ERT Parameters
Research Studies by T. Lebourg, Geoazur, UNS, Nice, France.

Friday 4 December 2009 by Thomas Lebourg

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Panagopoula

ERT1.

The ERT1 has been carried out along the slope axis (see the figure). It cuts the E-W normal faults and several contacts between calcareous units. The investigation depth is about 300 m with a metrical resolution. The first figure below shows the result of the inversion (RES2DINV, Geotomo Software) obtained after the data filtering from the pole-pole device (Root Mean Square value about 6%). The inverted pseudo-section, called "Electrical Resistivity Image" is the result of an iterative process which tends to minimize the difference between measured and calculated resistivity values. The Root Mean Squared (RMS) error gives a measure of this difference. Further details can be found in the paper by M.H. Loke and R.D. Barker (1996).

The resistivity range of the obtained model is included in the interval [30 to 300 Ω m], individualized in two units: (i) the first one with range value from 30 to 130 Ω m and (ii) a second one with values ranging between 130 and 300 Ω m. We observe also in the two units the evolution of the resistivity, one long vertical structure associated to fault, and a second one deeper. The decreasing of resistivity in the vertical structures can be interpreted by faults zones affected by weathering and water flows. The second zone affected by lower resistivity appears to be under the influence of saline water.

With regard to the ERT1 and geological setting, the range of high resistivity is related to the presence of Cretaceous grey limestone, whereas the smallest range corresponds to purple limestone. The low resistivity vertical zone described in the central part of the profile is connected to a complex zone. The northern E-W fault (on the geological cross section) corresponds with the structure observed on the ERT1. The two geometries are similar, which confirms the choice of the inflection on the faults layout. The contact between Cretaceous grey limestone and the red one is not visible on the ERT1, because, in dry conditions, there is a strong likelihood that these lithologies are electrically homogeneous (V. Chaplot et al, 2004).

ERT2.

The ERT2 was carried out at the foot of the Panagopoula slope, near to the coast on a recent alluvial deposit. According to the geological map and the regional stratigraphic sequence, the rock substratum should be made of Cretaceous grey limestone. The aim of the ERT2 is to characterize the contact between these limestones and red limestones observed 4 km to the west. The inverted model resulting from pole-pole measurements (300 m deep) is presented in the second figure.

Two resistivity units appear: (i) a first ranging between 1 and 130 Ω m covering and (ii) a second one with a range of resistivities from 130 to 350 Ω m. The model obtained is homogeneous with a 150 m thick mass (0 to 130 Ω m) covering the stronger resistivity unit (130 to 350 Ω m), with a minimal thickness of 150 m. This second unit seems however physically less homogeneous than the first one. It presents in its median part, a band of lower resistivities (130 Ω m), of 50 m thick and with a 15° dip toward the west not interpreted.

The figure shows the correlation between the low electrical resistivity values with the Cretaceous grey limestone and the strong values for the red one. The contact between the two physical units (ERT2) corresponds with the geometrical projection of the stratigraphic contact observed a few kilometers to the west. Contrary to the ERT1, grey limestone shows low resistivity values here. Considering the proximity to the sea, we think that the difference in resistivity is due to the infiltrations of sea water. It corresponds with a saline edge whose characteristic geometry (P. Henry, 1964) explains the anomaly of low resistivities in the northern extremity of the ERT1.

Conclusion.

Finally, coupling between surface observations and physical measurement at depth allows a good constraint of geological model and geometrical links between geology/faults and landslides. Particularly, it appears that two main slope deformations affect the slope:

(i) a superficial movement in weathered formations and a deeper,

(ii) a slower movement expressed by shearing outcropping in manmade structures.

Moreover, the upper E-W fault seems to be currently active and to control the western colluvium deposits. An instrumental survey was performed in order to quantify gravitational and tectonic components in these slope deformation processes.

ERT1 and ERT2 with Geological Interpretation