Graduate Thesis Or Dissertation
 

Tidal tilt observations in the Krafla geothermal area in North Iceland

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  • A brief tilt and strain survey was conducted in the Krafla-Namafjall area in the North of Iceland during August of 1979 in order to study the feasibility of applying solid earth tidal observations in the exploration of volcanic geothermal systems. The rationale of the survey was based on the expectation that various types of geological structures such as rift zones and magma chambers can lead to observable distortions of the local solid earth tidal amplitude field. The field procedure consisted in measuring the local tidal amplitudes and comparing them with normal theoretical amplitudes at the same location. The Krafla volcanic complex is a central volcano traversed by a N-S trending fissure swarm, that has been tectonically and volcanically active since December 1975. Accompanying this activity have been periodic inflations of the Krafla caldera, presumably caused by a flow of magma into a local magma chamber and resulting in long term tilts of the order 0.5 μrad/day that have been observed at two sites south and southeast of the caldera. In computing theoretical amplitudes the effects of the ocean tides need to be estimated. In the case of north Iceland they are found to be of the same magnitude as the solid earth tides. The amplitudes of the M₂ ocean loading tilt at Krafla are estimated to be 0.066 μrad and 0.032 μrad for the NS and EW components respectively. The most noteworthy result was obtained at a site in the Namafjall geothermal area inside the active Krafla fissure swarm. The ratios of the observed to the theoretical M₂ tidal tilt amplitudes at this site as estimated by a least squares spectral analysis method are found to be 0.9 ± 0.3 and 3.2 ± 1.5 for the NS and EW components respectively. On the basis of some simple order-of-magnitude estimates we can exclude one of the numerous nearby fractures as a possible cause for the EW tilt anomaly and conclude that it is most likely to be generated by a large body of magma below the Krafla fissure swarm. Due to thermoelastic noise other tilt data obtained during this survey turned out to be less reliable. However, our work at a site east of the fissure swarm and southeast of the caldera indicated a possible anomaly. The strain data are highly contaminated by thermal noise and could not be successfully analyzed. These results tend to confirm that tidal tilt observations can be of use in explorations of volcanic and geothermal systems. Our work indicates that a few improvements of the simple field techniques adapted may enhance the quality of data. These include (1) increasing the instrument resolution, (2) selecting sites with surface layers that are incapable of transmitting thermal stresses and (3) obtaining more extensive higher quality temperature recordings, that should enable the thermal noise to be largely removed.
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