I completed my PhD in June 2005. Below is a summary of some of the work carried out during my doctoral studies. Since my PhD I have been working as a post-doctoral research associate under Dr Nicky White investigating the new field of seismic oceanography.

The seismic expression of Paleogene basalts on the Atlantic Margin


The aim of my PhD is to investigate the purturbation of seismic waves due to sub-surface basalt. I started my PhD in October 2000 and submitted in June 2004. My supervisors are Prof Robert S. White, (Bullard Laboratories) Dr Richard Hobbs (Durham University) and Dr John Smallwood (Amerada Hess). My PhD was funded with a NERC CASE studentship, in conjunction with Amerada Hess Ltd. Click here to see my publications. Once I have passed by viva I will place my thesis on this site. For a non-technical description of my work click here.

Regional Geological Background

Early Paleogene rifting which eventually resulted in the opening of the Atlantic Ocean generated a huge volume of melt. The generation of such melt has been attributed to the combination of lithospheric stretching and the presence of a newly initiated mantle plume now under Iceland. The melt was partitioned into various areas of the crust. It is now believed that a significant proportion ponded at the base of the crust forming an underplate under much of Britain and Ireland. A significant amount also penetrated the crust as intrusive sills and extrusive basalts. The volcanic activity in the Paleogene has left a dramatic mark on the North Atlantic Margin landscape or example the Faroe Islands are composed of  over seven kilometres thickness of layered basalts. Basalt  subcrops in many basins in the region including the Faroe-Shetland Basin and the Rockall Trough and igneous centres are scattered along the margin. Subsequent subsidence of the margin and associated basins buried much of the basalt under sediment except in a few topographically high areas such as the Faroe Islands and northeast Ireland.

Sub-Basalt Imaging

Oil companies, in an attempt to locate new petroleum plays within British and European territory, began exploration of the Atlantic Margin in the late 1980s and early 1990s. Several oil fields were discovered namely Scheihallion and Foinhaven within the Faroe-Shetland Basin, however these are situated eastwards of the feather edge of the Paleogene basalt. It was soon discovered that the principle geophysical tool of oil exploration, seismic reflection  profiling was returning inadequate data from regions where a basalt layer was present.  Seismic energy seemed to be highly attenuated by basalt. Active hydrocarbon systems had been found in numerous places on the margin, therefore  there was no reason why the same should not be true beneath the basalt. The problem was obtaining the data of a quality high enough to calculate where to drill.

Over the last ten years a huge amount of money has been invested in this problem which has straddled the industry academia divide.  Various techniques have been developed to image beneath basalt. New seismic acquisition techniques have been developed, for example using long streamers and/or two ships and large deep sources. Advances in processing have also inproved the sub-basalt image. Additionally non-seismic techniques have been employed (gravity, gradiometrics, MT, compliance) often in combination with seismic data. My PhD is interested in the question of why Atlantic Margin basalts are so attenuative. Many igneous rocks are good transmitters of seismic energy but this does not seem to be the case for sub-aerially erupted basalt flows.

My Research

I have combined seismic and borehole data obtained from the Nothern Rockall Trough with 1D, 2D and 3D modelling strategies to investigate the seismic properties of Atlantic Margin basalts. Observations of the basalts both from outcrop in the Faroes and using borehole data from two wells drilled through significant basalt sequences reveal that they are made up of stacks of individual lava flows of characteristic thickness and large areal extent. Below is a photo of basalt flows from Eysturoy, Faroe Islands.

We might expect seismic energy passing through such a structure to result in reverberations between the layers generating many peg-leg multiples.  I investigate these ideas with 1-dimensional reflectivity modelling. However additionally the layer interfaces are not planar. Below is an image of the top-basalt surface picked from a 3D sesimic survey in the northern Rockall Trough.

The rough nature of basalt layers results in a large degree of scattering of sesimic energy during propogation. These effects are quantified with 3D modelling using the phase-screen modelling approach developed by Richard Hobbs.

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Copyright (c) 2005 Jenny Maresh. All rights reserved