Application of mobile electrical and electrostatic quadripoles during the past ten years has allowed a considerable increase in the size of the surveyed areas, together with keeping a high spatial resolution and a reduction of the total cost of a survey. Two new developments of towed arrays are illustrated here: (1) a pole‐pole array pulled by the operator provides a lightweight solution for mapping large surfaces at a unique given depth of investigation, as shown by the prospection of the Roman‐British city of Wroxeter; and (2) a multipole, multidepth system allows a 3-D investigation of the ground resistivity, as illustrated by the experiments undertaken on the test site of Garchy and on the archaeological site of Montbaron (Indre, France).
The soil‐gas 222Rn concentration had been monitored almost continuously from June 1993 till November 1996 on Taal volcano, Luzon Island, the Philippines. During this measurement period, a singular Mb 7.1 earthquake occurred on November 15, 1994, between Luzon and Mindoro, 48 km south of the volcano. Twenty‐two days before the earthquake, an anomalous increase in soil‐gas radon (peak to background ratio = 6) was recorded, unique in the whole time series. The possible generation of this anomaly by typhoon Teresa, which struck Luzon Island a few days before, was ruled out one year later when super typhoon Angela, the most powerful storm to hit the Philippines in ten years, crossed Luzon Island along almost the same track without triggering a similar disturbance in the radon signal. Consequently, there is strong evidence that the Taal radon anomaly originated in stress accumulation preceding the Mindoro earthquake.
S U M M A R YThe problem of 3-D inverse modelling in Direct Current (DC) surveys is addressed in this paper. First, forward modelling of the response of 3-D bodies in DC surveys is carried out by the moment method. It consists of dividing a volume into N small cells, equivalent to 3N dipoles. The numerical code is checked against published results obtained through algorithms that use either equivalent surface charge densities or a finite-difference approach. Good agreement is found between these methods and a maximum discrepancy of 3 per cent is computed on a widely published test model.Secondly, inverse modelling is carried out by a classical least-squares (LS) scheme that includes the Levenberg-Marquardt constraints formalism. We have tested two approximations: Born, and localized non-linear (LN). The difference between resistivities calculated with and without these approximations is found to be too large for inverse modelling, especially in the case of conductive bodies. We use this inversion scheme for different theoretical 3-D models that consist of two layers (34 cells) under an overburden. It is found, in the case of a vertical contrast, that, when a resistive feature overlays a conductive one, resistivities are resolved very accurately, with a low number of iterations and with a better accuracy than in the case where the conductive feature overlays the resistive one. Despite a slower convergence rate, in the case of both vertical and lateral constrasts, the shape of the body is well resolved, even if a slight discrepancy in the absolute values is noticed, especially for conductive cells. Finally, the stability of the inversion is tested with noisy data.
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