Combined Reflection and Refraction Measurements for Investigating the Geothermal Anomaly of Urach
After field work and first results from the combined seismic experiments in the Urach area have been described at the previous meeting (Meissner, Bartelsen, 1979), many new information velocities and structure in the Urach area are now available. Velocity data consist of (1) horizontal velocities from the first (refracted) arrivals from the reflection units and MARS stations, reaching depths down to 5 Km (2) velocities from the second and third arrivals from the MARS stations in the wide angle range, down to depths of 29 Km, and (3) near vertical stacking and intervals from the MARS stations in the wide angle range, down to depths of 29 Km, and (3) near vertical stacking and interval velocities down to 29 Km. The structural information is mainly based on the near vertical reflection work, but important additional data have been collected from the stationary refraction (=MARS) stations along the two lines.
Parallel to the interpretation of field data theoretical calculations based on experiments with gneiss and granitic samples in a high pressure — high temperature apparatus (Kern, 1978), (Meissner, Fakhimi, 1977), have been performed in order to distinguish the effect of temperature on the velocity from that of anisotropy and inhomogeneity of material.
Experiment with (1) constant pressure p and variable temperature T, (2) constant T and variable p, and (3) variable p and T for certain p-T function similar to that in the Urach area have been selected.
The experiments based on 3 component velocity measurements of four gneiss samples show the large influence of a possibility inhomogeneity which may reach up to 20% of the mean p-velocity of 14 gneiss samples investigated. The effect of anisotropy may reach vp values 13% different from the mean. Compared to these figures the pure T- effect is about 50 m/sec for a 50°C difference in T at constant p which is about 1% change in vp. It might be enhanced by certain hydrothermal alterations.
The horizontal velocity of the refracted arrivals from the reflection units can be compared with the velocity of the sonic log of URACH III showing the different degree of resolution and indications of the suspected anisotropy. Velocities from the evaluation MARS stations in the wide angle range give velocity depth functions which have to be compared with the interval velocities of the near angle-reflection work. In general velocities are lower than those in the vicinity of the Urach area. Velocities obtained from the slant ray paths of the wide angle survey tend to be larger than the near-vertical velocities, but a final correlation has not get been established. The first structural interpretation from the stacked records of the near vertical reflection survey of the long profile shown two unusual reflectors at the crust-mantle boundary, dipping slightly to the ENE with an indication of a disturbed region below the center of the heat flow anomaly. The crust-mantle boundary at 28–29 Km depth is slightly shallower than in the vicinity of the Urach area.
In general, the whole Urach area is marked by a shallow crust, by lower crustal velocities and by two strong reflectors in the lower crust, where velocities increase with depth and additional near-vertical reflections indicate a lammellar structure. Locally, near Urach, velocities show a minimum in the upper layers and a disturbed zone at greater depth.
KeywordsSeismic Anisotropy Stationary Refraction Interval Velocity Disturbed Region Refraction Measurement
- Meissner R., Bartelsen H., Field work and first results of seismic reflection and refraction measurements in the area of the Urach geothermal anomaly; summary and half yearly report, project No G/B 60 (1979)Google Scholar