Improvement in TDEM sounding interpretation in presence of induced polarization. A case study in resistive rocks of the Fogo volcano, Cape Verde Islands

A Time Domain Electromagnetic (TDEM) survey was carried out in and around t he caldera of the Fogo volcano, Cape Verde Islands, to detect the low resis tive structures that could be related to groundwater. A sign reversal in th e sounding curves was encountered in central-loop measurements for the soun dings located in the centre of the caldera along three main radial profiles . The negative transients are recorded in the early channels between 6.8 an d 37 mu s. Negative values in an early time transient is an unusual field o bservation, and consequently the first step was to check the data to ascert ain their accuracy and quality. In the second step, three-dimensional (3D) effects are evaluated and ruled out in this zone, while an Induced Polariza tion (IP) phenomenon is observed using Direct Current (DC) sounding measure ments. In the third step, the IP effect is called upon to explain the TDEM distortions; a Cole-Cole dispersive conductivity is found to be adequate to fit the field data. However, the more relevant one-dimensional (1D) model is recovered when both central-loop and offset-loop data are jointly taken into account, thus indicating that an effect of dispersive conductivity is necessary to explain the field data. The 1D electrical structure exhibits f our layers, with decreasing resistivity with depth. Only the first layer is polarizable and its Cole-Cole parameters are m = 0.85, c = 0.8 and tau = 0 .02 ms for chargeability, frequency dependence and time constant, respectiv ely. However, the Cole-Cole parameters deduced from TDEM forward modelling remain different from those deduced from DC/IP sounding. In this volcanic s etting, this IP effect may be caused by the presence of small grains of mag netite and/or by the granularity of effusive products (lapillis). As a conc lusion, it is shown that a modelling using different TDEM data sets is esse ntial to recover the electrical structure of this area. (C) 2000 Elsevier S cience B.V. All rights reserved.