Here we use data from the Life of Field Seismic (LoFS) surface array in conjunction with a migration-style approach to locate microseismic events in Valhall Field during 6.5 hours of hydraulic fracturing. We identify an event distribution that extends roughly 300 m vertically and 200 m horizontally with sources both within the chalk layer, containing the reservoir, and the overburden. However, the temporal distribution of the events is not consistent with the induced fracturing extending into the overburden. Instead, the synchronous increase in event activity with downhole pressure could be interpreted as evidence that elastic wavefronts originating from well activity trigger movement on faults where the in-situ stress is very close to that required for failure.

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