Assignment 2 - School of Ocean and Earth Science

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.
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.