resolution of internal basalt structures offshore sri lanka

Resolution of internal basalt structures offshore Sri Lanka

Noah Stevens1, Victoria Valler1, Juergen Fruehn1, Priyabrata Chatterjee2

1 ION GX Technology, 2 Cairn India

Abstract

Offshore Sri Lanka, basaltic flows and other intrusive bodies (sillsand dykes) hamper efforts to image deeper structures.

In this study, we demonstrate the ability of hi-resolution hybridgridded tomography to resolve the internal structure of an extensivebasaltic flow, revealing intercalated sedimentary units within thebasalt.

These findings are supported by well log measurements whichpenetrate one of the flows.

76th EAGE Conference & Exhibition 2014Amsterdam RAI, The Netherlands, 16-19 June 2014

Introduction

Basaltic flows offshore India are very extensive, especially to thewest where the well-known Deccan Traps form seals for variousreservoir units. Offshore Sri Lanka, there are also basaltic andother intrusive bodies (sills and dykes) which hamper efforts toimage deeper structures. In this study, we were able to resolve theinternal structure of an extensive basaltic flow, revealingintercalated sedimentary units within the basalt, and also to imagethe deeper sub-basalt structure. Well log measurements confirmedthis observation.

Hybrid gridded tomographic resultsFor the vast majority of the study area, anisotropic parametricpicking and ray-based hybrid gridded tomographic inversion (Fruehnet al., 2009; Jones, 2010) worked well for resolving the detailedvelocity field required for successful depth imaging. High-resolution tomographic inversion was able to resolve layering withinthe basalt and identify trapped sediment units (of about 200mthickness) within the basal flows.

During the initial stages of velocity model building, the overburdensediment field was determined. Figure 1 shows results from theseinitial stages: the top of the basalt structure is clearly visibleas a bright reflector at about and below 3.2 km depth.


Figure 1 overburden structure and interval velocity overlay

After picking a constraint layer representing the top of the basaltflows, high-resolution tomographic inversion was employed so as toresolve the internal structure of the basalt layers. Anisotropicautomatic 2nd and 4th order picking was performed on a 50m*50m grid ofCRP Kirchhoff and beam 3D preSDM gathers, using a tomographic cellsizes (adjusted at each iteration) with a minimum spatial size of250m*250m and minimum vertical size of 40m.

Figure 2 shows the interval velocity model and final image of thisprocedure after several iterations of tomographic inversion.


Figure 2 High-resolution tomographic inversion was able to resolve layering within thebasalt and identify trapped sediment units in the basal flows

The structure of the intra-basalt layering was verified with thewell sonic log available over one of the basalt flows (this log wasalso used to calibrate the anisotropic parameterisation). Details ofthe log and corresponding tomographic interval velocity profile areshown in Figure 3. We can discern a high velocity basalt layerencasing a lower velocity sediment layer. The sediment layer isabout 200m thick, with layers of basal of similar thickness bothabove and below it at the well location.

Figure 4 shows CRP preSDM gathers before and after the basalt-updateiteration. The bright reflector central to the image is the topbasalt.

The final anisotropic beam 3D preSDM results show good imaging ofthe deeper layering below the basalt, permitting enhancedinterpretation of the deep sub-basalt structure down to about 10kmdepth. In Figure 5, the top basalt lies at about 4km depth, with thehighly irregular base at about 5km depth (shallowing to the right of


the image). Continuous deep conformable layering is visible belowthis, down to the maximum imaged depth of 10km.

Figure 3 The intra-basalt layering was verified with sonic log ties


Figure 4 CRP gather during basalt update iteration showing improved flatness at top basaltand immediately below (the bright event central to the images). Top: before update, Bottom:after update.


Figure 5 Final 3D anisotropic beam preSDM showing good resolution of the sub-basaltstructures, down to a depth of about 10km.

Conclusions

Delineating sills, dykes, and basalt structures is notoriously difficult due to the irregular nature of these bodies, their propensity to scatter and absorb seismic energy, and the possibilityof them being formed during multiple eruptions with sedimentary layers being deposited between successive flows.

In this case study offshore Sri Lanka, we have obtained good imagingresults by employing hybrid gridded high-resolution ray-based tomographic inversion, producing final images which will facilitate more reliable structural interpretation of these data.

Acknowledgements

We thank Cairn India and ION Geophysical for permission to presentthis work and to Ian Jones for help in preparing the material.

References


Fruehn, J.K., I. F. Jones, V. Valler, P. Sangvai, A. Biswal, & M. Mathur, [2008], Resolving Near-Seabed Velocity Anomalies: Deep Water Offshore Eastern India: Geophysics, 73, No.5, VE235-VE241..

Jones, I.F., 2010, An introduction to velocity model building, EAGE,ISBN 978-90-73781-84-9, 296 pages.


resolution of internal basalt structures offshore sri lanka

Documents