Passive seismic monitoring waveform data collected at the Groningen gas field contain many interesting events besides direct P- and S-arrivals. We examine some of these arrivals in order to understand their nature. A combination of move-out analysis, raytracing and finite-difference simulations has revealed that the data contain converted phases from two shallow interfaces. By using a kinematic version of the Source Independent Converted Phase Imaging Condition, we can map those interfaces.