NiphNet: a self-governing environmental monitoring network

A high-precision and low-cost temperature and humidity logging device, called Niphargus and originally intended for environmental monitoring in caves, was developed at the Geological Survey of Belgium (Burlet et al., 2015). The Niphargus is designed as a standalone logger, with data to be retrieved manually whenever needed. This allows for a very small and simple electronic design, low power consumption and flexible placement. There are, however, a number of disadvantages for specific applications. For example, there is no feedback possible on malfunction or battery lifetime. To avoid loss of data during long-term measurement campaigns, regular inspection and data retrieval are necessary. Apart from the inconvenience, this manipulation also causes disturbance in the measurements. A new version of the Niphargus was therefore developed, including a wireless Digi XBee DigiMesh module. These modules communicate on a 868 MHz radio frequency, in a self-governing mesh network (Fig. 1). In such a network, every device is able to communicate to any other device within range. For data transmission, the most optimal pathway is chosen between transmitter and receiver. As such, in case of a single device malfunction, the connection between the other nodes can still be guaranteed. In case of the NiphNet, the receiving end includes a single-board computer with cellular network connectivity, from which data is uploaded to a cloud repository. From there, live monitoring data can be displayed online, downloaded and processed. A first successful test was conducted with a NiphNet of 5 devices in waterproof containers (Fig. 2) and online display at the GeoEnergy Test Bed in Nottingham, UK, in March 2018. Current and future efforts focus on the enclosure design and the automation of data readout over the network. There is a large array of possible applications. For environmental monitoring in caves, the individual nodes can ensure data transmission from a network of environmental sensors inside the cave to a station outside, allowing for continuous access to measurements and minimising the need for regular field inspection. This is currently being installed in the caves of Han. The geological storage of CO2 requires long-term monitoring to establish a baseline and detect leakage from the reservoir, both below and above ground. Such monitoring activities need to be maintained for several decades, and therefore need to be low effort and low cost. Near the surface, temperature is expected to be a good proxy for CO2 leakage when a network is set-up that can detect temperature anomalies in the range of 0.01°C. This is possible with a network of shallow buried Niphargus nodes. Then, wireless access to thesedevices is not only a matter of long-term and maintenance-free coverage of a large area. Detection of small temperature differences depends on not disturbing the shallow subsurface, and therefore on being able to download the data remotely.

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