# Subsurface variability of sandy coastal geosystems revealed by integrated georadar and cone penetrating testing approach: Insights from the Pays-de-Monts coast (Atlantic France)

Julie Billy, Vivien Baudouin, Gildas Noury, Manuel Garcin

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Julie Billy, Vivien Baudouin, Gildas Noury, Manuel Garcin. Subsurface variability of sandy coastal geosystems revealed by integrated georadar and cone penetrating testing approach: Insights from the Pays-de-Monts coast (Atlantic France). Geomorphology, 2026, pp.110153. 10.1016/j geomorph.2025.110153. hal-05450570

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# Subsurface variability of sandy coastal geosystems revealed by integrated georadar and cone penetrating testing approach: Insights from the Pays-de-Monts coast (Atlantic France)

Julie Billy<sup>a,*</sup>, Vivien Baudouin<sup>b</sup>, Gildas Noury<sup>c</sup>, Manuel Garcin<sup>c</sup>

## ARTICLE INFO

**Keywords:**

Marsh-foreshore transition

Coastal geological model

Geophysics

Geotechnics

## ABSTRACT

Beneath dune fields, the subsurface geology of sandy coastal systems can provide valuable insights for understanding coastal environment. While sandy coasts are often described and monitored through surface geomorphology, their internal structure and geotechnical properties remain poorly documented. To address this limitation, a combined ground-penetrating radar (GPR) and cone penetration testing (CPT) approach was applied along the Pays-de-Monts coast (Atlantic France). This multidisciplinary methodology yields complementary information on the internal architecture, stratigraphic variability, and soil behavior of coastal deposits.

Two sectors were investigated, both characterized by dune fields underlain by fractured limestone and bordered inland by low-lying marshes. Despite their apparent similarities, the sites reveal distinct features. Results provide new insights into (i) the sedimentary units anchoring the sandy coastline, (ii) the connection with adjacent marshes and low-lying environments, (iii) the relationships between dune systems and underlying deposits, and (iv) soil behavior properties linked to depositional environment or dune types themself.

From these findings, two conceptual models are proposed: (1) a prograding sandy coastline with an inland marshward transition overlain by dune field (La Parée Grollier, northern sector), and (2) a channel-fill and spit development sequence overlain by dunes (La Pège-Les 60 Bornes, southern sector). These results demonstrate the value of integrating geophysical and geotechnical methods to improve understanding of coastal geosystems and enabling the identification of site-specific characteristics. More broadly, this study demonstrates and discusses the benefits of integrating GPR-CPT approach for advancing coastal geology and highlights the relevance of local geological models for management and adaptation strategies under climate change.

## 1. Introduction

In the context of coastal evolution and adaptation to climate change, improving our understanding of coastal geosystems has therefore become increasingly essential. Sandy coasts are still predominantly described through their surface geomorphology, with particular emphasis on features such as nearshore bars, beaches or dune systems. Contemporary monitoring tools, including LiDAR (e.g., Aleman et al., 2017; Khan et al., 2025; Nicolae Lerma et al., 2022), satellite imagery (e.g., Castelle et al., 2022; Ford, 2013; Konlechner et al., 2020; Robinet et al., 2025), and video monitoring (e.g., Bouvier et al., 2017; Huisman et al., 2011), provide valuable insights into shoreline dynamics and morphological evolution, both in real time and across recent decades.

However, the internal architecture and intrinsic properties of coastal geosystems remain poorly documented, and sandy coasts may thus be often considered as relatively homogeneous geological environments. Consequently, their geological heterogeneity and site-specific characteristics are rarely considered in coastal management and remain largely absent from long-term adaptation strategies (e.g., Le Cozannet et al., 2022).

Ground-penetrating radar (GPR) offers a high-resolution geophysical method for investigating coastal environments. Over the past few decades, it has been increasingly applied to characterize sandy to gravelly deposits across a range of depositional settings and coastal contexts (Neal, 2004), including dunes (e.g., Billières et al., 2025; Girardi and Davis, 2010; Robin et al., 2021; Tamura et al., 2011), beach ridges (e.g.,

<footnote><sup>a</sup> BRGM, F-33600, Pessac, France</footnote>

<footnote><sup>b</sup> BRGM, F-44323, Nantes, France</footnote>

<footnote><sup>c</sup> BRGM, F-45060, Orléans, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: j.billy@brgm.fr (J. Billy).</footnote>
Billy et al., 2015; Schffer et al., 2023; Tamura et al., 2019; Weill et al., 2012), strandplain or barrier systems (e.g., Costas and FitzGerald, 2011; Hein et al., 2013; Jahnert et al., 2012; Konlechner et al., 2020; Tanigawa et al., 2025), etc. Yet, GPR remains under-used on many coastal sites and is rarely incorporated into observation-based monitoring programs. This is despite its ability to reveal subsurface geological structures, and to image the development phases of coastal systems, either past or recent and natural evolution or due to management plans.

Soil behavior is strongly influenced by both depositional and post-
depositional processes, and distinct sedimentary facies display specific
geotechnical properties (Campo et al., 2023; Sarti et al., 2012; Truong
et al., 2011). Cone Penetration Testing (CPT) is a geotechnical technique
that provides accurate and detailed information on subsurface proper-
ties (soil behavior, mechanical or hydraulic properties) and is widely
applied in geotechnical and engineering investigations (Robertson,
2009; Robertson and Cabal, 2022). A key advantage of CPT is its ability
to deliver high-resolution in situ measurements that can be directly
correlated with adjacent sediment cores, stratigraphic data, or
geophysical surveys. As highlighted by Zhang et al. (2018), CPT has
gained increasing importance in sedimentological studies of coastal
environments because it provides continuous, high-resolution records of
subsurface lithology and physical parameters, and it is possible to
investigate large areas (Lunne et al., 1997; Robertson, 2009; Tillmann
et al., 2008). It has been particularly effective in the investigation of
alluvial plains (e.g. Amorosi et al., 2017; Bruno et al., 2019) as well as in
coastal systems, both onshore and offshore (e.g. Gourvenec, 2024; Yang
et al., 2025).

Combining geophysical and geotechnical methods provides a
particularly powerful framework for coastal investigations. GPR pro-
vides two-dimensional imagery of internal architecture and spatial ge-
ometry of deposits, while CPT supplies complementary information on
soil behavior and subsurface variability that cannot be captured by GPR
alone. Together, these methods enable detailed characterization of
coastal areas.

This study evaluated the potential of a combined GPR - CPT
approach to characterize sandy coastal geosystems along the Pays-de-
Monts coastline (Atlantic coast, France; SIBLES Project). This area
consists of sandy beaches and dune fields, bordered inland by low-lying
marshes. The aim of this study is to investigate the subsurface geology of
nearby sandy coasts to examine sediment deposits, and particularly the
dune and underling units up to limestone bedrock. Site-specific features
are revealed through analyses of internal architecture, stratigraphic
variability, and soil behavior types, providing new insights into the past
evolution of each site. Ultimately, this approach allows the construction
of synthetic conceptual coastal geological models that highlight the
singularities of each coastal system, essential to the development of
future adaptation strategies.

## 2. Study area

The Pays-de-Monts coastal dune system is located on the Atlantic
coast of Vendée, in western France (Fig. 1). The morphology of this coast
is closely linked to the Breton-Vendéen wetland, a 36,000-hectare
coastal marsh influenced by the Baie de Bourgneuf to the north and
the Vie estuary to the south. The historical evolution of this wetland
reflects its gradual transformation from a coastal marine environment
into a managed marsh, driven by sedimentation, human intervention,
and hydraulic engineering. Prior to the Middle Ages, the area consisted
of tidal channels and mudflats, with natural sedimentation occurring in
sheltered areas behind sandbanks. Halophytic vegetation (Salicornia,
Obione) facilitated sediment trapping, contributing to progressive
infilling of the marsh. From the Middle Ages to the 17th century, this
process intensified. The construction of canals and dykes marked the
onset of land reclamation (Pinot, 1980; Sarrazin, 2007). During this
period, active dune migration contributed to significant landscape
transformations, burying parts of villages and overlaying sections of

<figure><img src="image_3.png" /><figcaption>Fig. 1. Location map of the Pays-de-Monts coastal dune system (Vendée, Atlantic Ocean France), and the two specific areas studied, N°1 - La Parée Grollier and N°2 - La Pège-Les 60 Bornes.</figcaption></figure>

marsh (Welsch, 1917). Between the 18th and 19th centuries, polder-
ization expanded and water management associations emerged, regu-
lating freshwater-and saltwater balance (Baudet et al., 1987; Rousseau,
1968). In the 20th century, salt production declined, and polderization
was completed in 1963. Meanwhile, oyster farming developed in
Bourgneuf Bay, and tourism expanded in the region. The long-term
management of freshwater and saltwater dynamics has played a
crucial role in shaping this landscape, which has been profoundly
influenced by centuries of human-environment interactions.

Pays-de-la-Loire region benefits from comprehensive geological
syntheses, providing a standardized knowledge base for the entire
coastal area (1:250,000 scales) and a consistent and standardized
geological reference for the entire region (accessible via GEOPAL portal;
https://www.geopal.org/). Indeed, the French Geological Survey
(BRGM) developed a harmonized land-sea geological mapping of the
region, encompassing unconsolidated sediment formations and sub-
strate geology, including lithology and major structural features (Thinon
et al., 2013). This work also produced public digital elevation models
(DEMs) including a sediment thickness map and a substrate depth
model. These regional-scale maps do not reflect site specificities, and
higher-resolution data on the coastal geosystem are not available.
Nevertheless, a local study was conducted at La Parée Grollier
(2016–2017) to test a multi-geophysical approach for characterizing a
land to sea transect within this coastal geosystem (Billy et al., 2020). A
key finding of this innovative approach was the identification of the
depth and morphology of the Eocene limestone bedrock (fractured to
highly fractured) by terrestrial seismic surveys (MASW). MASW profile
revealed that the bedrock is found at an elevation between –10 to –15
m relative to the French national vertical datum (NGF), and displayed
incised morphologies. This study further demonstrated that the bed-
rock's upper boundary is not flat, and that its spatial heterogeneity re-
mains poorly understood.

Focusing on the coastline, it has primarily been studied through
coastline evolution analysis conducted by the regional coastal observa-
tory (OR2C), relying on photographic interpretation (since the mid-20th
century and from the French National Geographic Institute database)
and assessments of the impacts of major storms, such as Martin and
Lothar (1999) and Xynthia (2010) which have significantly impacted
the coastal landscape. These studies provide indicators, including coastline evolution rate (OR2C; Robin et al., 2019), erosion exposure index (building exposure to shoreline retreat; Juigner et al., 2017) or coastal erosion vulnerability assessment (Audère and Robin, 2021), as well as beach/dune volume changes derived from LiDAR-based analyses (particularly before and after Xynthia, using three DEMs 2008–2010–2013; Le Mauff et al., 2018).

The Pays-de-Monts coastal area is a 27-km-long and 0.5–2.3-km-wide, composed of four generations of vegetated dune systems (Ters, 1956), with increasing elevation inland. These dunes are separated by large interdunal slack areas and bordered to the east by low-lying marshes (<2 m NGF). From inland to the beach, the dune system comprises (Fig. 2): D₁ - highly digitated and overlapping parabolic dunes (18–25 m high); D₂ - digitated parabolic dunes (10–13 m high); D₃ - series of dune with an irregularly shape (7–14 m high); and D₄ - longitudinal dunes connected to the modern beach (4–10 m high). This study focuses on two specific sectors of this coast, named La Parée Grollier (4.5 km of coastline in the northern sector; Fig. 2A), which include the four dune types (D₁–₄), and La Pège-60 Bornes (7 km of coastline in the southern sector), where dunes are primarily of types D₃–₄ (Fig. 2B).

## 3. Methods

The integrated geophysical and geotechnical approach, combining ground-penetrating radar and cone penetration testing has been experimented to characterize the coastal geosystem, focusing on dunes and underlying sedimentary units. Additionally, the study area benefits from several publicly available datasets, as a regional LiDAR dataset (2011, 1 m pixel) geological mapping of sediment formations and substrate geology, DEMs of sediment thickness and substrate depth model (1:250,000 scale).

### 3.1. Ground-penetrating radar

GPR is a non-invasive geophysical imaging technique based on the propagation and reflection of transmitted electromagnetic pulses (Cassidy, 2009; Neal, 2004). This method provides a high-resolution, continuous imaging of internal sedimentary structures, and is particularly effective for investigating sandy to gravel coastal systems (Billy et al., 2015; Costa et al., 2016; Oliver et al., 2017; Robin et al., 2023; Tamura et al., 2011; van Heteren et al., 1998).

For this study, a Mala ProEx georadar system was used, with three antennae centered at a frequency of 100, 250 and 500 MHz, respectively. Two GPR surveys were conducted: (i) in 2016, at La Parée Grollier, to assess GPR performance in the studied coastal depositional environment (500 MHz antenna), and (ii) in 2022, to complete the La Parée Grollier survey (100 and 250 MHz antennae) and to investigate the second study area at La Pège-Les 60 Bornes (250 MHz antenna). A total of 10.3 km of profiles were acquired at La Parée Grollier (area n°1) and 16 km at La Pège-Les 60 Bornes (area n°2; Fig. 2).

Data was processed using ReflexW v10.5, applying time-zero drift, background removal, band-pass filtering, gain, topographic correction. Geosections (georeferenced image of radar profile), 3D visualization of the survey and interpretations (including edit picking and surface DEM generation) were produced using DELPH GeoInterpretation v5. A constant radar velocity of 0.122 m/ns was estimated using the common mid-point (CMP) method (Billy et al., 2020) and applied for time-depth conversion and to estimated vertical resolution (0.30, 0.12 and 0.06 m for the 100 MHz, 250 MHz and 500 MHz antennae, respectively). While GPR data are commonly visualized in 2D plan view, the use of a 3D viewer significantly enhances data interpretation by improving the spatial integration of profiles, allowing better perception of sedimentary units, their dips or the direction of depositional progradation.

Groundwater level at the study site n°1 has previously been investigated using both GPR and electrical resistivity tomography (Billy et al., 2020). In sandy dune environments, GPR profiles commonly enable

discrimination between unsaturated aeolian sands and freshwater-saturated sediments. This transition is typically expressed by a geometrical attenuation of the radar signal below the groundwater surface (e.g. Fu et al., 2019; Rejiba et al., 2008; Robin et al., 2021; Tronicke et al., 1999 in dune systems). In addition, the geophysical expression of this interface also depends on antenna frequency. Higher-frequency antennae, characterized by shorter wavelengths, may not encounter sufficient dielectric impedance contrast within the capillary fringe above the groundwater table to generate a well-defined reflector (e.g. Harari, 1996). In contrast, lower-frequency antennae with longer wavelengths (ex: 100 MHz) are more likely to image this interface as a distinct, laterally continuous sub-horizontal reflector. In this study, GPR profiles acquired with the 250 and 500 MHz antennae display a high-amplitude response at the groundwater interface, followed by pronounced signal attenuation below it, while a clearer and more continuous reflector is observed with the 100 MHz antenna data.

### 3.2. The cone penetration testing and geotechnical data

CPT is a standardized geotechnical method (ISO 22476-1, TE1 class) widely used to determine geotechnical properties of soils and identify subsurface stratigraphy (Lunne et al., 1997). CPT is particularly effective for characterizing soft soils, sand to clay, making it well-suited for studies in coastal plains (e.g., Campo et al., 2023; Missiaen et al., 2015; Zhang et al., 2018). However, it may be difficult or unfeasible in gravel/pebble layers, or compacted soil, where test refusal can occur. The test is performed by advancing an instrumented cone at a constant speed of 2 cm/s, while recording soil resistance variations. Sensors measure two key parameters (in MPa) every 2 cm: (i) the cone tip resistance ($Q_c$) measure the soil's resistance to penetration; and (ii) the sleeve friction ($F_s$), which corresponds to the lateral friction exerted by the soil on the cone's sides. From these measurements, additional geotechnical parameters can be derived, such as the friction ratio ($R_f = 100 * f_s/Q_c$, %), which enables identify soil sediment type, and relative density ($D_r$, %), an indicator of grain packing (related to porosity). Using empirical soil classification charts (e.g., Robertson, 1990), these values enable the classification of soils, and distinct sedimentary units, based on their behavioral properties (Soil Behavior Type, SBT 1–9; e.g. dense sands, soft clays, etc.).

Three CPT campaigns were conducted in 2016, 2022 and 2024 using a track-trunk (20 t) equipped with an electronic piezocone (43.7 mm diameter). In total, 16 CPTs were performed in the coastal dune field at La Parée Grollier, and 41 CPTs at 60 Bornes-La Pège (Fig. 2). Each of these sounding was performed up to the refusal (50 MPa; on average up to ~10 to 15 m depth). Interstitial pore pressure (u) is measured at some points during the surveys (CPTu), to provide insights into subsurface hydraulic conditions and soil permeability. The analysis of this parameter is not included in this present work. CPT measurements were carried out by two distinct companies, involving their own calibration procedures. Different CPT obtained at the same location (less than 1 m distance; Fig. 2) have highly similar cone tip resistance, $Q_c$, profiles behaviors, but some differences have been identified for sleeve friction. Indeed, data from the third campaign (2024) were specifically used in this study for soil behavior classification and statistical analyses.

The $Q_c$ profiles from all sounding were plotted in function of the depth and then superimposed onto the GPR profiles (Fig. 3 and Fig. 4). Cross-interpretation of CPT curves and GPR profiles enabled the identification of distinct sedimentary units within the geosystem. For each sedimentary unit, CPT points were plotted on the Robertson's soil classification diagram (scatter plot). To characterize the sedimentary units and compare them with each other, covariance analysis was performed using a gaussian mixture model (GMM) to cluster data, with probability density function for GMM and fitted GMM contours (processed and plotted in MATLAB).

Previously, a major geotechnical data collection campaign was conducted between 2016 and 2018, gathering a large dataset from
<figure><img src="image_4.png" /><figcaption>**Fig. 2.** Maps of the data available on the two study areas A) La Parée Grollier and B) La Pège-Les 60Bornes, illustrating the location of the GPR profiles and CPT soundings.</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>Fig. 3. Processed (A) interpreted (B) GPR profile at La Parée Grollier (location map Fig. 2) and superposed profiles of Qc with depth of each CPT sounding and the BSS LOG existing along or near the profile. C) LOG of SC1 and SC5 cores and corresponding sedimentary facies.</figcaption></figure>

private geotechnical engineering firms (including about 1200 drilling, boreholes, CPT tests in the entire Pays-de-Monts area and Breton-Vendée wetland, eq. area of 25 km × 25 km). These “sleeping” data have been integrated into the French national BSS database (https://infoterr.e.brgm.fr/viewer/MainTileForward.do) and made publicly accessible by BRGM. This geotechnical dataset is a valuable resource, significantly improving knowledge of the subsurface and the spatial variability of the geosystem. Existing cores and borehole logs are used to support the interpretation of the soil classes identified through the CPT tests conducted in this study.

## 4. Results

The sedimentary sequence across the Pays-de-Monts coastal system is underlain by a fractured to highly fractured Eocene limestone sub-stratum, identified between −8 and −15 m NGF. Toward beaches and

nearshore zones, the limestone becomes progressively shallower with elevations reaching up to −5 m NGF. This spatial variability in sub-stratum elevation reflects structural controls and has implications for both sedimentary infill and coastal dynamics. The combined analysis of GPR and CPT data provides new insights into the internal sedimentary architecture and geotechnical properties of this coastal system. Across the two targeted study sites, La Parée Grollier and La Pège, the surveys highlight the properties of the dune systems and characterize several underlying units, including the foreshore-to-marsh transition, a filled paleo-channel, or a former coastal spit. The following sections present a detailed description of these features.

### 4.1. Dune field

The internal structure of each dune system is clearly imaged using GPR. The parabolic dunes (D1–2 types in Fig. 2) and dune with irregular
<figure><img src="image_6.png" /><figcaption>Fig. 4. CPT Qc profiles in function of the depth (MPa) at La Parée Grollier.</figcaption></figure>

shape ($D_3$) are primarily characterized by units with eastward-dipping reflectors (highlighted by yellow dotted-line in $U_{D3}$, $U_{D2a}$, $U_{D2b}$ in Fig. 3). This internal configuration reflects their successive development phases and inland migration pattern, driven by prevailing easterly winds, prior to their stabilization by maritime pine afforestation during the mid-19th century. In contrast, the longitudinal dunes ($D_4$) exhibit westward-dipping reflectors ($U_{D4}$ and $U_{D5}$; Fig. 3), showing seaward dune development and growth. These dunes show no evidence of migration or translation.

The dunes are composed of moderately dense, yellow, fine-to-medium sand ($D_{50}$: 0.2 mm; cores SC1 and SC5). CPT analyses (Fig. 4; soundings 325 to 328), interpreted using established soil behavior chart, indicate that dune materials are defined by Soil Behavior Types (SBT) 6 and 7 (Fig. 5-A1), corresponding to “clean sands to silty sands” and “dense sands to gravel,” respectively. These deposits appear normally-to-slightly over-consolidated. A more detailed analysis distinguishes two behavioral subsets. CPTs crossing the parabolic dunes ($N# 325, 326, 327$) are classified as SBT 7 (yellow dots in Fig. 5-A1) and show sparse data distributions, while the CPT from the irregularly shaped dune ($N#328; D_3$) is classified as SBT 6 (red dots). The parabolic dunes exhibit higher normalized cone tip resistance ($Q_d$), with median values around 500 ($D_2$), compared to 280 for the $D_3$ dune type (Fig. 5-B1). Although the friction ratios ($F_r$) are similar (median ~0.5–0.6 %), the data from the parabolic dunes display a broader spread toward finer-grained sediments, suggesting subtle internal textural variability within the aeolian deposits.

## 4.2. Interdune zone, foreshore and marsh deposits at La Parée Grollier

### 4.2.1. Radar units

Dune field is bordered at its east side by extensive managed marsh (Fig. 2). At La Parée Grollier, beneath the ~2 km of dune field, GPR reveals a non-uniform underlying deposit that can be split into three

domains, from beach (0 m) to land (2 km distance from the shoreline) it is therefore possible to identify:

Unit $U_{FM}$ from the beach to 850–900 m inland ($U_{FM}$: fluviomarine deposits). $U_{FM}$ is identified and defined by thick deposits (from –10 m to +2 m NGF) with homogeneous facies characterized by westward-dipping oblique reflectors (slope: 3–5°; Fig. 3). The deposits are interpreted as a continuum of shoreface, foreshore and beach deposits overlying Eocene limestone. $U_{FM}$ illustrates the progradation pattern of this coast and the successive seaward sediment accumulation over 900 m distance (subdivided into 5 sub-units from land to sea: $U_{FM1–5}$). Groundwater level is broadly close to the upper boundary of $U_{FM}$.

From 850 to 900 m to 1 km, a mound-shape structure is observed on GPR profiles and named $U_T$ (Fig. 3). $U_{FM}$ is adjoined to $U_T$ (U$_T$: transition). This unit is spatially restricted (150 m long), but it is also observed on profiles further north (1 km to the north) on The Lays Path.

From 1 to 2 km, deposits below aeolian sand cover are very distinct than unit $U_{FM}$ or $U_T$. Indeed, $U_M$ ($U_M$: mash) is defined by a 2 m thick draping unit characterized by transparent facies and a clearly visible sub-horizontal basal discontinuity. Below this layer, GPR profiles do not provide an image of deeper deposits ($U_{X0}$: uncharacterized fluviomarine deposits).

On either side of the $U_T$, units $U_{FM}$ and $U_M$ have a very distinct configuration. $U_T$ is therefore interpreted as the boundary between two distinct depositional environments, one prograding seaward ($U_{FM}$) and the other with aggrading deposits ($U_M$) and characteristic of more sheltered environments.

### 4.2.2. Interdune zone morphology

Interdune zones are low-lying areas between dune systems, typically ranging from 2 to 4 m NGF in elevation within the Pays-de-Monts dune field. A detailed analysis of these interdune areas reveals the presence of regular longitudinal morphological structures on the 2011 LiDAR dataset (Fig. 6A). Topographic profiles extracted from these zones display decimetric-scale oscillations in surface elevation (T1 and T2 in Fig. 6B; decimetrical topographic variations of less than 1 m). In the north of the Pays-de-Monts system, these longitudinal structures have a N-NE to S-SW orientation (around T1), while those at La Parée Grollier have a N to S orientation, which is roughly the same orientation as the current shoreline. Notably, these longitudinal features are absent in the easternmost part of the dune field, particularly within the area defined by overlapping parabolic dunes area ($D_1$).

Given their linear topographic configuration at the scale of the Pays-de-Monts system, these features are interpreted as natural structures, rather than anthropogenic remnants (i.e. not considered as relict of rural field boundary ditch, dry-stone wall or remnant bocage). When compared with the internal sedimentary architecture of the underlying unit ($U_{FM}$), imaged through ground-penetrating radar (Fig. 3A and in Billy et al., 2020), these surface morphologies align with westward-dipping reflectors, oriented seaward. This concordance supports their interpretation as former shoreline positions or beach-ridge systems, providing valuable insight into the paleo-geographic evolution of the coastal system.

### 4.2.3. Sedimentological and geotechnical properties

CPT refusals and the end of the cores highlight the boundary between sediment and limestone ($U_{Basal}$, Fig. 3), which is consistent in depth and spatial variability with the limit determined by geophysics (Billy et al., 2020). Although the fluviomarine deposits ($U_{FM}$) are sandy, two sub-units are distinguishable due to their sedimentological and geotechnical properties thanks to two cores (SC1 and SC5, located to the west and at the middle of the system; Fig. 3) and CPT sounding ($Q_c$ profiles and SBT; Fig. 4 and Fig. 5). Indeed, it can be identified a lower layer, from limestone substrate to –2 to –1 m NGF, overlain by an upper layer, up to the base of aeolian dune (close ~1 to 2 m NGF), defined as shoreface and beach deposits, respectively.

The main difference between the two cores is that the deeper
<figure><img src="image_7.png" /><figcaption>**Fig. 5.** CPT data analysis following A) the soil behavior type chart, and B) covariance analysis, illustrating dunes (yellow, D₂ and red, D₃), fluvio-marine (bleu) and marsh (green) deposits.</figcaption></figure>

deposits have a distinct sand color (cores LOG and facies in Fig. 3C): SC1 displays yellowish-brown sand ($U_{FM5\_Lower}$) while SC5 shows bluish-gray sandy deposits ($U_{FM2}$), but both are defined by fine-to-medium sand with presence of rare fragments of shells ($D_{50}$: 0.2 and 0.25 mm for SC1 and SC5, respectively). In coastal and wetland environments, dark gray coloration of sandy deposits can result from organic matter enrichment rather than mineralogical sources. Indeed, under anoxic conditions, the incomplete decomposition of plant material and microbial residues leads to the accumulation of humic compounds, which darken the sand matrix. Such processes are characteristic of marsh-influenced beaches, coastal wetlands, or paleo-lagoonal environments. Unfortunately, the cores taken in 2017 have not been preserved or banked in a core library, but exclusively photographed, and further analyses of the grains or organic matter present in the sediment cannot be carried out. Above lower layer ($U_{FM5\_Lower}$ and $U_{FM2}$), deposits are defined by medium yellowish-brown sand ($D_{50}$: 0.3 mm) for both cores (Fig. 3C).

CPT measurements of fluvio-marine deposits are sparsely scattered and mainly associated with the SBT 6 domain, clean-to-silty sands (Fig. 5). For example, $U_{FM1}$ displays median normalized tip resistance values of approximately 200 and a median friction ratio around 0.6–0.7 %. A more detailed analysis of the CPT values ($Q_c$ profiles and SBT), identify a distinct geotechnical behavior between the two sub-units, the foreshore and the beach deposits. For example, CPT 328 analysis shows distinct median normalized tip resistance values for former beach

deposits ($U_{FM4\_Upper}$) and foreshore deposits ($U_{FM4\_Lower}$), of approximately 350 vs 150–200, while median friction ratio is still around 0.6–0.7 %. $U_T$ has similar soil behavior domain than $U_{FM1}$.

Landward, $U_M$ deposits display a distinct sedimentological and geotechnical signature compared to both dune and fluvio-marine units. According to BSS data close to the study area (N#05345X0007/8/9, LOG and diagraph), this unit is considered as a single clay unit of several meters (from 2 to 4 m thick, and down to around –0.8 to –1.7 m NGF) and is hence interpreted as marsh deposits. This deposit is traversed by CPT sounding N#325 and its analysis shows two sub-units with quite distinct characteristics ($U_{M-1}$ and $U_{M-2}$) not especially mentioned in the BSS database and neither easily distinguishable in GPR profile. From 1 to –1 m NGF, $Q_c$ profile exhibits an abrupt change associated to very low value (inf. 2 MPa; Fig. 4). CPT analysis of $U_{M1}$ mark the SBT 3 and 4 domains associated to “clay with silty clay” and “clayey silt and silty clay”. This layer is clearly distinguishable from the other layers of La Parée Grollier area, with $Q_{dl}$ and $F_r$ medians at around 27 and 3.5 %, respectively. The upper layer $U_{M2}$ has a behavior similar to that of sandy parabolic dunes, with a SBT 7-6, and medians of $Q_{dl}$ and $F_r$ around 450 and 0.6 %, respectively (Fig. 5).

### 4.3. Filled channel and spit under the dune at La Pège

To the south, at La Pège (Fig. 1), the dune system is less developed
<figure><img src="image_8.png" /><figcaption>**Fig. 6.** A. LiDAR DEM at the north of the Pays-De-Monts, highlighting the interdune slack areas, B. Topographic profiles extracted from the DEM, and C. 3D view illustrating the foreshore-marsh transition, the foreshore/beach progradation direction and the dunes migration or development direction.</figcaption></figure>

inland than La Parée Grollier, and can be narrowest (~40 m, Fig. 2B). Here, dunes are a natural barrier between the sea and tourist areas (parking area, campsite, etc.), as well as critical infrastructure such as water treatment plants. CPTs are realized on this narrow part of the dune and along a north-west to south-east longitudinal profile and beach-land transversal profile (Fig. 7A). CPT analyses highlight that the sand dunes have similar soil behavior than those described at La Parée Grollier. The CPT behavior of the fluviomarine deposits is quite similar to that of the dunes, with both deposits corresponding to the SBT 7 domain: “dense sand to gravel”, but the fluvio-marine deposits appear to be coarser, i.e. with a lower $F_r$ (0.3 %; Fig. 7D).

Intercalated between the aeolian sand dune ($U_{D4}$) and fluviomarine deposits, a 1 to 2 m thick layer is identified on curves with very distinct $Q_c$ values. Indeed, $Q_c$ profiles (CPT N#234, 235, 202, 204 and 338; Fig. 7B–C) exhibit an abrupt change associated with very low-pressure values, inf. 2 MPa. This unit is associated with the SBT 3 domain: “clay to silty clay” (Fig. 7D). This clayed layer is observed from CPT N#234 to 338 (400 m-long distance) and is absent of CPT values beyond, on both sides (Fig. 7B). These observations suggest that the clay layer could extend over a distance of 500 to 600 m (NW-SE orientation); or at least greater than 400 m and strictly less than 750 m. Perpendicular, along the transverse axis, this clayed layer is identified on every CPT (N#201 to 203), both the one closest to the beach and behind the dune (110 m distance profile; Fig. 7C). BSS data (cores and their LOG) in the vicinity of this area of interest show the presence of clay deposits, of 2.5–3 m thick behind the dune system (exhibit in green in Fig. 7A), i.e. a base level of around –0.5 to –0.65 m NGF for points N#05602X0219 and 05602X0168, respectively. In fact, the base level of the clay deposits identified by cores is consistent with those recorded by CPT. The

location of this clay layer seems to be related to the low topography and the area defined as paleo-channel (Fig. 2B), so the layer is interpreted as channel-fill deposit (e.g. former meander).

North of this channel-fill deposit, the internal architecture imaged by GPR reveals a distinct coastline history of coastal development. Profiles image a northwestward-prograding sequence of sedimentary deposits, at least 5–6 m thick (from approximately –5 m to +1 m NGF). This sequence is anchored on a basal sedimentary unit characterized by a radar-transparent facies with a sharply defined, laterally continuous upper discontinuity of high amplitude (Fig. 8, south-east part). This discontinuity marks the onset of the prograding system, interpreted here as a sedimentary spit now completely overlain by a dune field. At present, no geomorphological evidence of this spit remains visible in the landscape.

Along this coast, CPT data identify both dune deposits overlying fluvio-marine sediments. North of the channel-fill deposit and the spit sector, at the “60 Bornes” site along a sea-to-land transect, all CPTs similarly stopped at refusal. Borehole data, from the BSS database, indicates the presence of sedimentary deposits extending below this refusal depth to approximately –10 m NGF before reaching bedrock. The soundings did not reach this bedrock surface but encountered a gravel-cobble layer that prevented further penetration, triggering the automatic shutdown of the device.

The site lies close to the former estuary mouth, now infilled, but its morphology is still discernible in the landscape (Fig. 2B and Fig. 7A). This horizon of sandy deposits with gravel/cobbles layers is interpreted as reflecting an older depositional phase, likely associated with the past dynamics of the estuary. Historical records indicate that this area was intensively used for navigation and trade (commercial port; i.e. the
<figure><img src="image_9.png" /><figcaption>**Fig. 7.** A. Location map of La Pège; B. Profiles of Qc with depth of each CPT along the NW-SE longitudinal profile and C. the beach-land transversal profile and D. analysis of CPT N# 338 data following the soil behavior type chart.</figcaption></figure>

Masse Map of 1707 in Suire, 2017); however, following marsh drainage and hydraulic-management works over the past two centuries, the estuary progressively infilled (Baudet et al., 1987; Pinot, 1980; Rousseau, 1968).

# 5. Discussion

## 5.1. Coupling GPR and CPT: complementarity benefits of both methods for coastal investigation

The integration of ground-penetrating radar and cone penetration testing exploits the complementary assets of both methods, providing a powerful approach to subsurface characterization of coastal system. CPT data, classified using the Robertson chart and overlaid onto GPR profiles, enhances the discrimination of sedimentary units (Aguila et al., 2022; Alexopoulos et al., 2025; Billy et al., 2024; Zhang et al., 2018). Cross-interpretation allows for precise delineation of stratigraphic boundaries.

The vertical resolution of GPR depends on the radar wave velocity and the antenna frequency, typically approximated as one-quarter of the dominant wavelength (Neal, 2004; Sheriff, 1977). For conventional GPR

antennas (hundreds of MHz) used in coastal settings, such as the Vendée coast, the vertical resolution ranges from a few centimeters to decimeters (e.g. Billy et al., 2014; Chandler et al., 2022; Hein et al., 2012; Nielsen et al., 2009; Rodriguez and Meyer, 2006; Weill et al., 2012). This enables, theoretically, the identification of medium to thick sedimentary beds (>10 cm thick beds; Boggs, 1995) and even thinner beds (3–10 cm) when using very high-frequency antennas; but for this resolution, it is useful to combine radargrams with trenches or cores (invasive method). CPT provides measurements at 2 cm intervals, theoretically allowing the detection of thin beds. While a single measurement may be insufficient to identify an individual bed, the concordance of CPT signatures with GPR reflectors yields a more robust interpretation of sedimentary unit boundaries. In this study, for example, these concordances enable (i) the delineation of the dune base and the upper boundary of fluvio-marine deposits (Fig. 3, Fig. 4), (ii) the distinction between sand and sand-gravel-cobble layers (Fig. 8) or (iii) the precise identification of the upper and lower boundaries of the clay layer (Fig. 4, Fig. 7).

GPR performance is, however, limited by signal attenuation with depth, which depends on antenna frequency and the properties of the sedimentary layers. Good penetration is typically observed in sandy deposits (e.g. Billières et al., 2025; Billy et al., 2025a; Bristow and
<figure><img src="image_10.png" /><figcaption>**Fig. 8.** A. 3D view of GPR profiles along the sedimentary spit at La Pège; B. Zoom on the longitudinal SE-NW GPR profile showing the interpreted phases of spit progradation, with Qc profile from CPT237 superimposed, and C. Transverse GPR profile to the north, at "60 Bornes" site, with superimposed Qc profiles and BSS LOGs for this sector.</figcaption></figure>

Pucillo, 2006; Costas and FitzGerald, 2011; Hein et al., 2012; Robin et al., 2023), whereas rapid signal loss occurs in the presence of clay-rich layers (e.g. Billy et al., 2024; Gómez-Ortiz et al., 2010). CPT soundings can penetrate beyond the GPR investigation depth, making it possible to extend stratigraphic interpretation to greater depths, down to the CPT refusal depth or to the targeted acquisition limit (Billy et al., 2024). This combined approach was successfully applied in the present study, enabling data acquisition beneath marsh, dune or fluvio-marine deposits beyond the GPR penetration window and up to the limestone upper boundary (Fig. 3, Fig. 7 or Fig. 8). Moreover, in beach environments where saline water induces significant GPR signal attenuation, CPT surveys effectively overcome this limitation (Aguila et al., 2022). In addition, CPT techniques are widely applied in marine geotechnics (for

instance to support site investigations for offshore wind farms or pipes routes; e.g. Gourvenec, 2024; Yang et al., 2025) and represent a valuable source of data to investigate entire sedimentary prism units. In addition, CPTu-derived interstitial pore pressure (u) measurements offer promising perspectives for investigating subsurface hydrological controls in coastal context. However, in coastal environments thick unsaturated zones (i.e. dune) may lead to partial sensor desaturation, underestimated pore pressure values, and potential interpretation biases. Combining CPTu data with geological and hydrogeological investigations is particularly relevant in coastal settings particularly for assessing the influence of subsurface hydrodynamics on coastal geomorphology and contemporary coastal evolution.
## 5.2. Sediment deposit characterization

The internal architecture revealed by GPR imagery enables the delineation of distinct units and sub-units that represent successive developmental phases of the coastal system. These features can be linked to specific depositional environments, such as nearshore zones, open or closed embayment, evidence of dune migration or erosional surfaces, thereby providing key insights into the processes that have shaped the coast and its shoreline to their present configuration. Sediment coring remains essential for obtaining continuous stratigraphic records and collecting samples for dating or laboratory analyses (Alexopoulos et al., 2025; Baumann et al., 2017; Billy et al., 2014; Tanigawa et al., 2025). However, in sandy coastal settings such as dunes, coring operations are often logistically challenging, time-consuming and costly. They frequently suffer from incomplete recovery, including sediment loss and gaps between core sections. In contrast, CPT surveys are generally more efficient, rapid, can reach tens of meters (up to 40 m depth), cost-effective, and provide continuous data acquisition without gaps or lags.

Sandy systems are typically characterized by their grain-size distribution (ranging from fine to coarse, unimodal or bimodal, including parameters such as sorting, skewness, kurtosis; e.g. Barusseau, 2011) and, in more detailed analyses, by grain morphology (rounded, angular, polished, matte). These characteristics provide insight into sediment-transport and depositional conditions and can improve the discrimination and classification of depositional environments (Barusseau, 2011; Blott and Pye, 2001; Flemming, 2007; Liu and Zarillo, 1993; Tanner, 1988). Typically, a single or a few reference samples are collected to represent a given unit. In the present approach, the analysis encompasses the entire unit, allowing for the characterization of all measurement points defining it. Statistical analyses of the point clusters corresponding to each sedimentary unit can thus be performed, such as the covariance analysis presented here as an initial step. More targeted assessments of point dispersion, for example, could also be developed to address specific requirements for unit classification.

In this study, integrated interpretation demonstrates that different sandy deposits exhibit distinctive geotechnical signatures, whether associated with nearshore sediments influenced by fluvio-marine dynamics or dune systems shaped by aeolian processes (Fig. 5). This study further reveals the ability to detect variations in geotechnical response even among individual dune systems with close sand population, reflecting differences in their formation history and sedimentary composition. Several studies have exploited this capability to characterize sedimentary layers in deltaic or coastal-plain settings from different periods. Examples include post-LGM stratigraphic sequences of deltaic systems (Styllas, 2014), the distinction of the Holocene-Pleistocene boundary (Campo et al., 2023), or the identification of stratigraphic tide-dominated successions (Zhang et al., 2018) in coastal plains. Here, the dune field includes modern dunes, longitudinal dunes, and various generations of parabolic dunes (digitated and highly digitated-overlapping; Fig. 2), which display distinct geotechnical responses despite being located within the same area, influenced by the same hydro-system, covered by a maritime pine forest and influenced by a common sediment source.

Looking forward, the systematic characterization of sedimentary units in coastal environments using CPT data would enable direct inter-site comparisons, providing insights into both geotechnical similarities and differences between distinct coastal settings (geographically close areas or areas with a similar context). Beyond site-scale characterization, the systematic integration of CPT-derived SBT into a regional or coastal database represents a promising perspective. When coupled with records of past erosion events and coastal evolution, such datasets could support the classification of sectors according to their sensitivity to erosion, for example. This would enable comparative analyses across sites, linking sediment mechanical properties to observed erosion patterns and improving the predictive assessment of coastal vulnerability. Such an approach could also facilitate the derivation or calibration of

site-specific parameters describing sedimentary and geotechnical properties, which could be incorporated into numerical models of coastal evolution. This would enhance the capacity of these models to represent spatial heterogeneity of coastal areas and improve their predictive accuracy for coastal ecological/morphodynamics forecasting. Finally, the development of geotechnical stability analyses based on CPT data would also provide an additional tool for identifying erosion-prone zones at the local scale.

## 5.3. Below the dune: coastal geological models

Along the Pays-de-Monts coast, although the surveyed sectors are geographically close and share broadly similar geological and geomorphological settings, this work enables the construction of two distinct integrated coastal geological models. The study area is underlain by a fractured limestone substratum and consists of sandy beaches fronted by active foredunes, vegetated dune fields, and inland marshes, yet exhibits significant differences. These schematic representations (Fig. 9) facilitate a clearer understanding of site-specific characteristics by considering (i) the sedimentary units that anchor the sandy coastline, (ii) the connection with adjacent marsh or low-lying environments, and (iii) the relationships with dune systems.

### 5.3.1. A progradational coast with an inland marsh transition overlain by dune field

In the northern sector, at La Parée Grollier coastline (area n°1; Fig. 2A), the coastal system is clearly defined by two domains influenced by open or sheltered conditions. The sandy coastal system has prograded seawards through the accumulation of westward-dipping sandy deposits (extending over several hundred meters) corresponding to nearshore and beach facies (blue-turquoise color in the model; Fig. 9A). Geomorphological markers (regular-longitudinal morphological structures with decimetrical topographic variations; Fig. 6) visible across low-lying areas of the Pays-de-Monts landscape attest to the presence of these paleo-shorelines. Significant sand supply to the beach combined with wind activity has promoted dune edification. Efficient sand remobilization by aeolian processes driving the development of transgressive dune field, migrating landward, mostly likely through sediment recycling (cannibalization) Seaward progradation of the coastline and landward dune migration may have occurred contemporaneously, however establishing the chronological relationship between these processes would requires dating of both aeolian and fluvio-marine units, which lies beyond the scope of this study. However, this study provides a robust framework for future investigations, supporting the identification of key targets (sedimentary units and/or sectors) suitable for geochronological dating. Existing historical cartographic sources, covering the last few centuries, do not provide a sufficiently detailed temporal framework to address the coastal dynamics evidenced here. Future research would therefore benefit from an interdisciplinary approach combining geochronological methods with paleoenvironmental, archaeological, historical and archival investigations, in order to better constrain the timing and rates of coastal progradation as well as dune-field evolution.

Landward, subsurface deposits are quite different, showing silty–clayey layer (2 m thick) that indicates a more sheltered environment associated with low-lying inland marsh (dark green in model; Fig. 9A). A key finding of this work concerns the transition zone between the sandy open coast and the sheltered marsh environment. The exact location of this transition zone was previously unknown. Until now, the position of the marsh/sandy coast boundary has been identified from aerial photographs or topographic DEMs, which identify changes in vegetation or in surface elevation between low-lying marshes and high dune systems, for example. However, this morphological boundary does not necessarily represent the true geological transition. In contrast, the combined GPR-CPT approach reveals that as dunes migrated inland they progressively covered the nearshore/beach (paleo-shoreline) deposits and
<figure><img src="image_11.png" /><figcaption>**Fig. 9.** Conceptual coastal geological models along the Vendean coast: A) a progradational coast and marsh transition overlain by dune field or B) a channel-fill and spit development overlain by dune field.</figcaption></figure>

the western part of the former marsh (Fig. 3 and Fig. 6).

### 5.3.2. A channel-fill and spit development sequence overlain by dune field

In the southern sector, at the 60 Bornes-La Pège (area n°2; Fig. 2B), the coastal system was previously influenced by both marine and fluvial processes. In contrast to the northern sector, the coastline here does not extend seaward (Fig. 9B). The main difference is the presence of a former channel within the landscape, indicating that the mouth was once open to the ocean and that exchanges of water and sediment occurred between the two environments. This interpretation is supported by historical records documenting intensive navigation and trade activities in the area, as well as historical maps as the Masse Map (from 1707; in Suire, 2017). A barrier spit, identified through GPR analysis, shows southeast-northwest progradation (longshore drift influence) and lies parallel to the paleo-channel. This progradation drove the northwestward migration of the channel mouth. Over time, the channel became progressively disconnected from marine influence and was ultimately closed, particularly due to marsh drainage and hydraulic management undertaken to regulate the freshwater-saltwater balance in the marsh area (Baudet et al., 1987; Pinot, 1980; Rousseau, 1968). By the 18th century, the channel had largely infilled with silty-clayey sediments and was effectively isolated from tidal influence. For roughly two centuries thereafter, the low-lying area of the paleo-channel has remained isolated and lacking new sediment input. As a result, accommodation space and sediment accumulation have been limited, and the paleo-channel topography has remained relatively unchanged and presently lies below the modern beach level. Aeolian processes have since led to dune systems covering the paleo-morphology of the former spit and mouth system (Fig. 7A). The combined GPR-CPT approach reveals that a meander of the paleo-channel is still preserved beneath the dunes, very close to the present beach level and previously unknown. Since 18th century this area has been dominated by marine processes, and its former fluvial influence has been progressively forgotten by the population and masked by coastal management plans. Indeed, remnants morphology of a former channel remain discernible in the landscape (topographic DEM; Fig. 2B and Fig. 7A), although part of it are now partially or entirely overlain by agricultural use, urbanization or recreational infrastructures.

### 5.3.3. Benefits of local coastal geological models

Coastal managers often assume that sandy sites are broadly homogeneous and can therefore be managed in the same manner as adjacent areas, overlooking their specific characteristics and inherent heterogeneity. Furthermore, coastal urban development has frequently smoothed or masked distinctive geomorphological features (e.g., landfills, polders, flattened dunes), and can accentuate disruption of land-sea sediment dynamics. Integrating geological knowledge makes it possible to establish a detailed 'identity card' for each site, facilitating site-to-site comparisons and highlighting specific features (Fig. 9).

In the context of climate change and shoreline retreat, a robust understanding of subsurface geology is even more essential, especially to complete the planimetric (2D-horizontal) analyses performed by aerial photographs interpretation (e.g., Boak and Turner, 2005; Cellone et al., 2016; Fellowes et al., 2021; Garcin et al., 2020; Laporte-Fauret et al., 2022; Robin et al., 2023, 2021). Moreover, shoreface-beach-dune evolution or retreat can expose previously unsuspected elements, such as sedimentary units with highly distinct compositions (clay layers to coarse sediment bodies), paleosols (with varying degrees of consolidation within sandy environments; e.g., Mehl et al., 2025; Tastet, 2019) or even historical (fortification of the Atlantic wall or shipwreck; Billy et al., 2025b) or archeological remains (e.g., Gorczyńska et al., 2023; Stéphan et al., 2019). At La Pège, for instance, the clay-rich layer associated with paleo-channel infill lies at an elevation of -1 to +1.5 m NGF, making it highly susceptible to exposure on the beach under ongoing coastal retreat (approximately -0.5 m/yr, and up to -0.85 m/yr in the southern sector since the mid-20th century; Juigner et al., 2017; OR2C).

Geological controls such as rock outcrops, paleosols or paleochannel infills may locally crop out along the shoreface, the beach, or at the dune toe and exert a first-order control on sediment availability and coastal morphodynamics (e.g. Billy et al., 2013; Jackson and Cooper, 2009; Kinsela et al., 2022; Robertson et al., 2008). These inherited geological features can strongly influence sediment availability and wave-sediment interactions, thereby inducing spatial variability in shoreline retreat patterns. However, classical shoreline change models, such as the Bruun rule (Bruun, 1983, 1988), are based on simplifying assumptions including uniform sediment characteristics, simplified cross-shore sediment exchanges, and laterally homogeneous coastal morphology (e.g. Cooper et al., 2020; Vousdoukas et al., 2020; Zhang et al., 2004).
Therefore, such simplified approaches overlook subsurface geological heterogeneities and inherited stratigraphic controls within the coastal domain (shoreface and below the beach). Consequently, incorporating site-specific geological heterogeneities is essential to produce more realistic and robust predictions of coastal evolution. Detailed local coastal geological investigations, including the characterization of depositional systems and simplified geological frameworks such as those presented here (Fig. 9), are therefore required to constrain boundary conditions and to develop more efficient predictions of coastal evolution relevant for coastal management.

Improved knowledge of coastal geology has also significant impli-
cations for understanding coastal ecosystems and hydrogeological sys-
tems (Barlow, 2003). Indeed, geological formations strongly influence
coastal waters, salt marshes and aquifers properties (e.g. the natural
cyclical dynamics of ponds, the seasonal water table fluctuations, the
influence of tidal signal, etc.). Geology has further implications for the
characterization of the freshwater-saltwater transition zone (Barlow,
2003), and determines whether aquifers are single-layer or multi-layers
(LeBlanc et al., 1986; Miller, 1990), etc. In addition, such hydro-
geological knowledge provides valuable insights into anthropogenic is-
sues, such as groundwater pumping, the integration of wastewater
treatment plants, and the management of daily water discharges into the
environment—issues that are particularly relevant to both study sites
(Baudouin et al., 2021; Castillo et al., 2024).

Finally, this approach makes it possible not only to focus on the beach-dune domain but the entire view of the coastal system (dunes, interdunal aeras, fluvio-marine deposits, etc.), including also the low-lying backshore and hinterland areas (marsh, paleo-channel), which are often overlooked in management strategies mainly addressing erosion or marine flooding. Due to their low topography, these areas are nevertheless at risk of inundation through groundwater or surface runoff (for example, like the Dutch coastline; e.g., Dufour, 1998; Stuyfzand, 1995); and could be amplified by subsidence (natural and anthropogenic effects; Thiéblemont et al., 2024). In addition, beyond climate-change-driven sea-level rise, the equilibrium between sea level and the coastal hydrosystem will also have to adjust to sea-level variations (Barlow, 2003). Beyond improving geological knowledge of coastal sites, these insights constitute valuable assets for coastal managers and will be essential for adapting these territories in the context of global change.

6. Conclusion

The multidisciplinary approach developed within the SIBLES project, integrating geophysical, geotechnical, geological, and geomorphological investigations; substantially improves coastal site characterization beyond landscape interpretations or traditional shoreline-change analyses. These environments are often poorly documented in terms of their geological structure or intrinsic parameters and are frequently considered by coastal managers, non-geoscientific, or in numerical coastal models, etc., as spatially homogeneous sand systems. GPR and CPT are a high-performance pairing for coastal investigation and characterization of sedimentary deposits. By identifying the internal architecture of sedimentary deposits, the spatial arrangement of stratigraphic units, and the geotechnical properties of sediments, this approach yields valuable insights into both the formation history and the intrinsic characteristics of the study sites. It can characterize sandy systems with high precision, revealing pronounced singularities of each coastal system, even proximate sites.

In the northern sector (La Parée Grollier), the combined GPR-CPT approach highlights a coastal system shaped by both open and sheltered conditions, with seaward progradation of sandy deposits and landward dune migration. A key outcome is the identification of the true geological transition between the sandy coastline and the inland marsh, previously masked by dune migration and undetectable from surface geomorphology alone. CPT interpretation shows distinct soil behavior types of sand deposits, between dune and fluvio-marine systems, as well

as nuances between each dune type.

In the southern sector (60 Bornes-La Pège), this approach reveals a coastal system shaped by a former fluvial-marine channel, establishing a previously unknown link between former fluvial deposits and modern coastal landforms. The progradation of the barrier spit drove the northwestward migration of the channel mouth. This channel was later infilled and disconnected from tidal influence due to natural processes and human interventions inland (water and marsh management).

One of the main findings of this study is the variety of sediment types beneath the dune system, including sandy fluvial-marine deposits, gravel-cobble deposits, and a 2-meter-thick layer of clayey silt, which is linked to a paleochannel meander. This finding is particularly significant in the context of shoreline retreat and coastal erosion, as the clay-rich paleo-channel may become exposed, and thus have an impact on coastal sediment dynamics. More generally, inherited geological features can strongly influence coastal behaviors and thereby inducing spatial variability in shoreline retreat patterns. Incorporating such geological heterogeneities is therefore essential to develop more realistic and robust predictions of coastal evolution.

Data availability

In-situ data (drillings, cores and boreholes) is public data available on the BSS database, managed by the BRGM, at http://infoterre.brgm.fr/page/banque-sol-bss. The CPTs of this study will also be transferred to the BSS database and will be made available on request. Raw georadar profiles will be made available on request (Billy et al., 2025a). LiDAR data is available on the regional portal GEOPORTAIL.

CRediT authorship contribution statement

**Billy Julie:** Writing – original draft, Visualization, Project adminis-
tration, Methodology, Investigation, Formal analysis, Data curation,
Conceptualization. **Baudouin Vivien:** Writing – original draft, Meth-
odology, Investigation, Funding acquisition, Data curation, Conceptu-
alization. **Noury Gildas:** Writing – review & editing, Investigation,
Formal analysis, Data curation. **Garcin Manuel:** Writing – review &
editing, Data curation, Conceptualization.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This paper is a contribution of SIBLES2 research project co-funded by the BRGM, French Geological survey, and the Pays de La Loire Regional direction of the environment, DREAL. The authors would like to thank Bastien Ruaud, Emeline Rame and Emeric Gautier, for their field assistance, Jacques Deparis for discussions and advice, SILT company for CPT measurement campaign, and Xavier Rachez, Benoit Squiban and Sarah Lamadhi for their support during the project. We also thank the local municipalities of Ocean-Marais-de-Monts and Saint-Gilles-Croix-de-Vie and the French National Forest Office (ONF) for granting access to the study sites. The authors thank the two anonymous reviewers for their comments and suggestions, which helped improve the manuscript.

Data availability

You will find more details in the MS : section “Data Availability.”
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