Adapting Controlled-Source Seismic Techniques for Earthquake Reflection Imaging in Complex Environments: Insights from Krafla Volcano, NE Iceland

Volcanoes generate complex seismic wavefields due to their heterogeneous geological structure, making it challenging to obtain accurate reflection images of their interiors. However, understanding the internal structure and dynamics of volcanoes is essential for enhancing monitoring capabilities and improving eruption forecasting. In this study, we apply controlled-source seismic techniques to passive reflection imaging at Krafla volcano, NE Iceland. Krafla is globally recognised as one of the few sites where magma was directly encountered during drilling at the IDDP-1 borehole at a depth of 2.1 km. Using the known magma location as a reference, we employ common-depth-point binning and stacking for preconditioned seismic data from over 300 earthquakes recorded by more than 100 short-period (5 Hz) seismic nodes. At first, we computed theoretical arrival times for various P- and S-wave reflections, including both primary and multiple reflections, using a ray-tracing algorithm and local 1D velocity models. The computational domain was discretized between the surface and a depth of 6 km, using a grid of 80x80x20 m cells. For each grid cell, common-depth-point gathers were generated, and amplitudes were stacked along the reflection trajectories. We further use synthetic waveforms to systematically evaluate the challenges and limitations of this approach in our study area. We observe that the specific distribution of earthquakes and stations causes direct waves (e.g., P- and S-waves) to constructively interfere, creating  spurious reflectors in the imaging results. To mitigate this effect, we exclude time windows corresponding to the arrival of direct waves. We also investigate the effect of various waveform attributes—such as absolute amplitudes, envelopes, and their derivatives—on the stacking process and the final imaging results. Preliminary results indicate that the second derivative of seismic trace envelopes might be particularly useful in complex environments characterized by a high degree of incoherent scattering and diverse earthquake source mechanisms. This approach has successfully revealed several discontinuities at shallow depths, offering new insights into the local structure of the Krafla geothermal system.