2D modelling of resistivity and magnetotelluric data from the Belvedere Spinello salt mine,Italy1

The Belvedere Spinello salt mine is located in the Catanzaro Province of Calabria in Southern Italy. An extensive mining program has caused the development of Underground cavities filled with brine and the migration of this brine has been of great environmental concern to the mine owners. This paper presents the results of a multidimensional interpretation of a two-phase resistivity and magnetotelluric (MT) survey that was performed in an attempt to determine the complex conductivity structure of the mine area and to gain information on brine development and migration pathways. Key resistivity soundings were interpreted using a 2.5D algorithm based on the Polozhii decomposition method. The MT data were interpreted using a 2D finite-element code. A conductivity model was developed, integrating available geological and drill-hole information. The interpretation of the MT data, collected five years after the acquisition of the resistivity data, shows a conductive feature of depth that is not resolved in the resistivity interpretation. This feature has been interpreted as a thick brine zone that has developed as a result of mining during the interval between the resistivity and the MT measurements.     

La prospection gravimétrique dans la modélisation d'un substratum sous formation sédimentaire : apport à l'hydrogéologie d'une zone semi-aride du Sud de Madagascar

A gravimetric survey has been carried out in a sedimentary basin, south of Madagascar, in order to define the bedrock morphology. The bedrock is made up of crystalline and volcanic rocks. The objective of the survey was to demonstrate whether a relationship could be established between bedrock morphology and groundwater mineralisation. Indeed, bedrock morphology has been successfully mapped and it is confirmed that most of the low mineralisation wells are located in areas where the slope of the bedrock is trending to the south-west, which ensures a higher hydraulic conductivity, i.e. a faster water flow, than in the other parts of the survey area. To cite this article: H. Rakoto et al., C. R. Geoscience 335 (2003).     

A model study of telluric fields in two-dimensional structures

Summary. In recent years telluric sounding has been replaced by MT (magnetotellurics). However, several new purely telluric parameters, besides the traditional Jacobian, have been shown to be efficient geophysical indicators of lateral conductivity variations. A set of typical two-dimensional structures is analysed to demonstrate the resolving power of the new indicators. For such telluric studies, a wide frequency band is a great asset, especially because the indicators are best displayed as pseudo-sections in the frequency domain. However, a wide frequency range is easily achieved when only the telluric field needs to be measured. In MT the magnetic sensing coils often severely reduce the available bandwidth.     

On the interference of man-made EM fields in the magnetotelluric 'dead band'

During processing of magnetotelluric (MT) data, acquired in a survey carried out in southern Italy, a problem was encountered, connected with the so-called 'dead band' of the MT signal (around 1 Hz). In the apparent resistivity curves of some MT soundings, a V-shaped minimum appeared, centred on the dead-band frequency. This phenomenon turned out to originate from a strong artificial source and was not due to a downward bias of the robust processing techniques adopted. The source distance from the MT sounding locations was such that the V-shaped minimum fell precisely in the dead-band frequency range. Theoretical considerations about fields generated by an electric dipole led us to the probable identification of the source as the d.c.-powered railway between Naples and Bari.