Introduction
Geological setting
Methods and results
Geological data
Geophysical data
Electrical resistivity tomography
Ambient noise
2D arrays
Geotechnical data
The subsoil model
The geological model
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The pyroclastic units form the low-relief ring of the crater (Fig. 5a). These pyroclastic deposits, which consist of stratified tuffs with intercalated lavas boulders, correspond to the high-resistivity domains (h1) located at the NW edge of the ERT1 and at both the E and W edges of the ERT2 (Fig. 3). This volcanic succession forms a tabular body dipping outward from the crater, as constrained by the dip of the bedding (Fig. 2a) and the geometry of the h1 domain in the NW edge of the ERT1 (Fig. 3a). Both ERT profiles show that these high-resistivity domains are bounded at depth by domains showing lower resistivity values, thus suggesting that the pyroclastic units correspond to a vadose zone. We attribute a maximum thickness of about 70 m to this volcanic sequence based on thickness of h1 domain in the NW edge of the ERT1 (Fig. 3a).
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A layer of lacustrine deposits, corresponding to the uppermost 15-m-thick sequence of dark and grey clayey silts drilled at SS1 (Fig. 1d), fills the uppermost part of the maar crater (Fig. 5a). This layer corresponds to sub-horizontal low-resistivity domains in both ERT profiles (l1 in Fig. 3). The attitude of these low-resistivity domains helps to constrain the lateral geometry and thickness of the lacustrine deposits. In particular, pinch-out and/or interfingering geometries can be envisaged for the transition from these lacustrine deposits to the epiclastic deposits.
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The epiclastic deposits form lenticular bodies rimming (in map view) the lacustrine deposits (Fig. 5a). The epiclastic deposits correspond also to the grey sands and volcaniclastic material drilled at SS1 (Fig. 1d). At depth, the thickness and the geometry of these deposits resemble the attitude of the low-to-medium resistivity domains constrained by both ERT profiles (l3 in Fig. 3).
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A chaotic material fills the central part of the crater, just below the lacustrine deposits (Fig. 5a). This intra-crater material can be attributed to slightly bedded syn-eruptive filling made of slumping and collapsed blocks within a finer hosting material. Both ERT profiles show that this filling corresponds to the recurrence of medium- and high-resistivity discontinuous anomalies (m1 and m2 in Fig. 3). Although the nature of this filling is still unknown, it is noteworthy an important variability in thickness and the pinch-and-swell geometries for these resistivity anomalies, which pass from lenticular bodies of high resistivity (> 200 Ωm) to mantling layers of low resistivity (< 60 Ωm).
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A succession of alternating lavas and tuffs, which pre-date the maar structure and extend from the base of capping pyroclastic deposits to about 450-m depth (De Rita et al. 1983; Sottili et al. 2012), can be considered as forming the pre-eruptive substrate. This substrate corresponds to the high-resistivity domains (h2 in Fig. 3) that are disposed in a staircase-like shape, dipping toward the crater centre.
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An undefined material fills the lowermost part of the crater, at depth below 100 m a.s.l. (Fig. 5a). We have no direct knowledge on the nature of this material, and we can only assume to be part of the fragmented, pre-eruptive, substrate (including country rock blocks and rafts) within the unbedded lower diatreme. Both ERT profiles have not enough resolution to detect the geometry and thickness of this fragmented substrate, and an alternation of high- and low-resistivity domain can be only envisaged. We only infer that this material may pass, to depth, to less fractured substrate.
The lithotechnical model
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A lithotype #3a and a lithotype #3b, both corresponding to the fine-grained material filling the central part of the crater (Fig. 5c and Table 1). These lithotypes also correspond to the irregular anomalies showing low-to-medium resistivity. A precise attribution of Vs values for these lithotypes is not possible on the base of our dataset. Anyway, considering a model of increasing Vs with depth, we assume Vs higher for lithotypes #3a and #3b than Vs attributed to lithotype #2. Therefore, we attribute values of Vs in the order of 400 m/s to lithotype #3a and values of Vs in the order of to 700 m/s to lithotype #3b. The highest Vs values have been attributed according to the array-02 (Fig. 3b).
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A lithotype #6b, corresponding to the fragmented, pre-eruptive, substrate filling the lowermost part of the crater (Fig. 5c and Table 1). This lithotype is completely unconstrained in terms of both thickness and Vs. Following our geological model, we consider this lithotype like the lithotype #4 in terms of geotechnical characteristics. To this lithotype, we attribute values of Vs in the order of 1000 m/s that may increase at depth with possible unfractured substrata (lithotype #6a at the bottom of the lithotechnical cross-section of Fig. 5c).
# | Lithotypes | γ (kN/m3) | Vs (m/s) | D0 (%) | G/G0-γc and D-γc curves |
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1 | Lacustrine silty clays/organic clays | 16 | 150 | 1.3 | DSDSS C3 (150 kPa) |
2 | Epiclastic debris and alluvial silty sands | 18 | 250 | 1.0 | DSDSS C5 (200 kPa) |
3a | Chaotic pyroclastic (-sedimentary?) deposit | 18 | 400 | 0.5 | Linear behaviour |
3b | Chaotic sedimentary-lacustrine silty clays (?) | 18 | 700 | 0.5 | Linear behaviour |
4 | Semi-chaotic pyroclastic deposit with dominant lithoid component | 19 | 1000 | 0.5 | Linear behaviour |
5 | Tuffs with lavas boulders | 19 | 700 | 0.5 | Linear behaviour |
6a | Alternation of lavas and tuffs | 21 | 1800 | ||
6b | Alternation of highly jointed/weathered lavas and tuffs | 20 | 1000 | 0.5 | Linear behaviour |