Seabed Monitoring

Seafloor monitoring plays an important role in improving our understanding of geological, oceanographic, and climatological changes. A new approach used by the University of Bremen conducts precise long-term pressure measurements on the seafloor. The central element is a pressure transducer specially developed for this purpose by KELLER.

Forces act inside our planet that shape mountains and continents. From time to time, accumulated natural forces suddenly release, causing earthquakes and volcanic eruptions. Monitoring magmatic and hydrothermal activity in the Earth's crust helps scientists understand these underground processes. The knowledge gained clarifies a number of questions. Understanding tectonic plate movements helps scientists assess the risk of earthquakes and tsunamis and analyze the life cycles of deep-sea ecosystems near mid-ocean ridges and island volcanoes. The data is used to assess the consequences of climate change, such as sea level changes and ocean currents, as well as to monitor underwater resource extraction operations.

An earthquake followed by a tsunami brings death and destruction (Palu, Indonesia, September 2018)

Comprehensive monitoring requires a huge amount of data collected by measuring stations—and data collected from space. On land, there are established processes and a network of monitoring stations, including those with pressure sensors. However, most of the Earth's surface is covered by oceans, which complicates the installation and operation of measuring equipment. Data from the ocean depths is scarce and less accurate. There are other methods that can be used in the oceans. For example, measuring the water pressure on the seafloor allows calculating the distance to the ocean surface. This, in turn, makes it possible to independently determine whether the seafloor at a specific point has risen or fallen regardless of initial positions.

Dr. Hans-German Gennerich is well acquainted with pressure changes on the seafloor. He works at the Faculty of Geosciences at the University of Bremen, where he is responsible for marine technologies and sensor systems. He has already developed and tested prototypes of two devices for measuring pressure changes on the seafloor. These devices are known as OBPs (Ocean Bottom Pressure sensors).

DR. HANS-GERMAN GENNERICH

Faculty of Geosciences

University of Bremen

 «Thanks to KELLER, I found a partner with the technology and experience necessary to meet my specific requirements for the pressure transducer used in the project.»

Valuable data was collected during the projects. However, there were many sources of interference, such as tides and general wave motion. Nevertheless, most of these could be eliminated by correlating the relevant data with other measurements from buoys and satellites.

The observed pressure difference caused by seafloor movement is millions of times smaller than the prevailing atmospheric pressure at depths of several kilometers underwater. This means that a measuring device for absolute pressure requires a perfect degree of long-term stability to distinguish the desired long-term measurement signal from zero drift. Therefore, for the next generation of OBP measuring devices, Dr. Gennerich applied a different approach, recording only the pressure change over time instead of measuring the water column above the seafloor. Zero drift is proportional to the sensor's total measurement range; interference caused by this factor can be reduced by a thousand times using this low-range measurement method. This makes the long-term signal unambiguously detectable.

Structure of the OBP measuring device with reference tank, differential pressure transducer, and data logger.

The new prototype of the measuring instrument is constructed as follows (see Figure 2): One of the ports of the differential pressure transducer is connected directly to the surrounding sea, while the other leads to a reference reservoir. The reservoir can also be opened to the external environment through a valve. When submerging or extracting the OBP, the valve is kept open so that the pressure on both sides is always equal and the sensitive sensor is not damaged. Once the measuring equipment reaches the seabed, the valve is closed. Now the contents of the reservoir are under exactly the same pressure as at the start of the measurement (P1). If the ambient pressure (P2) changes, the sensor records the difference. Pressure deviations in the reference reservoir caused by thermal expansion of the material are compensated by the precisely calculated volume of quartz glass. Additionally, temperature is measured for subsequent mathematical calculation of residual deviation. Measurement data is recorded using a data logger. During measurements, the system can be recalibrated at any time by opening the valve and re-measuring the zero point. Thus, any signal drift can be detected and compensated later during data evaluation. This design allows measuring changes in water pressure at the seabed level with maximum accuracy and almost no measurement errors over long periods of time.

For this purpose, piezoresistive pressure cells are suitable, to which pressure is applied on both sides. When pressure is applied to both sides of the same membrane, part of the pressure effect is leveled out, leaving only the difference. This applies both to the measured value and to the (one-sided) deformation of the silicon membrane. The crystal lattice structure of silicon is extremely resistant to pressure applied evenly on both sides, even within thin walls, as required for highly sensitive measuring cells.

Internal structure of the differential pressure transducer

The complex measuring system of the OBP prototype requires reliable and accurate transducers. The PD-23 differential pressure transducers from KELLER meet all the requirements for the instrument and are therefore suitable for this application. They measure the pressure difference on a single silicon membrane, which is separated on each side from the measured medium by a metal membrane, and transmit the result in a temperature-compensated and standardized form to the data logger. A design was developed for the project that meets the customer's requirements: The transducer is rated for line pressure up to 600 bar, corresponding to a sea depth of 6000 m. At the same time, the pressure difference can be measured with an accuracy of less than one thousandth of a bar. Specially designed parts for use on the seabed that contact the measured medium are made of Hastelloy C-276 alloy to prevent corrosion damage from salty seawater. Additionally, at the customer's request, the process connections were extended by two pipes so that the transducer fits into the design.

3D CAD illustration of the custom-made differential pressure transducer

New instruments for marine geology will be successful, largely thanks to close cooperation between transducer developers and Dr. Gennerich’s team at the University of Bremen. At KELLER, we are proud to have contributed to science. We wish Dr. Gennerich and other researchers great success in their future work. Thanks to hard work, we will soon better understand the fundamental forces of nature and be able to predict their behavior more accurately.

PD-23 Series

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