SOES6002: Modelling in Environmental and Earth System Science Assignment 2 Semester 2, 2004 Geophysical Modelling Deadline: Tuesday 16th March, 14:00 At the time of the opening of the N Atlantic ocean – about 60 Ma, during the earliest Tertiary – the location of continental rifting closely coincided with the position of a newly formed mantle plume (the same plume that now underlies Iceland). As a result, large regions of the N Atlantic continental margin were covered by flood basalts erupted at that time. Sedimentary sequences in the North Sea and west of Shetland dating from the time of this rifting contain important hydrocarbon reserves. It is therefore quite likely that other sedimentary sequences of similar age formed on the margin may also be oilbearing. In a large area around the Faeroe islands, though, sediments of the appropriate age have been buried beneath thick sequences of the plume-related flood basalts. This makes exploration of the sediments difficult. Seismic methods have great difficulty in penetrating to beneath the basalts, which tend to be opaque to seismic waves. Drilling in deep water, far offshore in the N Atlantic, is extremely costly. Consequently in may areas around the Faeroes, there is uncertainty both as to whether there are any syn-rift sediments beneath the basalts, and if so, how thick they are. The purpose of this exercise is to use 1-D Occam inversion to analyse a hypothetical data set, which we are told was collected in a CSEM survey in the deep water area SW of the Faeroe islands. You are to analyse the data using the codes ‘occam1d’ and ‘forward’, as per the instruction sheets provided. Instructions and practice will be given during the computer lab class on Thursday and Friday. The data set that you should analyse is contained in the file ‘faeroes.occ’. It consists of CSEM amplitude data from both inline and broadside configurations, at frequencies of 0.25, 0.75 and 8 Hz. You should try to determine: How thick the basalt layer is Whether there is a layer of low resistivity sediments between the basalts and the pre-rift basement If so, how thick the sediment layer is What the typical resistivity of the sediment layer (if any) is. Background geology: The water at the survey site is 5 km deep. The sea floor consists of superficial, young sediments, probably less than 500 m thick, and with resistivity less than 1 ohm-m. The basalts lie immediately beneath this layer. They may be up to 3 or 4 km thick, and should have a high resistivity – 200 to 500 ohm-m. At the base of the sequence, there is ancient pre-rift basement – consisting of rocks with resistivities of upwards of 500 ohm-m. Sandwiched between the basalt and the basement, there may be the layer of sediments in which we are interested. They are likely, if they exist at all, to have a resistivity of a few ohm-m. After completing your initial assessment, you could try adding some a priori information to your modelling. Two pieces of independent information are available. Seismic reflection profiling has identified that the top of the basalt layer lies at a depth of 120 m below the sea floor. Wide-angle seismic data suggest that the top of the pre-rift basement is at a depth of 3.7 +/- 0.2 km below the sea floor. Both of these boundaries are likely to coincide with large, abrupt changes in resistivity. The seismic data can provide no evidence, however, as to whether or not the sub-basalt sediment layer exists, or how thick it might be. References: Sinha, M.C., 1999. Controlled source EM sounding: survey design considerations for hydrocarbon applications. In: Singh, S.C., Barton, P.J. & Sinha, M.C. (Eds), Lithos science report, April 1999, 1, 95-101. Constable, S.C., Parker, R.L. & Constable, C.G., 1987. Occam’s inversion: a practical algorithm for generating smooth models from electromagnetic data. Geophysics, 52, 289-300.