# Airborne electromagnetics as a tool to image the land-to-sea sedimentary continuum: A complementary geophysical approach to improve coastal characterization

Julie Billy, Pierre-Alexandre Reninger, Anne Raingeard, Eric Chaumillon, Clément Bouvier

► To cite this version:

Julie Billy, Pierre-Alexandre Reninger, Anne Raingeard, Eric Chaumillon, Clément Bouvier. Airborne electromagnetics as a tool to image the land-to-sea sedimentary continuum: A complementary geophysical approach to improve coastal characterization. Marine Geology, 2022, 454, pp.106945. 10.1016/j.margeo.2022.106945. hal-03870156

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Distributed under a Creative Commons Attribution 4.0 International License
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Airborne electromagnetics as a tool to image the land-to-sea

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sedimentary continuum: a complementary geophysical

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approach to improve coastal characterization

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Authors

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BILLY Julie¹, RENINGER Pierre-Alexandre², RAINGEARD Anne²,

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CHAUMILLON Eric³, and BOUVIER Clément⁴

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Affiliation

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1 - BRGM French Geological Survey, Risks and Prevention Direction, 24-25 Av. Léonard de Vinci,

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33600 Pessac, France.

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2- BRGM French Geological Survey, Geo-Resources Direction, Orléans, France

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3- University of La Rochelle, UMR CNRS 7266 LIENSs, La Rochelle, France

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4- BRGM French Geological Survey, Regional Direction of Martinique, Fort de France, Martinique

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Corresponding author

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E-mail address: j.billy@brgm.fr (Julie BILLY)

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16 Abstract

17 Geophysical investigation of the entire coastal zone, including emerged and shallow marine areas, is
18 difficult to conduct especially due to method limitations. Therefore, the land to sea transition zone is
19 often few or not surveyed consequently blanked areas subsist in the backshore, foreshore and upper
20 shoreface domains. This study aims to evaluate the capacity of airborne electromagnetics to image
21 the coastal zone and particularly the land-to-sea sedimentary continuum. It focuses on the Galion
22 and Robert Bays of the Martinique island and is based on the regional survey MarTEM, conducted in
23 2013 with the SkyTEM 304 system. This survey offers coverage of the whole island and provides
24 information on the subsurface every 30 m along flightlines. Airborne electromagnetics results were
25 compared to topo-bathymetric data, seafloor sediment surface map, ortho-photographs, cores, as
26 well very high-resolution seismic profiles from the CARQUAKES French research program.

27 The seafloor appears to be relatively well imaged by the electromagnetic technique and corresponds
28 to a resistivity contrast around 0.3 ohm.m. The soft sediment deposits corresponds to the resistivity
29 values between 0.3 and 0.8 ohm.m. The top and the bottom of the soft sediment layer were
30 extracted from the resistivity model and thickness of sediment deposit was then calculated. Grids of
31 the top of the basement and of the sediment thickness were obtained (130 m pixel) for the two bays.
32 Thus, airborne electromagnetics allows imaging the land to sea continuum and giving a continuous
33 view of the emerged part of the coast as well as shallow water area, over the whole bays and up to
34 the penetration limit of the method (around 20 m for the system used in 2013). This is especially
35 advantageous close to rocky coast and over turbid areas, where marine surveys are difficult to carry
36 out. Advantages and disadvantages of the method are discussed. From this case study, it appears
37 that airborne electromagnetics is an efficient method to investigate sediment bodies lying in the
38 land-to-sea transition zone to complement to the other methods traditionally carried out.

39 Key words:

40 Onshore-offshore connection, large-scale investigation, sediment thickness, bay, AEM, Martinique
41 **1 Introduction**

42 Sedimentary coastal systems are composed of emerged landforms in the backshore (as dunes and
43 marshes) and in the foreshore (beaches and tidal flats) connected to the shoreface (sand sheet,
44 subtidal, bars, sandbanks, etc.) (e.g. Carter, 1988; Roy et al., 1994; Short, 1999). One key point when
45 studying coastal systems is to investigate both the marine and terrestrial domains, and connect
46 them, in order to have a view of the entire system (Figure 1A). The analysis of coastal morphologies
47 through aerial and satellite photographs, videos or timestack images, and through topographic and
48 bathymetric data are essential to characterize coastal systems and record their evolution (Trembanis
49 et al., 2021). Technological advances in the last decades of topo-bathymetric airborne LiDAR surveys
50 are widely recognized as robust and providing a large benefit in highlighting morphological changes
51 of coastal systems. In particular, they provide the ability to extend an join an onshore-offshore
52 surface mapping (e.g. Caballero and Stumpf, 2021; Levoy et al., 2016; Montreuil et al., 2020, 2014).

53 However, only few studies explore stratigraphic and sedimentological records for both onshore and
54 offshore domains (e.g. Billy et al., 2018b; Kirkpatrick and Green, 2018; Oliver et al., 2017; Timmons et
55 al., 2010). Although such kind of knowledge is essential (i) to determine the role of inherited
56 geological framework (e.g. Billy et al., 2018a; Cooper et al., 2018; Kirkpatrick et al., 2019; Kirkpatrick
57 and Green, 2021; Zaremba et al., 2016); (ii) to evaluate sediment volume (e.g. Billy et al., 2018b;
58 Johnson et al., 2020) ; (iii) to provide key insights into coastal dynamics and to highlight the relations
59 hips between shoreface and coastline evolution (e.g. Brunel et al., 2014); or (iv) to provide reliable
60 data of sediment thickness and substrate morphology to better inform coastal zone management
61 (e.g. Stronkhorst et al., 2018) as well as to inform realistic models (numerical of conceptual) of future
62 coastal behavior in relation to climate changes scenarios. Unfortunately, at this time, the desired
63 land-to sea sedimentological, stratigraphic and morphological continuum is rarely revealed,
64 especially because this transition zone suffers from a lack of data to connect shoreface and beach
65 domains.
<figure><img src="image_2.png" /><figcaption>Figure 1: Schematic of a coastal zone (marine domain, transition zone and terrestrial domain) and popular geophysics methods used for their investigations, highlighting difficulties in investigating the transition zone.</figcaption></figure>

Very high resolution (VHR) seismic surveys and Ground-Penetrating Radar (GPR) are the most
popular technologies used to investigate coastal Holocene sedimentary prisms and to define
architecture of coastal sediment systems in the marine and terrestrial domains, respectively. VHR
seismic technology is widely employed by the scientific community to characterize shallow coastal
areas (e.g. Aleman et al., 2014; Allard et al., 2009; Chaumillon et al., 2004; Cooper et al., 2019; Green
et al., 2015; Hinestrosa et al., 2014; Lericolais et al., 2001; Raynal et al., 2009), as well as GPR to
characterize dunes, beach-ridges systems, cheniers, etc. (e.g. Billy et al., 2014; Buynevich and
Fitzgerald, 2003; Engels and Roberts, 2005; Neal et al., 2002; Robin et al., 2021; Rodriguez and
Meyer, 2006; Tamura et al., 2008). Both allow investigation of chrono-stratigraphy, internal
architecture, sedimentary units (discontinuities, erosional surfaces, reflection, etc.). The main limit of
VHR seismic investigation in very-shallow coastal area is the limited water depth close to the shore
and the limitations imposed by waves, ship draught, etc.. Indeed, the seafloor signal multiple
overlays and masks the signal information. Other perturbations such as gas in sediment that induce
an acoustic blanking mask all the signal information (gas wipeout; e.g. Bertin and Chaumillon, 2005;
Cukur et al., 2013; Fanget et al., 2014). The main limit of GPR is the salt-water signal perturbation that masks the radargram, especially on or close to the beach. Finally, for both technologies, the signal could also be attenuated before reaching the lower portions of substrate. In this way, the entire sediment cover is not recorded, limiting the sediment investigation to the maximum geophysical penetration depth. Furthermore, the GPR and VHR seismic surveys are not completely connected; a white zone without data, up to several hundred meters depending on the nearshore gradient, persists at the junction of the marine and terrestrial domains (Figure 1). Other methods are also used for coastal investigation including geotechnics, essential for in-situ records (cores, cone penetration test), or terrestrial geophysics, such as seismic MASW (multichannel analysis of surface wave), ERT (electrical resistivity tomography) (Billy et al., 2020). Even if these geophysical methods are complementary in characterizing and defining the properties of the coastal zone (Figure 1A), they are less frequently employed in coastal research and similarly provide incomplete data of the coastal zone too.

Airborne electromagnetics (AEM) excites the subsurface, emitting a variable magnetic field with the function of time, which induces currents in it, and then measuring the response within a defined recording window (Figure 1B). Recorded data gives information on the electrical resistivity/conductivity variations as a function of depth (Schamper et al., 2013). The depth of local investigation depends on the emitted magnetic moment, the bandwidth used, the subsurface conductivity and the signal/noise ratio. This method has been extensively used for mining exploration for more than 70 years (Allard, 2007; Fountain, 1998). It has the capacity to obtain continuous cover of large areas. Over the last 20 years, the method has proved useful for environmental applications and is used in more and more work such as risk assessment (Rault et al., 2022; Thiery et al., 2021), hydrogeology (Dumont et al., 2021; Vittecoq et al., 2019), and land use planning (Harrison et al., 2021). Recently, Auken et al. (2017) published a review of the use of the AEM in hydrogeological or geotechnical studies.
108 The capacity of the AEM to not be hampered by relief, vegetation, land use and the land to sea
109 transition makes this method interesting for studying coastal areas. It has been already used in many
110 studies to image the salt-water intrusion (Teatini et al., 2011). Some studies have focused on the
111 variations of facies in the sediments and the impact on hydrogeological phenomena (Siemon et al.,
112 2009). Viezzoli et al. (2009) have also explored the use of the AEM to map bathymetry and seafloor
113 sediments as well as Kirkpatrick and Green (2018) to visualize the bedrock topography and sand
114 cover onshore.

115 In this work the aim is to investigate the potential of the AEM to provide a holistic picture of the land
116 to sea sedimentary continuum, in addition to the other methods generally employed in coastal
117 studies (ground and marine geophysics, LiDAR survey, bathymetric survey by boats), especially within
118 the transition zone, where few or no data are generally available. AEM further allows for the
119 obtaining of homogeneous and continuous records from the land to the sea. The very high
120 conductivity of the seawater is of course a barrier to the use of the method in terms of depth
121 imaging. However, a depth of investigation of about 20 m can be obtained with "standard" systems
122 and specifications of the latter can be adapted specifically to this end in order to image deeper. This
123 point will be discussed at the end of this study, but robust modellings have yet to be performed.

124 In this way, two bays along the Martinique Island, in the Caribbean Sea, were studied, based on an
125 existing AEM survey (acquired in 2013 and covering the whole island (Deparis et al., 2014) and
126 available public data (cores, sea-floor sediment distribution, topo-bathymetry). We also compared
127 AEM results with VHR seismic data recently acquired during the CARESSE cruise (CARaïbes
128 Enregistrement Sédimentaire Séismes tEmpêtes tsunamis, July 2021) in the frame of the CARQUAKES
129 research program. This study provides an example of a continuous onshore-offshore geophysical
130 record especially appropriate to large-scale investigation of the coastal zone. The AEM results enable
131 mapping of the substrate depth morphology and sediment thickness for the whole land-to-sea
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coastal areas. Adaptation of the parameters of an AEM system is discussed and suggestions provided
to optimize future AEM surveys in the coastal zone.

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## 2 Study area

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Facing the Atlantic Ocean in the Lesser Antilles, Galion and Robert Bays are located on the eastern side of Martinique Island (Figure 2). The entire study area includes these two bays and the surrounding of the Caravel Peninsula (Figure 3). Martinique is a relatively small (~ 1600 km²) volcanic island characterized by a remarkable variety of coastal environments composed of 26 % mangroves, 27% cliffs, 20% gentler-vegetated coastline, 14% sandy beaches, 13 % highly urbanized areas (Nachbaur et al., 2015b) and contrasting sea bottom sediment and morphologies (mud, sand, coral reefs, rock; Figure 3B). This island counts approximately 120 sandy beaches characterized either by volcanic black sand or white coral sand (Bouvier, 2021). Beaches are small in both length (50 m to 4 km) and width (less than 20 m in general), made up of limited volumes of sediment. Mostly due to the topography of the island, 77 % of the population live along the coast below the 20 m contour line (Hübner and Gössling, 2012; Schleupner, 2008), therefore, coastal areas are exposed to anthropogenic pressures.

<figure><img src="image_3.png" /></figure>

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The climate of the Lesser Antilles is tropical humid with alternating seasons, directly controlled by the respective positions of the Azores anti-cyclone, and the Inter-Tropical Convergence Zone (Marshall et al., 2014; Philander et al., 1996). A wet and hot season occurs between June and November, punctuated by tropical storms and cyclones (Enfield and Alfaro, 1999; Krien et al., 2017; Moraes et al., 2022) while a cooler and dry season takes over between December and May (Granger, 1985; Martinez et al., 2019). The offshore wave climate is characterized by an average significant wave height and peak period of 1.9 m and 8.9 s, respectively, mainly controlled by trade winds from the east to northeast that blow relatively steadily all year (Nachbaur et al., 2015a). Tropical depressions, that sometimes become cyclones, can generate higher waves during the wet season (up to 4 m) depending in their current locations. However, both bays remain relatively well sheltered from offshore swell and are dominated by wind-waves. Martinique's coasts have semi-diurnal microtidal tides, with a maximum tidal range of 0.7 m during spring tides

### 2.1.1 AEM data processing

AEM data were processed following the procedure described in Reninger et al., (2020). Over the land, the ground clearance of the system was obtained by resampling the available 1 m Digital Elevation Model to a 25 m grid (consistently with the AEM footprint) and subtracting it from the DGPS elevation. Tilt measurements were processed taking into account the local topography in order to consider an effective tilt at each TDEM data location (Reninger et al., 2015). As part of an environmental study in an anthropized area, particular attention was paid to properly removing the noise from the AEM data. They were processed using a singular value decomposition filter (Reninger et al., 2011). In addition, a stack adapted to the noise level on each AEM data was applied (Reninger et al., 2020). Finally, manual editing was performed mainly to remove inductive/galvanic coupling noises (Sørensen et al., 2000). In order to improve the coverage of the study area, transit lines were also considered during the processing.
Data were then inverted using the Spatially Constrained Inversion algorithm (SCI) (Viezzoli et al., 2008). Each TDEM data is interpreted as a 1-D earth model (EM sounding) divided into n layers, each being defined by a thickness and a resistivity. During the inversion, constraints are applied vertically and spatially on nearby soundings (independently of the flightlines and the ground clearance).

Ground clearance of the system is inverted for, and the depth of investigation is assessed, as a final step in the inversion (Christiansen and Auken, 2012). We thus define a depth from which the model has any influence on the modeled signal. We then obtain a pessimistic and optimistic limit, which correspond to the limit of interpretation and the optimal depth of investigation, respectively. The depth of investigation is assessed as a final step in the inversion and depends locally on the emitted magnetic moment, the ban-width used, the subsurface conductivity and the signal/noise ratio.

Due to the very high conductivity of the sea-water, the inversion parameters had to be adapted to this specific environment. Several tests were performed on the type of the inversion, the thicknesses of the first layers, and the constraints applied. Results were obtained with a “smooth” inversion consisting of 30 layers from 1 to 400 m depth. A high degree of freedom was set for the first layers and then constraints increased rapidly with depth. Over the sea, we set the starting resistivity of the first layers at 0.2 ohm.m.

The ‘smooth” inversion is characterized by multiple layers of fixed thickness. It is effective to image complex geological structures with the lowest dependency on the starting model, but only results in a smoothed view of the resistivity/conductivity variations with depth.

## 2.2 Data collections

Public data collected to characterize the two bays and to constrain the AEM data include (Figure 3):

1) In-situ data, as stratigraphic logs of former cores and boreholes, available on the BSS public database (French data are collected, organized and managed by the BRGM; http://infoterre.brgm.fr/page/banque-sol-bss). The compression or shortening of the cores are not given.
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2) Bathymetric data from the bathymetric DEM HOMONIM (res. 100 m, Shom 2018, https://diffusion.shom.fr/pro/mnt-facade-ants.html) and the Martinique Litto3D® product (Figure 3A), a continuous land-sea altimetric reference model using airborne laser surveys (LiDAR) in addition to multi-beam sounders to complete the bathymetry (res. 1m; https://diffusion.shom.fr/pro/risques/litto3d-mart2016.html). The Litto 3D® DEM is used preferentially and is completed by the HOMONIM DEM on blanked areas (no Litto 3D® data due to turbidity) - called the DEM in the following. To compare bathymetry with TDEM data, the DEM was resampled to 25 m. The DEM assists in distinguishing structures in the shallow areas, such as coral patches etc., and to define the real sea floor depth on the TDEM data.

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3) Seafloor sediment surface map (Figure 3B) from the Martinique Marin Observatory (OMMM is the Observatoire du Milieu Marin Martiniquais) from 2006 (Legrand, 2010, 2009).

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4) Ortho-Photographs of the Martinique Island (© IGN 2017), which help to delineating onshore sediment bodies (beaches, pocket beaches, ponds, etc.) as well as rocky coastal tracts.

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<figure><img src="image_4.png" /></figure>

<figure><img src="image_5.png" /></figure>

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Figure 3: Map of the Galion Bay and Robert Bay with A. HEM Survey and Altitude (m) from Litto 3D (©SHOM), B. Seafloor sediment surface from OMMM (Legrand, 2010, 2009), Cores (from the BSS database; Figure 6) and VHR seismic profiles location from the CARESSE cruise (Figure 10).

Very high-resolution seismic data used in this study were recorded in Galion and Robert Bays during the CARESSE cruise (June 23 to July 12, 2021), aboard the R/V Antea, in the frame of the CARQUAKES research program (funding from the Agence de Recherche Nationale, ANR). An IKB Seistec profiler, a boomer plate associated with a line-in-cone receiver (Simpkin and Davis, 1993) was used during this survey. The source frequencies range from 1 to 10 kHz giving a vertical resolution in the order of 20 cm (Billeaud et al., 2005) and a penetration of tens of meters. Ten seismic profiles spanning a total length of 7.4 km, and 11 seismic profiles with a total length of 16.1 km were recorded in Galion and Robert Bays, respectively. Seismic data processing consisted of swell filtering, bandpass filtering, gain correction and stacking of adjacent traces using the Delph Seismic Interpretation software (v3.0). Conversion from two-way travel time to depth is made using a p-wave velocity of 1500 m.s⁻¹ in water and 1600 m.s⁻¹ in fine sediments. The seismic interpretation was made following the general principles of seismic stratigraphy (Mitchum et al., 1977).

## 3 Results

### 3.1 AEM evaluation

#### 3.1.1 Depth of investigation

In this high-conductivity context, the penetration of the method is limited and the AEM system used in 2013 allows imaging the seafloor only up to about 20 m of water depth; by comparison, the penetration depth is generally between 200 and 400 m (or even more) on the terrestrial part of Martinique Island. The penetration and the resolution of the method depends directly on the seawater column. As the water column increases in depth, resolution decreases at the seafloor until the seafloor is not imaged anymore. The section AA' (Figure 4) shows a typical AEM profile in Gallion Bay from the beach to -20 m msl (intersection with the DEM is indicated by a black line on Figure 4A).
Litto 3D® data are not available in this area (Figure 3A). On each AEM section, the blanked area gives an estimation of the signal penetration over the profile. The imaged resistivity contrasts appear consistent with the DEM: above, the resistivity is very low, consistent with the presence of the sea water, below, when the seafloor is reached, more resistive bodies are observed. Thus, the seafloor appears to be relatively well imaged and corresponds to a resistivity contrast of about 0.3 ohm.m. It can be noted that AEM resolution increases with the conductivity. At very high conductivities, the resolution is theoretically very good and small contrasts can normally be discriminated. For shallow water depths, from zero to -5 m msl, AEM data provide information up to 30-40 m below the seafloor (Figure 4). At -20 m water depth, the penetration is reduced to the first 5 m below the seafloor. No reliable information is obtained beyond this depth. As mentioned above, this limitation is inherent to the AEM method but is also related to the system specification used in 2013(recording time, magnetic moment, etc.). Nonetheless, the AEM dataset acquired in 2013 gives a homogeneous view of the land to sea continuum up to -20 m msl.

Onshore-offshore cross-section A-A'

A. Airborne EM profile

<figure><img src="image_6.png" /></figure>

B. Interpreted cross-section

<figure><img src="image_7.png" /></figure>

Figure 4: Onshore-offshore cross-section A-A' (East-West) A) the resistivity profile (ohm.m), the black line shows the bathymetry extracted from DEM; and B) the interpreted land-to-sea sedimentary continuum of the Galion Bay (resistivity between 0.3 and 0.8 ohm.m).
255 3.1.2 Transition between water column and seafloor

256 As seen in Figure 4, the seawater column displays lower resistivity values than the seafloor; the
257 contrast seems to be relatively well imaged. In order to analyze the resistivity distribution
258 (occurrence) at the seafloor, at each AEM measurement location, both altitude (Zb in meter msl) and
259 resistivity (Rb in ohm.m) of the seafloor were extracted from the topo-bathymetric DEM and the AEM
260 results, respectively. Figure 5 displays both Rb as a function of Zb and Rb on a map. Values at the
261 seafloor have a resistivity close to 0.3 ohm.m (78 % of Rb values are < 0.4 ohm.m and 38 % have a
262 value of 0.30 ± 0.05 ohm.m). Resistivities higher than 0.5 ohm.m (12% of Rb values) are mainly
263 located close to rocky coast or outcrop, where very few or no sediments are present and where
264 bathymetry is highly variable. At these places, the imaged resistivity of the seafloor is higher (Figure 7
265 and Figure 8) explaining the observed higher resistivities. It is important to note that the resistivity
266 model was obtained achieving a smooth inversion; which means that variations in resistivity with
267 depth are smoothed, therefore a distinct interface cannot be imaged accurately.

268 With regard to the inversion algorithm used, the obtained results appear consistent and stable in
269 terms of value, with a slight difference if sediments are present or not, and in terms of imagery, as
270 seen on profiles AA' and as shown by the other profiles discussed in the following (Figure 6, Figure 7
271 and Figure 8). Thus, a resistivity contrast of 0.3 ohm.m was considered for the seafloor where
272 sediments are present, and it was assumed that the water column had a lower resistivity.
<figure><img src="image_8.png" /><figcaption>Figure 5: Resistivity values of the seafloor (black dots) and occurrences per altitude (left) and mapped resistivity of the seafloor (right).</figcaption></figure>

### 3.1.3 AEM and geological log

The AEM results were compared with existing cores and geological logs close to the shore (Figure 3B and see Table 1). The geological log n#6 (core BSS002NNVY) displays 10 m of soft sediments deposited in the Galion Bay above a hard basement (Figure 6). These comprise 2.5 m of sand and 7.5 m of clay, from -7.5 to -10 m and -10 to -17.5 m msl, respectively. This core was projected on the closest AEM profile i.e. the cross-section BB' (Figure 3). The resistivity contrasts appear to be consistent with the core results. The soft sediment deposits would correspond to the resistivity values between 0.3 and 0.8 ohm.m (in green). Higher resistivity values are associated with the basement (yellow to brown), consistently to what is observed onshore, over the basement (Figure 4 and Figure 6). The nature of the basal unit is not defined in the log, however it is assumed here that it is a consolidated unit (e.g.: rock, former reef). So, this result confirms that the transition between the water column and the seafloor is around 0.3 ohm.m.
<figure><img src="image_9.png" /></figure>

Figure 6: Geological log of core BSS002NNVY (right; short name n#6) and transposed projection on to the closest EM profile (location of the profile in Figure 3; black line shows bathymetry).

## 3.2 AEM Interpretation

### 3.2.1 Soft sediment deposit and basal boundary: profiles analysis

The four cross-sections (Figure 3) AA' (east-west orientation; Figure 4), BB', CC' and DD' (north-south orientation; Figure 6, Figure 7 and Figure 8, respectively) display different resistivity profiles obtained in Galion Bay. For each profile, the topo-bathymetric DEM is consistent with the AEM results. The interface between basement and soft sediment (yellow to green transition, 0.8 ohm.m) is imaged onshore, close to the shoreline and offshore. Onshore, basement deepens rapidly up to -25 m msl (Figure 6). In this bay, its morphology is highly variable especially (with depressions and high points) from north to south (Figure 7C and Figure 8C) which reflects the presence of the Galion Islet and the Pointe Jean-Claude (Figure 2). Sediment deposits are thick in the whole bay (Figure 9A), especially onshore and in the shallow to very shallow areas (< 2.5 m water depth). The cross-sections display the continuity of soft sediment deposit bodies from land up to 20 m water depth, without any perturbation related to environment changes, neither by brackish or salt water in sediment, beach, the zone of breaking waves or seawater column. The imaged seafloor also allows the identification of patches of coral reef. One of these patches is imaged on the section DD' (between 3700 and 4000 m of distance; Figure 8). However, coral reef has also a resistivity between 0.3 and 0.8 ohm.m along this profile like soft sediment. This resistivity is probably due to the shape, constitution and the porous texture of coral reefs. Therefore, identification of patches of coral reef should be done using both seafloor morphology and resistivity value.
Cross-section CC'

A. Topo-bathymetric profile

<figure><img src="image_10.png" /></figure>

B. Airborne EM profile

<figure><img src="image_11.png" /></figure>

C. Interpreted cross-section

<figure><img src="image_12.png" /></figure>

* msl: mean sea level

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<caption>Figure 7: Cross-section CC' with A) topo-bathymetric profile, B) the resistivity profile and C) the interpreted land-to-sea sedimentary continuum of the Galion Bay (location of the profile in Figure 3; black line in B) shows topo-bathymetric profile).</caption>

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Cross-section DD'

A. Topo-bathymetric profile

<figure><img src="image_13.png" /></figure>

B. Airborne EM profile

<figure><img src="image_14.png" /></figure>

C. Interpreted cross-section

<figure><img src="image_15.png" /><figcaption>Figure 8: Cross-section DD' with A) topo-bathymetric profile, B) the resistivity profile and C) the interpreted land-to-sea sedimentary continuum of the Galion Bay (location of the profile in Figure 3; black line in B) shows topo-bathymetric profile).</figcaption></figure>

### 3.2.2 Maps of coastal sediment cover and top of the hard basement

The top and the bottom of the soft sediment layer were extracted from the resistivity model. The thickness of sediment deposit was then calculated. Grids of the top of the basement and of the sediment thickness were obtained (130 m pixel; Figure 9) over the entire study area, including the two bays and the area north of Caravel Peninsula. One main advantage of the AEM derived results is the continuity between the land and marine domains, as highlighted on inland areas named a-e in Figure 9B.

The two bays display different morphologies, both in terms of the basement elevation and sediment cover. Galion Bay basement is defined by a basin-shape with a flat area at the center (from -20 to -
25 m msl) while Robert Bay displays a gentle slope with several incisions (Figure 9B). These incisions could be former incised-valleys, which are connected to the land, and progressively filled by sediments. Focusing on the sediment cover (Figure 9A), the sediment thickness in Galion's Bay is relatively uniform, up to 15-20 m in the shallow area, and from 5 to 10 m elsewhere. In contrast, the thickness of the Robert Bay sediment is more variable. The northeastern part of the bay is characterized by a thickness of about 15 m, while elsewhere, the thickness is around 5 m. In addition, the two grids provide significant information for the area around the Caravel Peninsula, where more classical surveys, using small boats, are difficult to conduct due to the coastal configuration and the proximity of rocky outcrops. North of the Caravel Peninsula, the substrate slope is steep and reaches -25 msl only 200 m from the shore. In this area, sediment thickness is significantly reduced (Figure 7), with the exception of Tartane Bay, where sediment thickness can reach 17 m (f. in Figure 9). Likewise, the small Treasure Bay, at the east of the peninsula, is characterized by a basement at -20/-25 m msl and a sediment thickness up to 12 m.

<figure><img src="image_16.png" /><figcaption>Figure 9: Grids derived from AEM results A. sediment thickness (sand and clay) B. Top of the basement substrate.</figcaption></figure>
The grids were compared to the eleven geological logs available close to the shore (Figure 3), and the results are summarized in Table 1. Such logs provide a sparse and localized information on sediment thickness. They give either the total sediment thickness at a given location, when the basement was reached, or a minimum sediment thickness when coring stopped before reaching the basement. Three logs reveal the total sediment thickness n#1, 2 and 6. Grids are highly consistent with cores n#1 and 2. For core n#6, grid values are a little bit higher (10 m from core vs 12 m from the grid, and from 10.7 to 13 if considering closer cells) but this remains fully acceptable given the resolution of the AEM method and the achieved interpolation. Moreover, looking at the closest resistivity profile, the AEM result was revealed to be highly consistent with core 6 results (Figure 6). Five 2 to 3 m-long logs only sampled superficial sediments (Table 1), while the grid displays sediment thickness of about 4.6 to 12.5 m. At Robert Bay, only one log is available (n#11) and is located inland. It shows a meter of soft sediment lying on very altered tuff lying itself on tuff and clay (11.5 m thick), nevertheless the area was not surveyed.

<table><caption>Table 1: Comparison of punctual in-situ cores log (Figure 3) and corresponding, or closest, sediment thickness (m) and basement depth (m amsl) from the grids (pixel size 130 m x 130 m).</caption><thead><tr><th>Short Name<br>Code BSS</th><th>1<br>002NNCL</th><th>2<br>002NNCM</th><th>3<br>004AFDA</th><th>4<br>004AFDB</th><th>5<br>004AFDC</th><th>6<br>002NNVY</th><th>7<br>002NNVZ</th><th>8<br>004AFAT</th><th>9<br>004AFBX</th><th>10<br>004AFBZ</th><th>11<br>002NNVW</th></tr></thead><tbody><tr><td>Z (m msl)</td><td>-0.8</td><td>-1.3</td><td>0.6</td><td>20</td><td>1.3</td><td>-7.6</td><td>1</td><td>3</td><td>1.1</td><td>4.4</td><td>922</td></tr><tr><th colspan="13">Geologic Log (in situ - punctual data available)</th></tr><tr><td>Soft sediment<br>thickness (m)</td><td>8.5</td><td>9</td><td>at least<br>2 m</td><td>at least<br>2 m</td><td>at least<br>3.4 m</td><td>10</td><td>10</td><td>at least<br>3 m</td><td>at least<br>3 m</td><td>at least<br>3 m</td><td>at least<br>1 m</td></tr><tr><td>Base of the log<br>(if description)</td><td colspan="2">Tuff &amp; Compact<br>Tuff/clay</td><td>Compact clay</td><td>ND</td><td>Altered<br>substratum</td><td>ND</td><td>Compact clay</td><td>ND</td><td>ND</td><td>ND</td><td>ND</td><td>Very altered Tuff<br>above Tuff/clay</td></tr><tr><th colspan="13">Grids from TDEM data (pixel size : 130 m x 130 m)</th></tr><tr><td rowspan="2">Sediment<br>thickness (m)</td><td>Grid<br>pixel value</td><td>8.5</td><td>8.5</td><td>ND</td><td>Not<br>surveyed</td><td>4.6</td><td>12.1</td><td>ND</td><td>8.2</td><td>ND</td><td>Not<br>surveyed</td><td>Not<br>surveyed</td></tr><tr><td>Closest cell(s)</td><td>-</td><td>-</td><td>9.9</td><td>Not<br>surveyed</td><td>-</td><td>10.7/13</td><td>5 to 9</td><td>8/11</td><td>12.5</td><td>Not<br>surveyed</td><td>Not<br>surveyed</td></tr><tr><td rowspan="2">Basement alt.<br>(m amsl)</td><td>Grid<br>pixel value</td><td>-9.9</td><td>-9.9</td><td>ND</td><td>Not<br>surveyed</td><td>-4.5</td><td>-19.7</td><td>ND</td><td>-5.7 m</td><td>ND</td><td>Not<br>surveyed</td><td>Not<br>surveyed</td></tr><tr><td>Closest<br>pixel(s)</td><td>-</td><td>-</td><td>-10.2</td><td>Not<br>surveyed</td><td>-</td><td>-17.8<br>to -20.8</td><td>-4.5<br>to -9.8</td><td>-6.1/-8.5</td><td>-12.75</td><td>Not<br>surveyed</td><td>Not<br>surveyed</td></tr></tbody></table>
356 3.2.3 Comparison with Seismic profiles

357 In order to compare AEM and seismic results, one seismic profile for each bay was selected:
358 Galion_04 and Robert_04 (Figure 3B). We focus on the upper part of the records, from 0 to 30 ms
359 twtt (millisecond two-way travel time), where most of the reflectors are observed.

360 From base to top, three main reflectors are observed, R1, R2 and R3 (Figure 10B-C). (i) R1 consists of
361 high amplitude, low frequency, discontinuous reflectors. R1 is characterized by a series of mounds
362 and troughs and is located between 22.6 and 25.6 ms twtt on Robert_04 profile, which is between 17
363 and 19.5 m below msl. On profile Galion_04, R1 is located between 23 and 27 ms twtt, which is
364 between 17.5 and 20.5 m below msl (Figure 10C and Figure 11). (ii) R2 is a low amplitude, moderate
365 frequency, discontinuous reflector and displays a mound shape on profile Galion_04. (iii) R3
366 comprises a low to high amplitude, moderate frequency, and discontinuous reflector. R3 pointes to a
367 significant subhorizontal to slightly inclined acoustic impedance change. The maximum dip of R3 is
368 about 0.6° on Galion_04. The profiles recorded during the CARESSE cruise were collected in
369 shoreward continuity to the sparker profile Mr08_20 published by Leclerc et al. (2015) (Figure 10A).
370 R1 can be correlated with the boundary between their U1R and U1L seismic units observed on this
371 sparker profile (Leclerc et al., 2015). Leclerc et al. (2015) interpreted U1R and U1L as post-LGM
372 fringing reefs and lagoon deposits (U1L in yellow in Figure 10A), respectively. This boundary is
373 consistent with the base of the sediment deposit imaged on AEM profiles and grids in these sectors
374 of the bays (Figure 9 and Figure 11) even with the average data obtained by the AEM Analysis (c.f.
375 130 m grid). Thus, based on this characterization, the sediment deposits above R1 are also
376 considered as lagoon deposits (Figure 10).
<figure><img src="image_17.png" /></figure>

Figure 10 : A. Seismic profile Mr08_20 modified from Leclerc et al. (2015), where U1L is interpreted as lagoon deposits (yellow). B. Galion_04 and BC. Robert_04 seismic profiles (Figure 3) from the CARESSE campaign and the observed discontinuities R1, R2 and R3 main reflectors. Sediment deposits above R3 are interpreted as lagoon deposits (yellow).
<figure><img src="image_18.png" /><figcaption>Figure 11 : A. Location map and B. 3D view of the seismic profiles (red lines), AEM survey (bleu dotes) and DTM of the substrate depth derived from AEM Analyses at Galion Bay thanks DELPH Road Map Software (V5.0). The vertical exaggeration (V.E.) is 10 and th. means thickness (in meter). For each geosection (seismic profile image), the conversion from two-way travel time to depth is made using a p-wave velocity of 1600 m.s⁻¹ in fine sediments.</figcaption></figure>

# 4 Discussion

## 4.1 AEM

### 4.1.1 Towards optimized specifications

As seawater is extremely conductive (lower than 1 Ohm.m), underwater AEM results are altered in terms of depth of investigation and resolution, compared to close in-land areas with the same acquisition parameters. In the scope of this study, some quick and preliminary tests were achieved to better comprehend how the system parameters affect the AEM imaging capability in this shallow marine environment. Models were tested to (i) evaluate the acquisition parameters used (MartEM) and (ii) estimate more suitable parameters for deeper imaging of the seafloor.

First, a homogeneous basement was modeled, to evaluate the maximum water depth, for identifying the bathymetry. Using similar parameters as those used in 2013, it would be possible, in perfect conditions, to image the bathymetry up to approximately 30 m of water depth. Recording the signal at later times (i.e. using a larger recording window) should improve the penetration. Then, with a constant water depth, the ground resistivity was changed to evaluate the limits to differentiate the nature of the seafloor. Under 10 m of water depth, similar parameters as those used in 2013 would be sufficient to evidence resistivity changes in the seafloor. At a depth of 20 m, recording at later
402

times turns out as necessary to achieve a good resolution. Thus, a recording window of at least 15-16 ms would better allow for the imaging of the contrast represented by the seafloor. Finally, a model of the ground consisting of two layers was computed in order to compare its resistivity to the resistivity of a homogeneous ground under the sea. Again, later times could allow better differentiating between the two layers, even with only 10 meters of seawater.

403

404

405

406

407

In the shallow marine environments, the amplitude of the signal is generally high, therefore the noise is generally not a problem and it is conceivable to record at later times. An increase in magnetic moment could still be interesting to improve the signal to noise ratio at later times, given that very early times are not very useful in such environments, where the decrease of the resolution with time is smaller. Results show that parameters used in 2013 can provide good results up to 10-20 m of water depth. However, recording the ground response up to 15-16 ms turned out more suitable for characterizing the sediment deposits and substrate under sea water. These recording times are reasonable and can be easily achieved with today's AEM systems. Recording times after 15-16 ms are less common but likewise perfectly achievable with some AEM systems, but this would make the system less versatile in terms of imaging, knowing that a compromise must be made to obtain an optimized result on land and still keep the best resolution throughout the whole survey.

408

409

410

411

412

413

414

415

416

417

418

As a result of these preliminary tests, some current AEM systems would already be quite suitable for studying the land to sea continuum and could be used without improvements. However, we must also keep in mind that whatever the system used, the flight plan is of first importance and contributes heavily in the success of the overall survey.

419

420

421

422

### 4.1.2 EM processing

423

The processing applied was designed to preserve the lateral resolution (i.e. between AEM soundings). This is particularly important in such areas where high contrasts are expected between the land and the sea and some stacking techniques could result in adding noise in the data (Reninger et al., 2020). Regarding the inversion, several tests were conducted in order to improve the results

424

425

426
and avoid artefacts. In this highly conductive environment, the thicknesses of the layers can be reduced. Attempts were made with several thicknesses for the first layer (the thickness of the other layers increases logarithmically). It was found that a thickness of 1 m was a good compromise to: (1) identify the shallow water column near the shore; (2) avoid artefacts due to too low a thickness, (3) obtain reliable results inland and; (4) remain consistent with the required resolution for the study of the land to sea continuum. A thickness higher than 1 m did not allow identification of a reliable water column close to the shore. The same observations were made if the constraints applied during the inversion were too high. The starting resistivity is also an important parameter. If the latter is too high for the first layers, this results in a resistive artefact for these layers. The value was therefore lowered.

The aim of this study was to show the capacity of the AEM method to provide useful and reliable information for characterizing sediment fill and substratum morphology in a coastal area. Therefore, we did not use bathymetry to constrain the AEM results in order to compare these results independently. It was shown that AEM is able to give a consistent global view on both the onshore and offshore domains. Nevertheless, if a good bathymetry exists, it could be helpful to improve the inversion. One could also imagine using existing conductivity measurements of the seawater in the starting model of the inversion.

Several inversion algorithms are also available. AEM data can be inverted in 1D, using a smooth inversion, like the one used in this work. A few layer (between 3 and 6 layers) or sharp strategy can also be used. This will result in more contrasted results allowing a better discrimination of the different horizons. However, these algorithms are more dependent on the starting models used and can easily lead to a resistivity model which is difficult to interpret and not necessarily representative of all the resistivity contrasts encountered. We suggest these algorithms could be used locally to obtain a more accurate results in terms of interfaces.
Some inversion algorithms exist for the 2D/3D. Many works are currently being carried out in this field. The 2D/3D inversion of several thousand AEM data, over a large area, is not easy to process.
Moreover, in highly contrasted areas, like the coastal setting in this study, a good starting model is essential, at the risk of having artefacts in the results, and is surely more complicated to manage and build than in 1D.

Finally, stochastic inversions exist and allow a better comprehension of the resolution of the method.
Such inversions could therefore be very interesting to study the seafloor and the contrasts below.
However, they remain difficult to achieve over large areas. Once again, this kind of inversions could be restricted to specific areas in order to better characterize the subsurface.

## 4.2 Improving knowledge across the land-to-sea continuum

### 4.2.1 A complementary method to provide a large-scale imaging of the coast

Very shallow water areas are difficult to investigate. Surveys using a boat need calm weather conditions, are functions of tide regime, tidal-range, and are time consuming. For example, VHR seismic could be limited in such domains (i.e. multiple hides the data, low signal penetration depth) or unworkable. It is also complicated for coring in such areas, because specific barges with low draught are required and a lot of time is needed to penetrate deeply. Even for studies exploring stratigraphic and sedimentological records for both onshore and offshore domains as Timmons et al. (2010), Billy et al. (2018b) or Oliver et al. (2017), the land-to-sea imaging is not complete and blanked areas exist with such usual methods. In our study, only three cores reach the basement and thus provide scant information on the basal unit (Table 1) and few seismic profiles are available to describe the bay at a suitable horizontal resolution. This is a remarkable example of the lack of data at the land-to-sea transition and more widely on the shape and depth of the basement on a coastal area (Figure 1).

In addition, very shallow areas (less than 4 m water depth) may cover large areas especially within bays and lagoons. In Galion and Robert Bays, such areas between isocontours 0 and -4 m (lines in red
on the Figure 12A), correspond to a significant proportion of the bay surface: up to a band of 1 km wide for each bay, and about 25% of their surface. To the north, the Caravel Peninsula is characterized by rocky coasts, steep foreshores, small beaches or small bays (as the Treasure Bay). Surveys using a boat are also difficult in this configuration, and this increases uninvestigated surfaces. Indeed, it can be assumed that with conventional methods at least one third of the surface of the studied bay could not have been surveyed. Here, the spatial coverage of AEM (Figure 12B) encompasses the marine and terrestrial domains, including shallow water areas which are difficult to investigate with the other methods. Therefore, AEM gives a continuous view of the emerged part of the coast, including sectors close to rocky and poorly navigable coast and the shallow turbid areas of the bays, up to the limit of the signal penetration close to 20 m depth (Figure 9). Most importantly, it results in a homogeneous coverage of the bays, with a unique method, onshore as well as offshore therefore allowing to image the land to sea continuum (Table 2). In this way, Kirkpatrick and Green (2018) show the first order geological framework control on the coastal and nearshore morphology (west coast of Namibia), in studying the onshore-offshore domains via AEM. They have identified fault locations with this method, and have confirmed these with boomer seismic profiles. They show that the EM is really useful in a full onshore-offshore continuum.
<figure><img src="image_19.png" /><figcaption>Figure 12: A. Evidence of shallow water area difficult to investigate (from 0 to -5 m in red). B. Coverage of the AEM survey for the Galion and Robert Bays.</figcaption></figure>

### 4.2.2 Advantage and difficulties of the AEM method on coastal context

A non-exhaustive list of the advantages and disadvantages of the AEM method are listed in Table 2.
One of the main advantage is that the capacity of AEM is not hampered by relief, vegetation, land use, land to sea transition or waves, and it gives a good characterization of conductive environment like a coastal system. From these results, it appears that AEM is a good imager of such areas providing a homogenous view of the land to sea continuum offering valuable results under the sea (at least up to 20 m below msl) to characterize sediment, thickness and basement morphology.
Moreso, it provides a large-scale investigation with a high acquisition rate (1000 km in ~4 days).
Indeed, it could offer a first image of the coastal zone at a regional scale, especially valuable for areas not yet explored or difficult to explore. However, the method cannot be carried out over urban areas. So, in seaside resorts conventional approaches, as ERT or GPR, must still be used for onshore investigations. A model of the bathymetry could also be derived from these data. It could give a first insight into the bathymetry, where no accurate acoustic measurements are available. However, AEM
508 is not intended to replace LiDAR or bathymetric surveys, the vertical and lateral (along and between
509 flight lines) resolutions being less in the former case.

510 Results from AEM will be useful for research in various fields, coastal management at a regional
511 scale, as well as industrial or applied studies on many issues i.e. underwater works, sand extraction,
512 submarine cable laying, coastal geology (identification of incised valleys or rocky outcrop close to or
513 at the seafloor, etc.). In addition, it appears that these kinds of data are more and more required to
514 provide realistic numerical models of the coastal zone, and are needed at a large spatial scale.
515 Indeed, sediment deposits are not unlimited (in depth and inland) and geological framework can play
516 a main role in coastal changes (e.g. Cooper et al., 2018; Kirkpatrick et al., 2019). Such knowledge
517 must be taken into account by stakeholders to properly anticipate coastal adaptation to climate
518 changes.

519

520

<table><caption>521 Table 2 : Advantage and difficulties of the AEM method on coastal context</caption><thead><tr><th></th><th></th></tr></thead><tbody><tr><td>AEM Advantages</td><td>Not Hampered by<br>Relief<br>Vegetation<br>Land use<br>Land to sea transition<br>Waves<br>Good characterization of conductive environment<br>Complementary with conventional methods used in coastal investigation<br>Shallow areas as lagoons, bays, turbid areas can be investigated<br>Land to sea survey is possible on a same profile with the same technology<br>Investigation is possible through 0 to 20 m depth of the seawater column, and maybe more with a suitable configuration<br>Large scale investigation<br>Homogeneous cover of the studied area<br>High acquisition rate : 1000 km in ~4 days</td></tr><tr><td>AEM Difficulties</td><td>Survey is not allowed in urban area such as seaside resorts<br>Needed local sedimentological (cores, drilling) to constrain interpretation of the AEM results<br>Shallow marine environment is characterized by a very high conductivity presenting slight contrasts (due to salt water saturation of the sediment). All useful information is in the first ohm.meter</td></tr></tbody></table>

522
It appears that AEM is a good complement to the other conventional methods. For example, the AEM results provide the outer shape of sedimentary units, but not the internal architecture like GPR or VHR marine seismic methods. Therefore, it is useful to consider an integration of these complimentary methods into a tool box of investigation and use it according to the objectives and scale of any given study. For instance, to face the challenge of global change (e.g. sea-level rises, morphological changes), a good knowledge of the coastal zone (morphology, structures, geological constraints, dynamic and internal forcing factors; see Cooper et al., 2018) is a necessity, and the AEM could allow covering large areas in order to define areas where more accurate measurements, like VHR seismic profiling, GPR, ERT and coring, could be conducted.

## 5 Conclusion

Much attention is paid to the development and use of airborne methods, and the AEM offers undoubtedly much potential for coastal investigation. Although, the AEM survey used in this study was not specifically designed for studying coastal areas, it provided consistent and reliable information about the sediment thickness, outer shape of sedimentary unit and substratum morphology within two bays of Martinique Island. Moreover, the results display a continuous series of information from the emergent part of the coast, into the shallow bays up to the limit of signal penetration (close to 20 m water depth). As shown, the top and the bottom of the soft sediment layers can be extracted from the resistivity model, and then grids of the top of the basement and of the sediment thickness produced (130 m pixel) for entire bays, especially important in areas unnavigable and difficult to access. Most importantly, this method provides data over the whole bays and particularly the land to sea transition, which is always difficult, if not impossible, to explore with traditional geophysical methods used in coastal studies.

AEM provides a large-scale investigation and is complementary to the methods conventionally used for the study if coastal environments. Moreover, such types of survey can highlight specific areas to be explored in more detail later with traditional methods such as topo-bathymetry, in-situ
548 investigations, marine seismic, ERT, GPR, etc. In some specific cases, AEM can provide useful
549 information where others methods can not conducted such as sectors close rocky coast and shallow
550 turbid areas, or where result in altered for example by bioturbation gas in sediments. AEM can thus
551 replace traditional methods in areas where they cannot be used. Finally, this approach is a good
552 opportunity to quickly obtain large-scale knowledge of the coastal zone required for coastal
553 management, to nourish realistic numerical models, to better anticipate coastal evolution or
554 adaptation.

## 6 Data availability

555 Most of the data are public data available for free from the BRGM, SHOM, IGN or the Martinique
556 Marin Observatory and listed with the appropriate directory in the section 3.2 *Data collections*. AEM
557 surveys are available through *Deparis et al. (2014)*. In-situ data (cores and boreholes) is available on
558 the BSS public database (managed by the BRGM) at http://infoterre.brgm.fr/page/banque-sol-bss.
559

560 HOMONIM Bathymetric data is available at https://diffusion.shom.fr/pro/mnt-facade-ants.html) and
561 the Martinique Litto3D® product is available at https://diffusion.shom.fr/pro/risques/litto3d-
562 mart2016.html. Seafloor sediment surface map is available through the Martinique Marin
563 Observatory reports (Legrand, 2010, 2009). Ortho-Photographs of the Martinique Island (2017) is
564 available at https://geoservices.ign.fr/documentation/donnees/ortho/bdortho. Seismic data are not
565 the property of the BRGM and are part of the CARQUAKES research program (funding from the
566 Agence de Recherche Nationale, ANR), they cannot be made available until the end of research
567 program, contact the chief scientist of the CARESSE cruise and co-author Eric Chaumillon for
568 information.

## 7 Acknowledgments

570 This paper is a contribution of ELECTROLIT project from the RISQNAT and GEOMOD BRGM research
571 programs. The heliborne geophysical survey of Martinique used is this study, from the MartEM
572 program, was co-funded by BRGM, the FEDER funds for Martinique, the Regional Office for
573 Environment Planning and Housing (DEAL), the Regional Council and the Water Office of Martinique
574 (ODE). Authors would also like to thank the CARQUAKES research team (project funded from the
575 Agence de Recherche Nationale, ANR) and especially the participants to the CARESSE cruise.

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812
813
<table><tr><td>841</td><td>CARESSE campaign and the observed discontinuities R1, R2 and R3 main reflectors. Sediment deposits above R3 are interpreted as lagoon deposits (yellow).</td><td>22</td></tr><tr><td>842</td><td>Figure 11: A. Location map and B. 3D view of the seismic profiles (red lines), AEM survey (bleu dotes) and DTM of the substrate depth derived from AEM Analyses at Galion Bay thanks DELPH Road Map Software (V5.0). The vertical exaggeration (V.E.) is 10 and th. means thickness (in meter). For each geosection (seismic profile image), the conversion from two-way travel time to depth is made using a p-wave velocity of 1600 m.s<sup>-1</sup> in fine sediments.</td><td>23</td></tr><tr><td>843</td><td>Figure 12: A. Evidence of shallow water area difficult to investigate (from 0 to -5 m in red). B. Coverage of the AEM survey for the Galion and Robert Bays.</td><td>27</td></tr><tr><td>844</td><td></td><td></td></tr><tr><td>845</td><td></td><td></td></tr><tr><td>846</td><td></td><td></td></tr><tr><td>847</td><td></td><td></td></tr><tr><td>848</td><td></td><td></td></tr><tr><td>849</td><td></td><td></td></tr><tr><td>850</td><td></td><td></td></tr></table>

Tables Captions

852

<table><tr><td>853</td><td>Table 1: Comparison of punctual in-situ cores log (Figure 3) and corresponding, or closest, sediment thickness (m) and basement depth (m amsl) from the girds (pixel size 130 m x 130 m).</td><td>20</td></tr><tr><td>854</td><td>Table 2: Advantage and difficulties of the AEM method on coastal context.</td><td>28</td></tr><tr><td>855</td><td></td><td></td></tr><tr><td>856</td><td></td><td></td></tr></table>