkel 1988). It has also been observed in several impact structures like Dellen and Siljan (Henkel 1992).

The resistivity measurements were made in-situ using a signal from a distant Very Low Frequency (VLF) radio wave transmitter. Typical transmitted VLF frequencies lie in the 10-30 kHz range. The instrument, Geonics EM16R, is a VLF receiver equipped with a 10-meter reference antenna. This equipment was used to receive the signal with a frequency of 16.0 kHz from the GBR transmitter located in southern England. The signal induces secondary Electro-Magnetic (EM) fields in conductors in the ground or on the surface. The instrument EM16R (Geonics 1979) measures the ratio between the horizontal electric and magnetic field as the phase angle and the apparent resistivity (pa) of the subsurface in Om by adjusting the resistivity dial and phase dial for signal minimum. As the readings are determined by listening to sound variations, the precision is about 5° in phase angle and about 10% in apparent resistivity. The penetration depth of the resistivity measurements depends on the pa of the measured site and is inversely related to the signal frequency. The penetration depth at frequency 15 kHz is 60 m at p = 200 Om, 200 m at p = 2000 Om, and 600 m at p = 20 000 Om. The p measured are in the range of 300 - 10 000 Om, giving a penetration depth much larger than the depth to the ground water level. The rock volumes measured are thus saturated with water.

As the earth is not a perfect conductor, the electric vector of the EM-field is tilted near the earth's surface; thus it also has a horizontal component. The phase angle represents the tilt of the electric vector compared to the magnetic vector caused by the vertical anisotropy of the resistivity within the penetration depth. A two-layer model can be applied to the readings, to estimate the overburden thickness and resistivities of the overburden and bedrock. One of these three model parameters must, however, be assumed. This procedure is put into practical form in the two-layer nomograms constructed by Geonics (1979). If the subsurface has constant electrical resistivity at least down to the penetration depth, the phase angle is 45°. When applied to two-dimensional conductive structures, the two-layer model results in several typical distortions that must be accounted for. When a highly conductive vertical structure, like a fault zone, has a strike direction parallel to the VLF-R antenna, the phase angle measurements give a false and too high value over rather large distances. Above thick conductors, the resistivity can be estimated, but the depth indication becomes unreliable (Hjelt et al. 1985).

It is known that the electric resistivity in fractured crystalline rock has rather low values from studies of mapping fracture zones using an electric method, Slingram, and the electromagnetic technique VLF (e.g. Eriksson 1980; Henkel 1988). Typical values are around 2000 Om in fractured rocks, while normal crystalline rock resistivities are >10 000 Om. Even lower values are found for fault gouge, down to 30 Om (Henkel 1988).

Investigation of Impact-Induced Fracturing

The fracture frequency measurements were made within an 8 km wide area between the two large shear zones mentioned previously but not across them (Fig. 1). The study was made on outcrops along two forest roads, section A and B in Fig. 1. Section B follows the eroded path of one of the mentioned resurge gullies formed through the rim of the crater. Each crystalline rock outcrop was investigated using window-mapping technique to estimate the fracture frequency.

Fig. 2. A nearly vertical surface of Tandsby Breccia similar in appearance to the horizontal surfaces on which the fracture frequency was measured. The hammer length is 0.6 m and the location of the outcrop can be seen as a triangle in Fig. 1.

The outcrops used were smooth, glacially eroded, horizontal surfaces having a homogenous lithology. Figure 2 shows a nearly vertical surface of Tandsby breccia, the same rock type for which the fracture frequency was measured.

The surface was cleaned of all loose materials including lichen or moss growth. Because of the glacial scouring, the surface weathering was minor, but had often reinforced the contrast between the fractures and the edges of the clasts, thus making the counting easier. The general size of each outcrop was 10-50 m2.

At each site, the measurements were divided into two groups, one for large fractures (with a trace length from 0.25 to 5 m) and one for small fractures (with a trace length of 0.1 to 0.25 m). The large fractures were counted over the entire outcrop, whereas the small fractures were counted within 0.25 m2 due to the large number of fractures.

The maximum distance between large fractures was restricted to 5 m because of the size of the outcrops, which had a width of about 5 - 10 m.

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Fig. 3. Projection of the fracture frequency (number of fractures per m2) to the northern (section A) and southern (section B) sections. For the location of sections and individual measurements, see Fig. 1.

The minimum distance between small fractures was truncated at 2 cm; fracture sets with less than 2 cm between fractures were not considered. Fractures smaller than that were too difficult to distinguish from the rock structure. The numbers of fractures were counted including the sides of individual clasts that were considered as fractures. The sum of the small fractures and the large fractures normalized to 1 m2 is the fracture frequency presented in this study.

In-situ Resistivity Measurements on Rock Outcrops

The resistivity measurements were preferentially made over a flat part of an outcrop that contained the fracture frequency sampling area, with no steep edges around. All nearby highly conductive features, such as power lines, railroad tracks, buried electricity cables and wire fences, were avoided. Tectonic fault zones parallel to the transmitter direction, as well as steep slopes in the terrain were also avoided.

At each measurement site, the phase angle and the apparent resistivity values were measured with the Geonics 16R instrument and a short description of the rock type and its soil or moss cover was made.

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