Testing vertical seismic profiling (VSP) as a subsurface mapping method at the Krafla volcanic geothermal field in Iceland

Vertical seismic profiling (VSP) was tested for mapping volcanic stratigraphy, fractures, dykes, fluid and steam in the geothermal area of Krafla in Iceland. Seismic imaging in magmatic environments is very challenging, largely due to the intense scattering of seismic waves traveling through the highly heterogeneous volcanic rocks. VSP offers means to image structures beneath and away from the well in complex volcanic environments. The VSP survey at Krafla was carried out in two wells, for each of which a zero offset, a far offset and a walk-away experiment were recorded. The zero offset data is of good quality, with the observed reflections corresponding to stratigraphic boundaries that can be explained by a simple 1D velocity model. The corridor stacks of the synthetic and field data look similar to each other, apart from a constant time shift and amplitude differences. High scattering in the subsurface leads to low amplitude reflections from deeper horizons. The walk-away data shows little coherent reflectivity. Furthermore, a complex 2D velocity model involving heterogeneities in the horizontal as well as vertical directions will be required to explain the observed seismograms.