46 Water / Wastewater MONITORING THE SEABED – THE SUBTLE DISTINCTION
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Seabed monitoring plays a key role in improving our understanding of geological, oceanographic and climatological changes. A new approach employed by the University of Bremen enables the precise long-term measurement of pressure on the seafl oor. A central element here is a pressure transmitter developed especially for this purpose by KELLER Pressure.
T
here are violent forces at work inside our planet that shape mountains and entire continents. For the most part, we remain oblivious to them. But now and then, pent up tensions are suddenly released, resulting in earthquakes and volcanic eruptions. Monitoring magmatic and hydrothermal activity in the earth’s crust helps scientists better understand these subterranean processes. The knowledge gained here can clarify a broad range of questions. Understanding the movements of tectonic plates helps scientists estimate the risk of earthquakes and tsunamis and analyse the life cycles of deep sea ecosystems in the vicinity of mid-oceanic ridges and island volcanoes. The
data is also used to evaluate the effects of climate change, such as changes to sea levels and ocean currents, and to monitor undersea raw material extraction operations as well.
Comprehensive monitoring requires an enormous amount of data collected by countless measuring stations – and even data collected from space. There are well tested processes and a dense network of sensors in place on land. However, the majority of the earth’s surface is covered by oceans, which makes setting up and operating measuring equipment considerably more diffi cult. For this reason, data from the deep sea is more scarce and often less accurate. Still, there are other methods that can be used in oceans. For example, measuring the water pressure on the seabed makes it possible to precisely calculate the distance to the ocean surface. This in turn makes it possible to determine independently of reference positions whether the seafl oor has risen or sunk at that particular point.
Dr. Hans-Hermann Gennerich is well acquainted with pressure changes on the seafl oor. He works in the Faculty of Geosciences at the University of Bremen, where he is responsible for marine technology and sensor systems. He has already developed and tested prototypes for two devices for measuring pressure changes on the seabed. These devices are known as OBPs (ocean bottom pressure meters).
Valuable data was collected in the course of these projects. However, there were also many sources of interference such as
Figure 2: Structure of the OBP measuring device with reference tank, differential pressure transmitter and data logger © University Bremen
tides and general swell. Nevertheless, the majority of these can be cleared up by carefully aligning the associated data with other measurement data from buoys and satellites.
The observable pressure differences due to the movement of the seabed are millions of times smaller than the prevailing ambient pressure at several kilometres underwater. This means that a measuring device for absolute pressure would require an unrealistically perfect degree of long-term stability in order to be able to distinguish between the long-term measurement signal being sought and the zero drift. For the next generation of OBP measuring devices, Dr. Gennerich has therefore taken a different approach by only recording the pressure change over time instead of measuring the entire water column above the seafl oor. As the zero drift is proportional to the sensor’s total measuring range, interference caused by this factor can be reduced more than a thousand-fold by using this method with a low measuring range. This makes the long-term signal unambiguously detectable.
Figure 1: An earthquake with a subsequent tsunami brings death and devastation (Palu, Indonesia, September 2018) © aditya bintang pradana/
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The new prototype for the measuring instrument is constructed as follows (see fi gure 2): One of the differential sensor’s pressure connections is connected directly with the sea around it, while the
Dr. Hans-Hermann Gennerich Faculty of Geosciences University of Bremen
“With KELLER, I have found a partner with the technology and expertise needed to implement my specifi c requirements for a pressure sensor that can be used in my project.”
IET MARCH / APRIL 2023
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