Fibre-optic technology maps the subsurface
What happens beneath our feet? Using innovative monitoring techniques, we're mapping the groundwater-bearing layer beneath the Deltares campus in Delft. This aquifer forms the basis of our Aquifer Thermal Energy Storage (ATES) system, which provides heating and cooling for several Deltares buildings. Our experts in applied geology and applied geophysics collect information about subsurface behaviour, enabling us to maintain and optimise the ATES system more effectively. The knowledge gained also supports the wider use of the subsurface as a foundation for a sustainable energy supply.
As part of Deltares’ ambition to make the campus Paris Proof by 2040, several buildings have been heated and cooled since 2022 using an Aquifer Thermal Energy Storage (ATES) system. This sustainable energy solution is a highly efficient way of heating and cooling buildings by using groundwater-bearing layers in the subsurface as a large thermal reservoir.
During summer, a cold well supplies cool groundwater that is used to cool buildings through a heat exchanger. The water, warmed in the process, is then injected into another part of the aquifer through a warm well. In winter, the process is reversed to provide heating. Together, the warm and cold wells form what is known as a doublet.
Fibre-optic cables
The subsurface layer used for an ATES system is known as an aquifer: a permeable layer of rock, sand or gravel saturated with groundwater. When the first two ATES wells were drilled on the Deltares campus in 2022, suitable aquifers were encountered at depths between 120 and 180 metres. Using geophysical borehole logging, our experts in applied geology and applied geophysics identified the sediment types present at different depths. This information was essential for installing the well screens at the correct levels.
The ATES well was subsequently installed in the open borehole. At the same time, fibre-optic cables were attached alongside the well screens that transport warm and cold water. These cables enable us to monitor temperature, groundwater flow and acoustic signals while the system is operating. This approach, known as Distributed Fibre-Optic Sensing (DFOS), allows us to detect clogging of the well screens at an early stage.
Optimising system performance
Applied geophysics expert Roeland Nieboer has been involved in these measurements from the outset. "Long fibre-optic cables make it possible to continuously monitor conditions throughout the aquifer, enabling rapid detection of changes. Improved insight into groundwater flow will help prevent clogging of the wells and contribute to more efficient operation of the system."
During groundwater abstraction and reinjection, fine sediment particles such as sand can be transported with water, explains Roeland. These particles may clog the well screens or the pores of the surrounding sandy formation. "By identifying reductions in flow velocity at specific depths, potential clogging can be detected early and corrective measures can be taken in time. This helps optimise the performance and efficiency of the well."
Fusion fibre
One of the key advantages of fibre-optic monitoring is that temperature can be measured continuously along the full length of the aquifer without interfering with the operation of the ATES system. In addition, the system enables long-term monitoring. "We have combined several monitoring techniques within a single cable", says Roeland. "Groundwater flow velocity is derived from temperature measurements. A heating cable integrated between the optical fibres generates a heat pulse. Fast-flowing groundwater cools the cable more quickly than slow-flowing groundwater. By analysing temperature changes, groundwater flow velocities can be determined."
The system also records the sound of sand particles striking the plastic well casings. Deltares refers to this combination of monitoring techniques within a single cable as Fusion fibre. The workflows for processing and visualising the data are being developed as open source software, enabling knowledge to be shared widely."
Expanding the knowledge base
The new ATES system on the Deltares campus not only contributes to more sustainable operations, but also creates opportunities to expand Deltares’ knowledge of the subsurface. For that reason, the development of these monitoring techniques is funded through Deltares’ strategic research programme.
Roeland: "At Deltares, we are constantly working on innovative methods to gain a better understanding of the subsurface. This research contributes to the organisation’s knowledge base on water and the subsurface as resources for sustainable energy supply and supports the acceleration of the energy transition."
Scaling up
"The monitoring technique is still at an early stage of development. As the Netherlands increasingly relies on subsurface energy systems such as ATES for heating and cooling, it is important that these systems perform as efficiently as possible", explains Roeland.
As more buildings on the Deltares campus are connected to the ATES installation, new opportunities arise to further improve monitoring at aquifer depth. "Each newly drilled well provides additional possibilities to refine and optimise the monitoring techniques, resulting in even greater understanding of subsurface processes", says Roeland.
An underground weather forecast
Roeland: “I chose this subject because I find it fascinating to shed light on things we cannot see. I think it would be great to show my colleagues what’s happening beneath our feet at different times – a sort of ‘underground weather forecast’. At Deltares, we’ll soon be displaying this live on the screens in our buildings via an online dashboard.”
