In the Netherlands a seismic network is in place to monitor both induced and natural seismicity. Most natural seismicity occurs in the south, over an extensional tectonic regime that can be seen as an extension of the Rhine Graben. Most induced seismic activity occurs in the north of the country and is primarily related to gas extraction and reactivation of existing faults at reservoir level (Spetzler and Dost, 2017; Willacy et al., 2019). In Groningen, in the north east of the Netherlands, an especially dense network is in place. The network is operated by the Royal Netherlands Meteorological Institute (KNMI). Both event data and continuous recordings are publicly available (KNMI, 1993). In the Nineties, a seismic network has been installed to monitor seismicity from the Groningen field and a string of surrounding gas fields (Dost et al., 2017). Since 2014 this network has been expanded with a dedicated network to monitor seismicity from the Groningen field (the G-network, Figure 1), and two gas storage plants (the N- and GK- networks, Figure 1). The area has soft soil and high seismic noise conditions. As a remedy, most of the seismic sensors have been installed in boreholes - in a set-up shown on Figure 1(c). This set-up yields a seismic power reduction up till about 30 dB in the relevant bandwidth (Ruigrok and Dost, 2019). Besides induced seismicity and natural seismicity, these networks pick up arrivals from all kinds of other seismic sources: sonic booms, explosions, piling works, etc. Events that are detect-ed at multiple stations are analysed. All non-earthquake events end up at a separate list. A subset of the non-earthquake sources are the controlled explosions. These events are compiled into the Groningen explosion database. In Groningen and surroundings, KNMI has detected three types of explosions (Figure 2): 1. Most of the onshore explosions are part of seismic surveys. Buried dynamite charges are used to illuminate subsurface targets as part of seismic acquisition. In recent years, a sur-vey was done to improve the model for the unconsolidated sediments, which make up about the first 800 metres of the subsurface below Groningen. Seismic characteristics of these sediments are relevant for assessing the seismic wave amplification in the near surface, which is one ingredient of the seismic hazard model for the region (Rodriguez Marek et al., 2017; Bommer et al., 2017). 2. In the Dutch subsurface, remnants from the Second World War ordnance are still present. Some of the explosives that were released from bombers did not detonate when hitting SPECIAL TOPIC: NEAR SURFACE GEOSCIENCE the Dutch soft soil. These unexploded ordnance (UXO) are actively sought prior to construction works, if there are indications that there have been bombings in the area. Also they are found by chance, e.g., by farmers ploughing their fields. A division of the Dutch army (EOD) is mobilized whenever an UXO is found. What follows is a controlled explosion by adding an additional explosive charge. The controlled explosion is typically done at the spot where the UXO is found. When this yields potential damage, the UXO is first moved to a place with more favourable near-surface and infrastructure conditions. 3. Also on the sea bottom, a large amount of UXOs exist, in both Dutch and German territorial waters close to Groningen. For example, sea mines that were not cleaned up, torpedoes and aircraft bombs that missed their target and lodged in the sea bed and ammunition that was dumped at sea. In recent years, many offshore construction works have taken place. Electricity cables have been placed to connect the Dutch grid with the Norwegian grid (NorNed) and with the Danish grid (COBRA). Wind turbine parks have been constructed north of Groningen (e.g., Riffgat, Riffgrund and Gemini) and many new offshore wind farms are under construction or on the drawing board. Prior to all this activity on the sea bed, geophysical surveys are carried out to find UXOs (e.g., van der Baan, 2019). When found, also these UXOs are typically detonated at, or close to, the place where they are found. Figure 2 shows locations of controlled detonations. For the KNMI, these explosions are part of the ambient field. Nevertheless, they have found their way in various work flows. Their accurate location makes them suitable for different kinds of studies. We will show how the explosions are distinguished from local earthquakes. Moreover, we will exemplify the use of ‘ambient’ explosions for sensor orientation, deep crustal imaging and near-surface tomography.@en

Induced earthquakes due to gas production have taken place in the province of Groningen in the northeast of The Netherlands since 1986. In the first years of seismicity, a sparse seismological network with large station distances from the seismogenic area in Groningen was used. The location of induced earthquakes was limited by the few and wide spread stations. Recently, the station network has been extended significantly and the location of induced earthquakes in Groningen has become routine work. Except for the depth estimation of the events. In the hypocentre method used for source location by the Royal Netherlands Meteorological Institute (KNMI), the depth of the induced earthquakes is by default set to 3 km which is the average depth of the gas-reservoir. Alternatively, a differential traveltime for P-waves approach for source location is applied on recorded data from the extended network. The epicentre and depth of 87 induced earthquakes from 2014 to July 2016 have been estimated. The newly estimated epicentres are close to the induced earthquake locations from the current method applied by the KNMI. It is observed that most induced earthquakes take place at reservoir level. Several events in the same magnitude order are found near a brittle anhydrite layer in the overburden of mainly rock salt evaporites.

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