GNGTS 2016 - Atti del 35° Convegno Nazionale

GNGTS 2016 S essione 3.3 585 On the other side, VED CSEM geometry is indicated when interested in characterizing the conductivity of a local area. Comparisons can be extended also to the resolution of the different geometries (not analyzed here). Key (2009) found that an inline horizontal electric dipole provides better resolution than either broadside or vertical electric dipoles and that the horizontal electric and magnetic fields for any transmitter orientation have better resolution than vertical fields. Proposal of a layer stripping resistivity inversion for TSEM geometry. In the towed streamer configuration the receivers and the transmitter are both at some tens of meters above the sea bottom. For the higher frequencies (e.g. 10 Hz) the EM wavefield cannot reach the sediments, so that the model simplifies to air/water: a first inversion loop obtains the sea water conductivity. We step down to lower frequencies, comparing real data and computed data for the simple air/water model, until the data residual goes over a given threshold. At that step the shallower portion of the sediments are entering the model, and we need to invert the conductivity log for a single unknown layer at the sea bottom. The procedure is repeated until the lowest frequency and the last deepest layer are recovered. Examples of this procedure will be presented at the conference. References Anderson, C., and J. Mattsson, 2010, An integrated approach to marine electromagnetic surveying using a towed streamer and source. Frist Break, 28(5), 71-75. Constable S.; 2010: Ten years of marine CSEM for hydrocarbon exploration. Geophysics, 75, A67–75A81. Constable, S., and C. J. Weiss, 2006, Mapping thin resistors and hydrocarbons with marine EM methods: Insights from 1D modeling: Geophysics, 71, no. 2, G43–G51. Folke E., Mattsson J., Linfoot J. 2012, Simultaneous acquisition of towed EM and 2D seismic – a successful field test: 22nd ASEG Conference & Exhibition 2012. Key, K., 2009, 1D inversion of multicomponent, multifrequency marine CSEM data: Methodology and synthetic studies for resolving thin resistive layers: Geophysics, 74, no. 2. Fig. 3 – Sensitivity plot for VED at frequency 0.2 Hz and SBL acquisition technique. Magenta line is the sensitivity limit. Blue line is sea-air interface.