Improving passive reflection seismic imaging in complex geological settings through site effect reduction: application to Krafla volcano, Iceland

SUMMARY Reflection imaging at volcanoes presents significant challenges due to the highly heterogeneous subsurface, which generates complex wavefields characterized by substantial wave scattering. These scattered waves obscure coherent energy, such as reflections from geological structures in the subsurface. In this study, we develop processing strategies to address the limitations of high-frequency (5–20 Hz) passive reflection imaging at Krafla, a volcanic caldera in NE Iceland. Krafla is among the few locations worldwide where magma has been encountered at 2.1 km depth when drilling the IDDP1 borehole. We analyse over 300 local microearthquakes and industrial noise recorded during five weeks in the summer of 2022. We show that wavefields lack coherency even between stations spaced at 30-m intervals due to the dominance of site effects beneath the stations. However, data coherency improves in the common-station domain, where different earthquakes recorded by a fixed station are analysed, thereby stabilizing the site effect. Spectral analyses in this domain reveal that site effects are partly due to resonances at the stations, likely caused by lava flows and cavities in the heterogeneous near-surface. By constructing a resonance removal filter, we successfully deconvolve resonance effects from the data, revealing previously masked coherent energy. We further reduce site effects by applying linear stacking of clustered earthquake traces and nonlinear amplitude weighting. Our approach significantly enhances coherency between stations and enables the identification of reflections in microearthquakes likely originating from the known magma–rock interface beneath the IDDP1 borehole.