On the NAM Seismic Monitoring Network in Groningen

Seismic monitoring is a vital element in case of induced seismicity because it provides information that can relate the causes to the effects. It is useful for licensee companies, governments, and especially for the people who are concerned about the safety of their houses.

There are two main seismic monitoring networks on the Groningen gas field: KNMI network and the NAM network that was operated by TNO until recently. KNMI network is a strong ground motion (SGM) network, meaning that it reports how much the earth moved during an earthquake. In the simplest way of explaining it, the KNMI does not measure the structural response, that is the impact of earthquakes on houses. The NAM network was meant to look into the structural response in fact, but the sensors are installed in an uncommon and a rather strange way, they were placed on the walls at arbitrary positions. A detailed discussion can be found in a booklet I had written more than a year ago (see Chapter 4)

There are doubts on the installation of the network by other parties as well. A recent study by Ntinalexis et al. (2019) presents the efforts made by NAM itself to explore the possibility of using the data of this network for further developing the Groningen GMM (ground motion model). This recent article can be found here.

NAM has decided to unplug the network and eventually dismantle it. I do not know the motivations behind it. Despite the fact that I am critical to the installation of the sensors, I still believe that the network in general is a useful tool for the people in the region as long as the earthquakes continue. I was asked in the last couple of months multiple times what my views are on the issue. I also saw that these discussions were reflected by the media.

Below is my position in brief

1. The network is of little use as it is. If we can make certain changes and adjustments to make it more in line with the international state-of-the-art, then it would be possible to make more sense out of the data produced. I am not talking about revolutionary changes but some limited and smart modifications. One way of doing that could be to divide the monitoring locations to certain clusters and ask building owners to volunteer to place the sensors in their houses down on the ground (i.e. just change the location from wall to the floor). This can give us the opportunity to correlate the data from this network with the KNMI network, and also to correlate the on-ground measurements with the rest of the measurements within the same network. This is still not the most ideal case, but it is a technical solution that can render the entire dataset much more meaningful and eventually useful.

2. Furthermore, if there are extra sensors or some of the building owners want to leave the monitoring program, we could use these sensors to monitor some of the structures fully, as we do at Fraeylemaborg. We could select structures that represent the typologies in the region and this would be tremendously useful information to everyone.

3. The network should be run by a board that is a combination of technical people (as Hanze, with the help and networking capacity of BuildinG for example, we can fill that gap in), local government and NGOs in the region that represent people's interest. The idea is to continue monitoring and provide transparent information to the people as long as the earthquakes continue. The outcomes should also be anonymized for protecting the privacy but made public (i.e. publicly available heathmaps) after every earthquake of certain magnitude and above.

4. There are technical challenges in operating such as system. It is hard to stream, collect and process such a big amount of data. Setting this up needs time and money investment. It is not easy.

5. The network is getting old and there may be some more need for maintenance by the time. This also needs money investment. I am not sure how much, but NAM may be willing to share some figures with us to give an idea.

6. The critical point here is a sponsor. Due to the high relevance and impact of the topic, at Hanze as a regional role player in the topic we can do our best, but still we need a sponsor for maintenance and for setting up the data collection system explained in Item #3 above. Province or NCG are the main candidates that come to my mind. We know that there are resources for that. Really, If NPG fund is not used for this purpose, for what actually it can be used better?

Despite the issues in the installation methods, the sensor network itself is an important asset for the region and it should stay. Hanze University of Applied Sciences Groningen has vast international experience in running such sensor networks.

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