Mapping sea cliff fracturation using passive seismic and self potential responses : case study of the Socoa flysh cliff (Basque Country, France)

EZPONDA is a FEDER funded project, which aims to study both the mechanichal and chemichal processes related to the erosion of the coastal area in the Basque country, France. In the Socoa flysh cliff, the presence of fractures roughly perpendicular to the shoreline control the nucleation and the growth of underground erosion cavities. The ‘Socoa Semaphore cavity’ is the most striking one, with a propagation of the void up to 30 m inland. Mapping sea cliff fracturation extent around this cavity is a critical aspect to anticipate possible future erosion processes.

Assuming the permeability of the fractured material is higher than the permeability of the nonfractured material, mapping water infiltration in the subsurface may be used as a proxy to map the fracturation extent. In this work, we propose to monitor the sea water infiltration during high tide using passive seismic listening and self potential electrical response to illuminate fractures in the surrounding of the ‘Socoa Semaphore cavity’.

72 vertical component autonomous 5 Hz seismic sensor were deployed at the surface over 5000 m2 with an average interstation distance of 10 m. Continuous records were collected between 19/09/2020 and 22/09/2020 (4 days) during a large tidal event to include 8 high tides with a coefficient higher than 100.  It should be noted that the first two days of measurements were carried out over the weekend. The self-potential signals were recorded at the ground surface using a set of 20 nonpolarizable Pb/Pbcl2 electrodes. Data were recording using a Campbell Scientific CR1000 datalogger, with multiplexer chips used to switch between the pole electrodes. Voltage were measured between the 19/09/2020 11am to 21/09/2020 16pm.

The seismic spectrograms show that between 5-20 Hz, anthropological activities such as trafic and harbour modulate the seismic energy for all sensors. In the 20-40 Hz frequency range, the sea height modulates the seismic energy for all sensors, with a seismic energy decreasing as a function to the distance to the coast.  For a large frequency range between 10-50 Hz, we observe that the relative change in median spectral amplitude during high tides with respect to the median amplitude during the full observation period exhibits highest value over a restricted area (400 m2) located east to the the ‘Socoa Semaphore cavity’, which extends far beyond the known void extent. We argue that this area with a singular geophysical signature may be related to the presence of fracturation. Self potential measurement shows a lower noise during the night (around 4 mV) compare to the day (about 10 mV). In addition the noise is higher on Monday (about 20 mV). Self potential measurement show periodic oscillations with a period of 6.4 hours approximately, corresponding to half the tidal cycle. Amplitude variations of self potential signal is more delicate to be interpreted and need further development.