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Feasibility of using the magnetoteliuric method for sub-basalt imaging at Kachchh, India

Applied Geophysics 2008年第1期5卷 74-82页

作者： Dhananjai Pandey lucy macgregor Martin Sinha Satish Singh Bullard Laboratories Madingley RoadUniversity of CambridgeCambridgeUKCB3 0EZ Presently at National Centre for Antarctic and Ocean Research Goa.SOESNational Oceanography CentreSouthamptonUKSO14 3ZH Offshore Hydrocarbon Mapping Plc. AberdeenUKAB23 8GD SOES National Oceanography CentreSouthamptonUKSO14 3ZH Labomtoire de G’eosciences Marines IPG Paris 4 Place Jussieu75252 Paris Cedex 05France

The magnetotelluric （MT） method has been among the favorite supporting tools for seismic imaging of sub-salt and sub-basalt targets. In this paper we present an example from Kachchh, India （where basaltic rocks overlie Mesozoic sedimentary rocks）, and discuss the feasibility of using MT method as an exploration tool in this geological setting. Our results highlight the difference in magnetotelluric response caused by the thin intrabasalt layering. The key issue addressed in this paper is what MT can and cannot provide in such geological settings. First, we compute apparent resistivity and phase response curves using representative resistivity-depth models and borehole data from the study area. Later, we compare these results to assess the plausibility of using MT to image the sub-volcanic sediments at Kachchh. Finally, we substantiate our discussion through one-dimensional inversion of the field observed MT data from this region that exhibits poor sensitivity of MT for thin basalt layers.

关键词： Sub-basalt imaging Deccan Traps Kachchh magnetotelluric.

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The effect of intra-trappean heterogeneities on seismic data： A case study from the Deccan Traps

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Applied Geophysics 2007年第3期4卷 183-193页

作者： Dhananjai Pandey Satish Singh Martin Sinha lucy macgregor Bullard Laboratories Madingley Road University of Cambridge Cambridge UK CB3 0EZ Laboratoire de G＇eosciences Marines IPG Paris 4 Place Jussieu 75252 Paris Cedex 05 France SOES National Oceanography Centre Southampton UK SO 14 3ZH Offshore Hydrocarbon Mapping Plc. Aberdeen UK AB23 8GD

Hydrocarbon exploration interests have renewed the need for developing new sub basalt imaging techniques. One of the most important problems encountered today is seismic imaging below basalt. In recent years, this problem appears to have been overcome partly by using long offset seismic data. However near offset data are yet to be fully utilised due to the complex waveform caused by the surface as well as internal heterogeneity of the basalts. The near normal incidence data, which influence the sub-basalt imaging, are highly useful to understand the internal structure within a basalt layer. The use of converted waves for such targets has been proposed as an alternative in a rather homogeneous basalt layer. With a few synthetic modelling exercises here we highlight the practical difficulties in dealing with more realistic and heterogeneous basalt flow. Full waveform seismograms are computed to understand the effects of intra-trappean sediments on the seismic data. A case study from the Deccan Traps of India is presented in this paper. First, we discuss the effects of intercalated sediments on the overall seismic image. Later, the sonic log data from the field are used to compute the full wave-field response using the reflectivity method and compared with the field data. The feasibility of using mode converted waves （P to S and vice-versa at the top and bottom basalt interfaces） for sub-basalt imaging in Kutch region is discussed through a series of velocity-depth profiles. By comparing with the field data we demonstrate that the effects of multiple thin layering within the basalt can strongly deteriorate the image we seek to interpret and exploit.

关键词： Sub-basalt imaging Kutch Deccan Traps Reflectivity Full-waveform modelling

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