Contribution of the Seismic Reflection Method to the Location of Deep Fractured Levels in the Geothermal Fields of Southern Tuscany (Central Italy)

Abstract

For more than twenty years ENEL has been employing the seismic reflection method for geothermal exploration in Central Italy. Initially, this methodology was used for geological-structural goals, but subsequently it has been more and more adopted to locate productive zones in the deep reservoirs. The large amount of data (about 600 km of 2D seismic traverses, about 20 km2 of 3D seismics and more than 30 VSPs), unique in geothermal research, has enabled us to increase our understanding of seismic signals in the particular geologic environment of the deep Tuscan geothermal reservoirs mostly made up of metamorphic and intrusive rocks. The hypothesis that some seismic reflections could correspond to fractured rocks was investigated with two different methodological approaches: one strictly theoretical, the other empirical-statistical. The former tries to reconstruct the synthetic seismograms of fractured rocks in a steam reservoir and compares them to the experimental signals. The latter investigates the statistical relationships between reservoir data (injectivity, productivity) and reflection signals. This statistic study was carried out on 83 deep geothermal wells. Both these approaches suggest that some velocity anomalies (i.e. seismic reflections) be linked to fractured and thus potentially productive layers.