Seismic imaging of shallow magma bodies at Krafla, Iceland.

Abstract

Imaging the subsurface geology at volcanoes is crucial for understanding their structure and dynamics. Knowledge about the existence of magma chambers, fault and fluid systems improves natural hazard assessment and geothermal energy exploitation. However, the highly heterogeneous subsurface at volcanoes complicates the identification of geologic layers and objects. A strongly scattered seismic wavefield is typically recorded that masks coherent energy reflected at interfaces of interest. In addition, small geologic features such as magma bodies are often smeared out by tomographic techniques. A well-known example that highlights this problem is Krafla, a volcano caldera in the north-east of Iceland. In 2009, a magma body was unexpectedly found at a depth of 2.1km during drilling for geothermal purposes. Even though Krafla is one of the best-studied volcanoes worldwide, the shallow magma body remained undetected. In the summer of 2022, we conducted a six weeks long field experiment at Krafla as part of the IMPROVE project. We deployed densely spaced short seismic profiles comprising 114 seismometers in a passive experiment. Our goal is to image the magma body at 2.1 km depth. At first, we carry out a comprehensive data characterization in order to better understand the seismic wavefield and the influence of source, propagation, and near-station effects. In a subsequent step we apply targeted imaging methods including local earthquakes and high-frequency industrial noise. Krafla provides an optimal setting to test and calibrate seismic imaging, because the location of the magma body is known (through drilling). By combining different methods, we seek to improve seismic imaging techniques in order to obtain a high-resolution image of the subsurface in complex media.