# MASW-2d seismic as a tool for investigating the architecture of Holocene coastal deposits: examples in the North of France (Boulonnais and Flemish coastal plain)

Jean-Yves Reynaud, Rachid Ouchaou, Alain Trentesaux, Roa Seif Dine, Nicolas Robin, Olivier Raynal, Franz Rohmer, Thomas Buchwald, Pierre Antoine

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Jean-Yves Reynaud, Rachid Ouchaou, Alain Trentesaux, Roa Seif Dine, Nicolas Robin, et al.. MASW-2d seismic as a tool for investigating the architecture of Holocene coastal deposits: examples in the North of France (Boulonnais and Flemish coastal plain). Jean-Yves Reynaud; Eric Armynot du Châtelet. Short Papers from the 19th French Congress of Sedimentology, Lille, 84, Association des Sédimentologistes Français, pp.81-85, 2025, 2-907205-83-8. 10.70665/HRYX3362. hal-05005438

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# MASW-2D SEISMIC AS A TOOL FOR INVESTIGATING THE ARCHITECTURE OF HOLOCENE COASTAL DEPOSITS: EXAMPLES IN THE NORTH OF FRANCE (BOULONNAIS AND FLEMISH COASTAL PLAIN)

Jean-Yves REYNAUD<fnref n="1" />, Rachid OUCHAOU<fnref n="1" />, Alain TRENTESAUX<fnref n="1" />, Roa SEIF DINE<fnref n="1" />, Nicolas ROBIN<fnref n="2" />, Olivier RAYNAL<fnref n="2" />, Franz ROHMER<fnref n="3" />, Thomas BUCHWALD<fnref n="4" /> and Pierre ANTOINE<fnref n="4" />

Corresponding author: jean-yves.reynaud@univ-lille.fr

**Abstract:** A 24-channel seismic display recording on a DaQLink is used to carry out tomographies of P and S waves down to 40m deep in the Holocene deposits of the coastal Flemish plain west of Dunkirk and the Slack estuary in the Boulonnais. The flat topography along the profiles collected along a road, on the beach, or in farmed fields, makes it possible to avoid complex processing of topographic correction. The results, validated by a number of cores, provide details on the morphology of the incision at the bottom of Holocene transgressive sediments, either infilling estuarine valleys or onlapping bedrock fault scarps. Vertical and horizontal heterogeneities in shear-wave velocity, well expressed in the upper layers, may reflect lateral changes in lithology or water content.

**Keywords:** MASW-2D, seismic tomography, Holocene, incised valleys, coastal plain, France.

## 1 INTRODUCTION

Coastal plains and lowlands are at stake in the context of global warming, which cause coastal erosion and salt intrusion, and the growing pressure on groundwater resources. While lidar and GNSS surveys help assessing the submersion hazards, stratigraphic data on the shallow subsurface are often limited by the availability of sediment cores, thousands of which are necessary to reconstruct the 3D heterogeneities of the related estuarine and deltaic depositional systems and their evolution through the Holocene. In this short paper, we report on a seismic survey held on two sites of Northern France, the Flemish Coastal plain and the Slack River estuary, as part of two PhD projects (Ouchaou, 2023; and Buchwald & Antoine, 2025).

The Flemish Coastal plain is the westernmost part of the Holocene deposits that fringe the Southern Bight of the North Sea from Northern France to Denmark. It results from overfilling of a network of incised valleys by transgressive, dominantly sandy sediments primarily supplied by the Rhine-Schelde system. Hundreds of cores have accounted for the mapping of the valley incision down to 30m below the present-day coastline (Fig. 1A).

<figure><img src="image_2.png" /><figcaption>Figure 1 – A: Isobath map of the base of Quaternary deposits of the Western Flemish Coastal plain (Desoutter, 2020, after Sommé, 1977 and Sommé et al., 2004). Red and green patterns indicate possible pre-Holocene deposits. B: Satellite view of the study area, showing the new cores and the two seismic profiles. A paleovalley has its thalweg 500m to the East of the profiles.</figcaption></figure>

<footnote n="1">¹ Laboratoire d'Océanologie et Géosciences (LOG), UMR 8187, Univ. Lille, CNRS, Univ. Littoral Côte d'Opale, France.</footnote>

<footnote n="2">² Centre Européen de Formation et de Recherches en Environnements Méditerranéens (CEFREM), UMR 5110, Univ. Perpignan Via Domitia, CNRS, France.</footnote>

<footnote n="3">³ GEOREVA, Rue du Puisatier, 35310 Cintré, France.</footnote>

<footnote n="4">⁴ Laboratoire de Géographie Physique (LGP), UMR 8591, Univ. Paris 1, Univ. Paris Est Créteil, France</footnote>
Based on a dense grid of new cores collected as part of a geotechnical survey prior to the southward extension of Dunkirk container port, the stratigraphy of the valley fill was recently specified (Desoutter, 2020). This provided the opportunity of calibrating two seismic profiles performed along and across the western border of one of those valleys (Fig. 1B).

The Slack River estuary, in the Boulonnais Coast, is a much smaller system (Fig. 2A) but the depth of incision is similar, as evidenced by a few boreholes (Fig. 2B). However, the nature of the valley flanks, either terraced fluvial banks or fault scarps, remains to be determined from a better subsurface mapping.

<figure><img src="image_3.png" /><figcaption>Figure 2 - A: Location of the Slack valley on a DEM of the Boulonnais coast (green: elevation below 10m asl; red: up to 50m asl; offshore: light blue above 30m bsl, deep blue: down to 50m bsl). B: Geological map of Marquise (1:50000). Yellow: Holocene dunes. Blue to green: Jurassic to Cretaceous. Location of the two seismic profiles S1 and S2 along drillholes 6-1, 6-2 and 7-4.</figcaption></figure>

## 2 MATERIALS AND METHODS

Two seismic profiles of 650m and 750m were collected W of Loon-Plage (respectively L1 and L2, Fig. 1B), and two others of 230m and 1350m in the Slack valley (respectively S1 and S2, Fig. 2B), using a 24-channel acquisition performed on a DaQLink with 5m interval between receivers. Seismic source was an ABS hammer plate with integrated trigger and a 11kg (for Loon-Plage) or 4kg hammer (for the Slack). Shots were located between receivers. The profiles L1 and L2 were acquired in non-cultivated meadows, the profile S1 along an asphalt road, and the profile S2 along the beach. The flat topography along L1 to S1 profiles was such that no topographic correction was necessary. The S2 profile was acquired during two successive days, from S to N. The first day was windy and rainy, bringing about a lesser quality recording of the first 200m. Therefore, during the second day the profile was acquired higher on the beach, and impinging the coastal barrier at its northern tip. It was thus necessary to integrate the elevation model for processing the P-wave data of this profile. The data were processed using Pickwin© and Crossrefa© modules of the Seislmager© suite software. Both time arrival of P waves and S waves were picked on the wiggles, and the primary mode of surface-wave dispersion, in order to create P-wave and S-wave (MASW-2D) tomographies. The MASW-2D analysis makes it possible to track low-velocity horizons below high-velocity horizons, which is not possible with a classic P-wave refraction processing. The tomographies revealed vertical and lateral changes in velocity that could be compared with the lithologies in cores, and with velocities expected for such lithologies based on a literature review (Tab. 1).

<table><caption>Table 1 - Values of P- and S-velocities for sediments based on Christensen & Mooney (1995), Telford et al. (1990), Lucius et al. (2007), Foti et al. (2013), Coe et al. (2018).</caption><thead><tr><th rowspan="2">Lithology</th><th colspan="2">P-wave velocity (m.s<sup>-1</sup>)</th><th colspan="2">S-wave velocity (m.s<sup>-1</sup>)</th></tr><tr><th>Non-saturated</th><th>Saturated</th><th>Non-saturated</th><th>Saturated</th></tr></thead><tbody><tr><td>Gravel</td><td>300-1200</td><td>1700-2500</td><td>150-600</td><td>200-800</td></tr><tr><td>Sand</td><td>200-900</td><td>1500-2000</td><td>100-350</td><td>150-600</td></tr><tr><td>Silt</td><td>150-500</td><td>1400-1900</td><td>50-200</td><td>100-300</td></tr><tr><td>Clay</td><td>100-400</td><td>1450-1800</td><td>50-150</td><td>50-200</td></tr></tbody></table>
## 3 RESULTS AND DISCUSSION

All profiles show a progressive increase at depth of P and S velocities. The profiles in Loon-Plage show an overall seaward deepening of the surface horizons with P velocities below 2500m.s⁻¹ (Fig. 3). This limit delineates the boundary between saturated, non-cohesive sediments, and rocks (Tab. 1). In the surveyed area, this limit is found around 20-25m, coinciding in the cores with the bottom of Quaternary deposits above the Ypresian claystone (Fig. 3A). The MASW-2D tomography shows that this boundary, less well defined, has S-wave velocities around 300 m.s⁻¹, which is less than expected in saturated gravels and sand, but better coincides with fine-grained deposits (silts, plastic clays). This could correspond to the Eemian clayey Loon Formation, or overlying loessic hillslope facies identified in the cores (Desoutter, 2020). The ups and downs of the valley floor expressed on the S-wave tomography, and not imaged on the P-wave tomography, can be questioned. If not resulting from inversion issues, the lows could point to gullies or tributaries of the main paleovalley, as they are visible only on the NS profile. Internal lithological heterogeneities of the Holocene infill are restricted to thin clay deposits (the non-recovery intervals in the cores) and are not evidenced in both tomographies. No grain-size data along the cores are available to discuss the overall increase at depth of velocity, that likely reflects the increasing compaction.

<figure><img src="image_4.png" /><figcaption>Figure 3 – A: P-wave tomography of profile L2, Flemish coastal plain (location in Fig. 1B). South is to the left. B: S-wave tomography of the same profile obtained by MASW-2D. The lithology of intersected cores is reported: yellow-sand, green-silt, brown-clay, black-peat. The clay at the bottom is the bedrock (Ypresian claystone). Blue: penetrometer profile.</figcaption></figure>

The two profiles obtained in the Slack estuary allow to discuss further the link to lithology, geometry of the valley floor, and also water saturation. The S2 profile shows P-wave variation in depth that is consistent with identification of the non-saturated (coastal barrier / beach berm) to saturated transition, around 1500 m.s⁻¹. The rise of rock-type P-wave velocity values (around 3000 m.s⁻¹) between 50 and 200m along the profile is consistent with the outcropping of Jurassic bedrock at the Petite Pointe aux Oies cape. Similarly, the abrupt rise of the bedrock up to 20m beneath the northern tip of the profile is consistent with the expected scarp created by the Slack fault at the northern border of the valley. There is an overall northward deepening across the valley fill of the 3000 m.s⁻¹ boundary, supposedly matching the deepening of the valley floor towards the fault. However, the grain-size changes tracked in the boreholes cannot be evidenced from neither the P-wave nor the S-wave tomography. The S-wave velocity of the corresponding deposits (sand with interstratified layers of gravel to rock fragments) has a wide range in the saturated zone (150 to 800 m.s-1), possibly explaining local velocity inversions where the coarse layers occur (see borehole 6-1, Fig. 4B).
<figure><img src="image_5.png" /></figure>

Figure 4 – A: P-wave tomography of profile S2, Slack valley (location in Fig. 2). South is to the left. B: S-wave tomography of the same profile obtained by MASW-2D. The grain-size profile of intersected boreholes is reported (coarser to the right).

The S1 profile (Fig. 5) is close to the southern valley border, as indicated by a Jurassic outcrop 300m to the South (Fig. 2B). The borehole 7-4 intersects the Jurassic bedrock (well cemented sandstone and limestone) at a P-wave velocity of 4500 m.s⁻¹, a value consistently higher than for the Ypresian claystone beneath the Flemish plain. This boundary gradually rises to the South, across possible terraces, as evidenced by the 7-3 boreholes (Fig. 5A). By contrast, the S-wave tomography has a dominantly flat or staircase layering, suggesting the profile was too short for a geometric assessment of the intersect layers. The values of S-wave velocities below 150 m.s⁻¹ in the 5 upper meters are consistent with unsaturated fine-grained sediments corresponding to the saltmarsh and eolian dune covers.

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

Figure 5 – A: P-wave tomography of profile S1, Slack valley (location in Fig. 2). South is to the left. Boreholes labelled as in the geological map (Fig. 2B). Inferred lithology: grey-Jurassic limestones with sandstone levels; yellow-Quaternary sand with gravel (7-3) or shelly layers (7-4). B: S-wave tomography of the same profile obtained by MASW-2D.

4 CONCLUSION

These experiments confirm that the bottom of the coastal paleovalleys can be detected with refraction and surface wave analysis of relatively short displays powered with simple hammer. A core control is necessary,
however, to discuss the results. With such control, details of the bottom geometry can be revealed, such as scarps or gullies. More work would be necessary to locate precisely the velocity shifts associated to changes in lithology of the valley fill. The average velocities fall within the span of velocities determined for the expected sediments, with a strong control of compaction and water saturation.

## 5 ACKNOWLEDGMENT

This work was funded by the LOG, CEFREM and SFR Campus de la Mer. The authors would like to thank Noé Frisch for his help on the field, GEOREVA for time spent in processing the Boulonnais data, and Bernadette Tessier for her careful review of the manuscript.

## 6 REFERENCES

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Christensen, N., Mooney, W., 1995. Seismic Velocity Structure and Composition of the Continental Crust: A Global View. Journal of Geophysical Research, 100, 9761-9788.

Coe, J.T., Brandenberg, S.J., Ahdi, S., Kordaji, A., 2018. Geophysical methods for determining the geotechnical engineering properties of earth materials. California Department of Transportation Report Number CA-17-2111. 305 p.

Desoutter, S., 2020. Gravelines-Loon-Plage, Volume 2/3 – Études. Rapport de diagnostic D128113, INRAP Report, 256p.

Foti, S., Lai, C.G., Rix, G.J., Strobbia, C., 2013. Surface Wave Methods for Near-Surface Site Characterization. CRC Press. 458 p.

Lucius, J.E., Langer, W.H., Ellefsen, K.J., 2007. An introduction to using surface geophysics to characterize sand and gravel deposits: U.S. Geological Survey Circular 1310, 33 p.

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Sommé, J., 1977. Les plaines du Nord de la France et leur bordure. Étude géomorphologique. Thèse d'état de l'Université de Lille III, 2 volumes, 810 p.

Sommé, J., Cunat-Bogé, N., Vanhoorne, R., Wouters, K., 2004. La Formation de Loon: Les dépôts pléistocènes marins profonds de la plaine maritime du Nord de la France [The Loon formation: The marine pleistocene deep deposits in the coastal plain of northern France, English abstract]. Quaternaire, 15, 319-327.

Telford, W. M., Geldart, L. P., Sheriff, R. E., 1990. Seismic Methods. In: Applied Geophysics. Cambridge University Press. pp. 136-282.