SIBLES: a method to better understand coastlines with sandy beaches
SIBLES project: better understanding coastlines with sandy beaches
We are at Saint-Hilaire-de-Riez, to acquire geophysical data, for the SIBLES project. The project is developing a methodology for geomechanical characterization of the properties of sandy shorelines. The project is innovative in uniting three disciplines, sedimentology, geotechnology and geophysics, to study the capacity of sandy shorelines to resist attack by weather and sea. This involves several types of geophysical data acquisition: seismic, electrical and radar acquisition. The aim is to test the three systems to see which is the most suitable for achieving our goals.
The study site is the Pays-de-Monts, extending from La Barre-de-Monts to Saint-Hilaire-de-Riez. This is a group of sandy structures, mostly aeolian, with a dune cordon by the sea, linked to the interaction of oceanic and continental processes.
Here, we're acquiring seismic data. With our system we can measure long stretches, such as the seashore. This prototype, developed at BRGM, combines terrestrial and marine seismic exploration. Every time we want to move, we make a "shot", whereby we generate waves. The waves go into the subsoil and are recorded by sensors. Knowing the distance travelled by the wave and the time it takes to travel between gyrophones and behind the gyrophone, we can calculate wave-propagation speed in the subsoil. Knowing the propagation speed, we can define, by approximating density, a subsoil's elasticity parameters.
I'm using a mobile device called an OhmMapper. A current is sent by a transmitter at the other end of my system. Five receivers measure difference in potential, to give, with Ohm's law, the subsoil's resistivity. The goal is to measure resistivity variations, the subsoil's resistance to electric current. We can thus define the connection of dunes one with another, and also examine how seawater infiltrates the subsoil.
This ground-penetrating radar is a sort of underground scanner. We use it to record delicate dune structures and study the interrelationship of the various dune systems. It works very well behind the shoreline. On the foreshore or beach, it works less well, because the ground is very conductive, which weakens the signal. You see nothing on the device, whereas back here, we see the dune structure in fine detail.
We're taking several transects, either parallel or perpendicular to the coastline. With these, we will map the subsoil to see the lateral variations of the dunes and the saltwater-wedge interaction.
We can then combine drilling and existing structures to arrive at correct interpretations. Besides that, we'll also use geotechnical data, meaning the measurement and geomechanical parameters of the properties of the various terrains we find. We're now at a second stage, a few months after acquiring the data. It's not so hot! Here are the results of the geophysical data acquisition with the electrical-property results of the OhmMapper, showing the apparent resistivity, the subsoil's capacity to conduct an electrical current. It's highly variable. Similarly, for seismic measurements, the propagation of acoustic waves, which accelerates the deeper the subsoil is penetrated. We're carrying out six probes, using static penetration, whereby a powerful machine drives a probe into soft soil, down through the entire sequence of quaternary sediments.
Here, we are inside the static penetrometer. The principle is relatively simple, involving driving a boring rod into the ground, at constant velocity, recording parameters such as depth, angle, resistance at the tip and lateral friction force, as well as interstitial pressure, which is linked to the amount of water in the soil. From these parameters we'll deduce the nature of the soil and other relevant geotechnical parameters.
After analysing two datasets, we'll make two core borings, to sample and observe the actual lithology, and we'll continue taking sample measurements in the lab.
From Saint-Hilaire-de-Riez to La Barre de Monts, BRGM Pays de la Loire is leading the SIBLES project, in conjunction with the Coastal Observatory of the Pays de Monts and with the support of its local stakeholders and managers (ONF, Océan-Marais de Monts Community of Communes).
This research project aims to improve our understanding of coastlines with sandy beaches and their future. It focuses on the capacity of a dune barrier to resist aggressive marine weather: storms, extreme events, rising sea level, etc.
Helping to shape strategies for coastal planning and management
The objective of this project is to implement a multidisciplinary methodology combining sedimentology, geotechnics and geophysics, using several types of data acquisition (seismic, electrical and radar). Eventually, this method should make it possible to cover larger areas and to produce 3D maps.
The project's results are expected at the end of 2017. The goal is to inform discussions by experts on strategic choices for coastal planning and management.