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Jökull - 01.06.2000, Blaðsíða 36

Jökull - 01.06.2000, Blaðsíða 36
Foulger and Field ons at the surface. The model contains no informati- on on near-surface areas between stations. This factor explains why the refraction proíiles of Brandsdóttir et al. (1997), which involved sensors deployed at 200 m intervals across the caldera floor, detected much lower surface velocities than are obtained by lateral extra- polation of the LET model at the surface. Other significant discrepancies between the LET model and the shallow refraction results, highlighted by Brandsdóttir et al. (1997), are at large distances from Víti along the profiles. Because rays from earth- quakes may travel for part of their paths outside the study volume (Figure 4) a velocity structure there must be defined for the LET inversion. However, this velocity structure is so poorly sampled by rays that it is held fixed in the inversion process. The perimeter nodes of ,the study volume, by their very nature, are also very sparsely sampled by rays and only at a few of them are the velocities resolved (Figure 4). For these reasons it is invalid to calculate by extrapolation velocities either outside the grid or in the outer rows of blocks in the study volume. This means that the parts of the refraction profi- les of Brandsdóttir et al. (1997) that are resolved by the LET are limited to distances from Víti of up to ~5.5 km to the west, up to ~1 km to the east, up to 3 km to the north and up to ~10 km to the south. With the exception of the lower velocities detected at very shallow depth by Brandsdóttir et al. (1997), explained above, the agreement between the LET model and the refraction results is remarkably good within these ranges. The LET study of the Krafla area was rather typical of this kind of study. The volume of int- erest was parameterised at 2-3 km intervals in the horizontal and 1 km intervals in the vertical. The inversion process determined the best fit velocities at nodes spaced at those intervals, assuming linear variations in velocity in between and a smooth gener- al model. Such a method returns a broad, average model of the velocity variations in the area. It is not designed to resolve small bodies nor velocity discont- inuities. This must be appreciated when comparing velocities determined by interpolation between grid nodes with methods that involve relatively localised and precise sampling such as small-scale seismic refraction experiments or drilling. Notwithstanding the imperfect earthquake distribution, a reasonable inversion result was obtained, with a data variance reduction of 84% from the starting model. The primary features imaged were high-velocity bodies beneath the caldera ring fault (Figures 4a-c). These were interpreted as gabbroic bodies intruded up the caldera fault. A smaller, high-velocity body was detected at shallow depth beneath Leirhnjúkur, and a low-velocity body to the SW of the caldera. GRAVITY DATA Hengill-Grensdalur Þorbergsson et al. (1984) measured gravity at 315 stations with average spacings of ~1.5 km covering an area ~450 km2 in the Hengill-Grensdalur area. The measured values of gravity at most stations have estimated uncertainties of <0.5 mGal. The data have been discussed in detail by Hersir et al. (1990). Krafla A gravity survey of 393 stations in the Krafla caldera is described by Karlsdóttir et al. (1978). The estimated accuracy is 0.5 mGal for most of the stations. Many of the stations are clustered around geothermal featur- es rather than being uniformly distributed throughout the area and the survey design was thus not ideal for comparison with the LET results. METHOD The approach adopted here is to convert the LET velocity field to density and then to calculate a “simulated” Bouguer anomaly field. This is compared with the “observed” Bouguer anomaly field calculated from the gravity data. The velocity-density relations- hip derived from measurements in the Reyðarfjörð- ur drillhole was used here (Christensen and Wilkins, 1982), which is p = 1530 + 230Vp (1) where Vp is the P-wave velocity in km/s and p is the rock density in kg/m3. The LET models were divided into cubes 0.25 km on a side and velocities interpola- ted linearly to determine an average velocity for each 34 JÖKULL No. 48
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