Listening to the Earth Move / / by Jim Whiteley & Arnaud Watlet

Jim Whiteley and Arnaud Watlet are geophysicists working on the Multihazards and Resilience challenge area at Keyworth, focusing on using geophysical approaches to better understand landslide hazards. Along with other members of BGS, and partners at the Royal Observatory of Belgium, they were successful in winning a £55k grant from the Natural Environmental Research Council (NERC) in the form of a loan of seismometers, which will be used to monitor an unstable slope in the UK. Here, they first explain why geophysics can be useful in monitoring landslides, and secondly how they went about deploying their seismometers to a BGS site in North Yorkshire...

Painting of the Hollin site by Dave Huntley from the Geological Survey of Canada
In Part 2 of this Geoblogy post, we’ll take a look at what seismometer installations entail, having looked at why we use seismometers to monitor landslides in Part 1. Our seismological adventure began with a visit to SEIS-UK, who administer the seismological portion of the NERC GEF, and who are handily located in Leicester, just down the road from BGS headquarters in Keyworth. Over two days, Victoria Lane and Lucy Finch provided us with all the logistical, technological and practical know-how needed to install a seismometer. The training was comprehensive to say the least, covering everything from how to position the sensors, download the data and service the instruments, including what to do if a sheep eats through one of your data cables!

Providing training is just a small part of what SEIS-UK do; the rest of the time they maintain a pool of over 60 seismometers and accompanying solar panels, digitisers and laptops, provide field support for installations, ship equipment around the world, and assist in seismic data processing. All of this training and support comes as part of a successful application to the NERC GEF, and Victoria and the team at SEIS-UK do a fantastic job in supporting UK researchers using seismological equipment around the world.

Figure 3: before (left) and after (right). The map on the left shows monitoring equipment installed at the Hollin Hill Landslide Observatory up until 2019. The map on the right shows the planned expanded monitoring network, including the location of the seismometers installed in March 2020.

Dig for victory

Two weeks after our training at SEIS-UK (just enough time to almost forget some of it) we headed north to Hollin Hill to install the seismometers. Our plan to install a new seismic network is part of a much larger equipment installation at the landslide, including a brand new ERT monitoring system developed by the GTom team, a complete refresh of our geotechnical wireless sensor network, and a fibre optic distributed acoustic sensing (DAS) system (more on these in future blog posts!) (Figure 3).

The seven G├╝ralp 6TD seismometers that we have all require burying in the ground, both in order to isolate the sensor from the kinds of seismic noise we don’t want to record (e.g., from stampeding sheep), and also to protect the equipment from damage (e.g., from stampeding sheep). And so, our installation begins in a very similar manner to many geological investigations – by digging a hole!

Figure 4: a typical seismometer setup of Hollin Hill Landslide Observatory. The sensor (bottom left) is buried in the ground, and the cables fed through waterproof tubing to a metal box that houses the battery (right), which is in turn charged by a solar panel (top). A big thanks to BGS workshops for manufacturing all the equipment we need to make everything waterproof and safe from sheep! 

I’m picking up good vibrations…

Each of the holes needs a sand base before the seismometer can be installed and levelled, after which all of the power, GPS and data cables can be fed out from the seismometer and back to a box on the ground surface in which the battery that powers the sensor is kept. A solar panel charges the battery, and everything is carefully buried and secured to keep those sheep away. Easy! On our first day, three of us managed to install one senor, having been confidently told that we would expect to install three to four a day. We’re obviously not naturals at this, but our perseverance paid off, and by the final day of our fieldwork, we managed to get four seismometers installed in a single day – not bad!

Fig5: a completed installation, with the sensor safely buried and battery ox sealed, keeping the equipment safe from marauding sheep

A waiting game

We’re looking forward to getting back up the Hollin Hill when we can, and getting some data to process to see what continuously monitoring seismic signals can tell us about the landslide’s movements, but this is a bit of a waiting game as we gather data over different seasons. It is also difficult to retrieve data in the current lockdown conditions (at the time of writing) that we are all sticking to help deal with the Covid-19 crisis. The lockdown, however, will have an interesting and unknown effect on our science at the Hollin Hill Landslide Observatory. One area of our research relies on using the seismic noise generated by people and vehicles to see how seismic waves travel through the ground, and scientists, including Brian Baptie (from the BGS) and Thomas Lecocq (one of our Hollin Hill project partners from the Royal Observatory of Belgium), have seen that this noise is now significantly quieter than normal. On the other hand, we have a better chance than ever of recording the noises that the landslide itself makes as it moves, which usually we might struggle to observe because of all the other noise. It is yet another example amongst many of how much of an impact the Covid-19 pandemic is having on society, and shows how challenges and opportunities in science rarely arrive individually.

By installing seismometers in different zones of the landslide (including in zones that are moving or stable, underlain by mudstone or sandstone, and in areas that are wet or dry) we will be able to see what the individual seismic signals on each seismometer tell us about the landslide. In addition, looking at all the data together, gathered across a relatively dense, closely spaced network, should tell us something about how seismic waves propagate between different sensors. For example, if seismic waves change in velocity between two sensors at particular times, this may point to changes in the water content in the subsurface. Continued innovations such as installing seismometers, and combining their observations to our high-resolution images from ERT monitoring, alongside weather and geotechnical data are what make Hollin Hill a world-class landslide monitoring observatory, and a fine example of the multi-disciplinary and practical science that we love doing here at BGS!

Uhlemann, S., Chambers, J., Wilkinson, P., Maurer, H., Merritt, A., Meldrum, P., Kuras, O., Gunn, D., Smith, A. & Dijkstra, T. 2017. Four-dimensional imaging of moisture dynamics during landslide reactivation. Journal of Geophysical Research: Earth Surface, 122, 398-418.

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