Research ArticleAirborne electromagnetics as a tool to image the land-to-sea sedimentary continuum: A complementary geophysical approach to improve coastal characterization
Introduction
Sedimentary coastal systems are composed of emerged landforms in the backshore (as dunes and marshes) and in the foreshore (beaches and tidal flats) connected to the shoreface (sand sheet, subtidal, bars, sandbanks, etc.) (e.g. Carter, 1988; Roy et al., 1994; Short, 1999). One key point when studying coastal systems is to investigate both the marine and terrestrial domains, and connect them, in order to have a view of the entire system (Fig. 1A). The analysis of coastal morphologies through aerial and satellite photographs, videos or timestack images, and through topographic and bathymetric data are essential to characterize coastal systems and record their evolution (Trembanis et al., 2021). Technological advances in the last decades of topo-bathymetric airborne LiDAR surveys are widely recognized as robust and providing a large benefit in highlighting morphological changes of coastal systems. In particular, they provide the ability to extend an join an onshore-offshore surface mapping (e.g. Caballero and Stumpf, 2021; Levoy et al., 2016; Montreuil et al., 2020; Montreuil et al., 2014).
However, only few studies explore stratigraphic and sedimentological records for both onshore and offshore domains (e.g. Billy et al., 2018b; Kirkpatrick and Green, 2018; Oliver et al., 2017; Timmons et al., 2010). Although such kind of knowledge is essential (i) to determine the role of inherited geological framework (e.g. Billy et al., 2018a; Cooper et al., 2018; Kirkpatrick et al., 2019; Kirkpatrick and Green, 2021; Zaremba et al., 2016); (ii) to evaluate sediment volume (e.g. Billy et al., 2018b; Johnson et al., 2020); (iii) to provide key insights into coastal dynamics and to highlight the relations hips between shoreface and coastline evolution (e.g. Brunel et al., 2014); or (iv) to provide reliable data of sediment thickness and substrate morphology to better inform coastal zone management (e.g. Stronkhorst et al., 2018) as well as to inform realistic models (numerical of conceptual) of future coastal behavior in relation to climate changes scenarios. Unfortunately, at this time, the desired land-to sea sedimentological, stratigraphic and morphological continuum is rarely revealed, especially because this transition zone suffers from a lack of data to connect shoreface and beach domains.
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 (Fig. 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 (Fig. 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 (Fig. 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.
The capacity of the AEM to not be hampered by relief, vegetation, land use and the land to sea transition makes this method interesting for studying coastal areas. It has been already used in many studies to image the salt-water intrusion (Teatini et al., 2011). Some studies have focused on the variations of facies in the sediments and the impact on hydrogeological phenomena (Siemon et al., 2009). Viezzoli et al. (2009) have also explored the use of the AEM to map bathymetry and seafloor sediments as well as Kirkpatrick and Green (2018) to visualize the bedrock topography and sand cover onshore.
In this work the aim is to investigate the potential of the AEM to provide a holistic picture of the land to sea sedimentary continuum, in addition to the other methods generally employed in coastal studies (ground and marine geophysics, LiDAR survey, bathymetric survey by boats), especially within the transition zone, where few or no data are generally available. AEM further allows for the obtaining of homogeneous and continuous records from the land to the sea. The very high conductivity of the seawater is of course a barrier to the use of the method in terms of depth imaging. However, a depth of investigation of about 20 m can be obtained with “standard” systems and specifications of the latter can be adapted specifically to this end in order to image deeper. This point will be discussed at the end of this study, but robust modellings have yet to be performed.
In this way, two bays along the Martinique Island, in the Caribbean Sea, were studied, based on an existing AEM survey (acquired in 2013 and covering the whole island (Deparis et al., 2014) and available public data (cores, sea-floor sediment distribution, topo-bathymetry). We also compared AEM results with VHR seismic data recently acquired during the CARESSE cruise (CARaïbes Enregistrement Sédimentaire Séismes tEmpêtes tsunamis, July 2021) in the frame of the CARQUAKES research program. This study provides an example of a continuous onshore-offshore geophysical record especially appropriate to large-scale investigation of the coastal zone. The AEM results enable mapping of the substrate depth morphology and sediment thickness for the whole land-to-sea coastal areas. Adaptation of the parameters of an AEM system is discussed and suggestions provided to optimize future AEM surveys in the coastal zone.
Section snippets
Study area
Facing the Atlantic Ocean in the Lesser Antilles, Galion and Robert Bays are located on the eastern side of Martinique Island (Fig. 2). The entire study area includes these two bays and the surrounding of the Caravel Peninsula (Fig. 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.,
AEM survey
An AEM survey was conducted for geological and hydrogeological purposes from January 29th to March 16th 2013 with the SkyTEM 304 system (Sorensen and Auken, 2004) over the entire Martinique island. This survey is described by Deparis et al. (2014) and was supervised by BRGM (French Geological Survey). The survey was flown mainly in a N-S orientation with a 400 m line spacing, and along the W-E direction with a 4000 m line spacing. Locally, it was refined with a line spacing of 200 to 100 m to
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
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
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
Declaration of Competing Interest
None.
Acknowledgments
This paper is a contribution of ELECTROLIT project from the RISQNAT and GEOMOD BRGM research programs. The heliborne geophysical survey of Martinique used is this study, from the MartEM program, was co-funded by BRGM, the FEDER funds for Martinique, the Regional Office for Environment Planning and Housing (DEAL), the Regional Council and the Water Office of Martinique (ODE). Authors would also like to thank the CARQUAKES research team (project funded from the Agence de Recherche Nationale, ANR)
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