Jökull


Jökull - 01.06.2000, Blaðsíða 47

Jökull - 01.06.2000, Blaðsíða 47
Comparison of tomographic crustal models with gravity data or in error sources 4-6 listed above, which were not quantified for inclusion in the error budget, but which could account for several tenths of a mGal locally. An error in detrending the observed gravity field would produce a systematic effect and cannot explain the isolated misfits. The two areas of highest residual cor- respond to the topographic highs of Skarðsmýrarfjall and Hrómundartindur which rise to 600 m and 520 m respectively. An error in the Bouguer density of about 900 kg/m3 would be required to explain all of the residuals - an unrealistically high value. The most likely explanation for the misfits are variations in the velocity : density relationship. Scatter in the data used to derive Equation (1) shows that for a given seismic velocity, the density may vary by up to ±100 kg/m3 from that predicted. The Reyðar- fjörður drillhole is sited in Tertiary rocks, whereas the Hengill-Grensdalur and Krafla areas are in the neovolcanic zone, and the rock types are thus rather different. In addition, the velocity : density relations- hip is constrained by data only down to velocities of 3.8 km/s. Densities corresponding to velocities lower than this were determined by downward extrapolati- on and are probably associated with larger errors. Densities of 100 kg/m3 lower than those predicted in the material between sea level and 1 km depth could explain the zone of residual low gravity, as could densities 50 kg/m3 lower, extending down to 3 km depth. The zone of low gravity residual cor- responds to the zone of greatest recent volcanism and much of the present geothermal area, and relatively low densities in this zone are thus to be expected from high- porosity volcanic rocks and geothermally- altered material. Poor resolution in the LET model in the form of a systematic underestimate of the amplitude and extent of low-velocity volumes is also a possible explanation. LET may be poorer at imaging low- velocity volumes than high-velocity volumes because rays follow minimum time-paths which by-pass low- velocity material. Krafla Comparison of the LET model with the gravity field was poorer for the Krafla area, probably because the observed gravity field is dominated by anomalies that originate from density variations at very shallow depth where LET is insensitive. Small, shallow anom- alies such as these will not be detected by the LET, which will return a smoothed, average velocity field for volumes of low resolution. Other methods, e.g., shallow refraction profiling with closely-spac- ed seismic recorders, are more suited than LET to detecting small-scale, shallow anomalies. Furt- hermore, only the seismic structure inside the well- resolved volume was modelled, and thus unresolved, neighbouring bodies might contribute to the gravity field. It is interesting to note that the primary feature of both the gravity field and the LET model are high density/velocity anomalies beneath the caldera ring fault. The average elevation of the Krafla area is ~50() m above sea level, and thus the Bouguer slab is about 500 m thick. The Bouguer correction assumes this slab to have a uniform density. However, rock types are in reality highly diverse as a result of intensive volcanic activity and this may result in errors in the Bouguer anomaly of up to ~ 1 mGal. These errors will result in errors in the calculated density variations for the material below sea level. In circumstances such as these, gravity data are clearly a feeble test for LET models, but they can contribute to revealing structure where LET is insensitive, i.e. at shallow depth. Nevertheless, it is clear that there are substantial lateral variations in density in the Krafla area. The exact shapes and depth extents of the bodies ima- ged are not well resolved because localized averag- ing of velocity and smearing of spatial characteristics is a feature of the inversion process. Some compari- sons with the observed geology may be mentioned, however. The volume beneath the Leirhnjúkur area in the centre of the caldera contrasts with the caldera fill material that surrounds it in being of slightly higher seismic velocity and density. There is lso evidence from boreholes, located 2.5-3 km southeast of Leir- hnjúkur, that the basaltic intrusives that underlie the caldera fill may be elevated to a depth of 400-500 m b.s.l. within the caldera, compared with ~1000 m b.s.l. just south of the caldera (Ármannsson et al. 1987). This could explain the velocity/density high observed in the centre of the caldera. JÖKULL No. 48 45
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