Seismic monitoring and site-characterization with near-surface vertical arrays

The northeast of the Netherlands is covered with a dense network of seismic stations. The prime use is monitoring of induced seismicity from the Groningen field and a string of surrounding gas fields. The near-surface consists of unconsolidated sediments. After extensive testing, a standard station design was chosen with an accelerometer at the Earth's surface and geophones at 50, 100, 150 and 200 m depth. Clearly, these vertical arrays cost more than single sensors and do not add in the network coverage. However, they also bring multiple advantages. The combination of a strong motion sensor and at least one weak-motion sensor is needed to record a wide spectrum of earthquake magnitudes. The installation of weak-motion sensors at depth reduces the seismic noise up till about 30 dB. Sensors are placed at different depth levels to enable near-surface characterization and to facilitate phase identification. Furthermore, the installation of multiple buried sensors provides redundancy in case one of the geophones breaks down.