Photo of a tTEM survey in the Netherlands, 2023, by TNO.
Note: the tTEM system shown has since been upgraded – the current version looks different from what’s pictured here.
01 – The challenge
Following new flood-defense standards introduced in 2017, the Aa en Maas Waterboard faced a 100 km reinforcement programme along the river Maas – and one 21 km earth-dike stretch needed answers no borehole alone could give.
The dike improvement between Cuijk and Ravenstein makes up roughly 21 km of that barrier, most of it earth dike. During the exploratory phase, it became clear that additional soil research was required to optimise both the reinforcement task and the new dike design – specifically, understanding the lateral continuity of clay layers, something traditional point-based drilling and probing can’t reliably reveal.
02 – The context
An investigative tTEM survey to assess the technology’s usability for dike safety assessment, carried out along the Maas river between Cuijk and Ravenstein in the Netherlands.
PARTNER
TNO (Geological Survey of the Netherlands)
COUNTRY
Netherlands, Europe
TERRAIN
Clay, Sand (river floodplain / earth dike)
FIELD DAYS
3
TECHNOLOGY
tTEM
YEAR
2021
Survey carried out by TNO (Geological Survey of the Netherlands), WSP, and the Aa en Maas Waterboard in December 2021 — an exploratory investigation into tTEM’s usability for dike safety assessment, ahead of the Waterboard’s wider reinforcement programme following the 2017 flood-defense standards.
03 – The solution
Ground-Based Transient Electromagnetic Survey
TEMcompany’s tTEM system was towed continuously along the ground surface across three sections of the Maas river, on top of and in the immediate vicinity of the dike. Built for very high near-surface resolution with early time gates and a fast repetition frequency, the instrument measures resistivity continuously rather than at isolated points – critical for tracing how far a clay layer actually extends, not just confirming it exists at a single borehole.
Methodology
01
Survey Planning
tTEM profiles were planned along three sections of the Maas river, covering both the crest of the dike and its immediate surroundings.
02
tTEM Acquisition
The system was towed continuously across the survey area in December 2021, collecting high-resolution near-surface resistivity data along each profile.
03
Data Quality Review
Data collected directly on top of the dike was found to be too disturbed by nearby infrastructure and cables to be usable, and was excluded from the interpretation.
04
Hydrogeological Interpretation
Resistivity results were interpreted using the region’s known soil contrast – clay reads as low resistivity, sand as high – to map the lateral continuity of the clay layer across the surveyed area.
04 – The results
The survey revealed a continuous clay layer, approximately 4 metres thick, present almost everywhere across the surveyed area. Unlike traditional drilling or probing – which only sample isolated points — tTEM’s continuous measurement made it possible to confirm the lateral continuity of this layer across the entire stretch, information that point-based methods alone could not have provided.
21Km
Dike stretch investigated
3
River sections surveyed
4m
Continuous clay layer thickness
The Region
01
Right along the Maas
This stretch sits between Cuijk and Ravenstein, along the Maas.
02
Scale
This 21 km stretch is part of a larger 100+ km dike reinforcement project across the Netherlands.
03
Not an isolated case
Similar dike safety concerns exist across much of the Netherlands, not just this stretch.
04
What’s at stake
The clay layer beneath this farmland is what stops water from seeping under the dike toward the area behind it.
05 The impact
A demonstrably safer dike – and one less failure mechanism to worry about.
Partly on the basis of these tTEM measurements, a significant sub-area of the Cuijk–Ravenstein dike stretch was shown to have a sufficiently thick clay layer in its foreland – meaning no further task remains for the piping failure mechanism in that section. Insight that drilling alone couldn’t have delivered, applied to one stretch of the 100+ km of dikes now being reinforced across the Netherlands.
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