GNGTS 2016 - Atti del 35° Convegno Nazionale

584 GNGTS 2016 S essione 3.3 The sensitivity is plotted as a 2D image for a transmitter-receiver pair (examples in Fig.1 to Fig. 3, panels a, b and c). For a multichannel streamer we compute also the total sensitivity as the maximum sensitivity at each subsurface point for all the receivers (panel d in the figures). The polarization is indicated, in every panel, by a pair of axes directions (e.g. xz ), the first being the polarization of the transmitter (e.g. x ), the second of the receiver (e.g. z ). The blue line is the air/water interface. The magenta line in the subsurface indicates the sensitivity limit, derived from a realistic estimation of the acquisition noise level: blue colors below the sensitivity limit correspond to a “non-observable” region, i.e. a resistive anomaly within this region produces a variation in the reference data below the noise level. We have built “movies” of sensitivity maps for varying frequency, TX-RX position, polarization: they are a very effective way to observe the exploration limits. Results . According to the skin depth effect, higher frequencies have a lower penetration in the subsurface. Regarding the acquisition geometries, TSEM provides an extra propagation of the scattered field in the marine water, slightly decreasing the investigation depth with respect to the SBL method. In general Horizontal Electric Dipoles CSEM with a vessel pulled transmitter (SLB and TSEM) are primarily suited for areal investigation: in shallow water TSEM is becoming very attractive due to the velocity of acquisition, avoiding the necessity to deploy receivers on the seabed, and even permitting to collect simultaneously EM and seismic data (Engelmark et al., 2012). Fig. 1 – Sensitivity plot for HED at frequency 0.2 Hz and SBL acquisition technique. Magenta line is the sensitivity limit. Blue line is sea-air interface. Fig. 2 – Sensitivity plot for HED at frequency 0.2 Hz and TSEM acquisition technique Magenta line is the sensitivity limit. Blue line is sea-air interface.