The OCTek Seismic project applies quantitative analytical techniques, including OCTek gravity inversion, to specific ION-BasinSPAN seismic surveys, in order to aid the interpretation and understanding of the seismic data. The techniques and their application are described below.
This is an established method, which has already been applied worldwide to produce the OCTek Gravity Inversion reports listed on the right. Key to the success of the OCTek gravity inversion method is the incorporation of:
• A correction for the lithosphere thermal-gravity-anomaly resulting from rifting and breakup
• A prediction of the thickness of new volcanic crust produced by decompression melting during breakup
OCTek gravity inversion allows us to predict depth-to-Moho and crustal-structure and display this information on PSDM seismic sections.
Sediment-corrected (backstripped) depth-to-basement is compared with both calibrated age-dependent oceanic-water-depths and a synthetic bathymetry profile derived from the results of the gravity inversion. Discrepancies between the backstripped and the predictive models are known as Residual Depth Anomalies. RDAs can be used to:
1) Identify the presence of oceanic crust and the location of the continent-ocean boundary
2) Identify the presence, magnitude and polarity of any present-day mantle dynamic topography
Backstripped subsidence of the basement is used to determine thinning-factors and beta-factors along profiles. This is done using a modified version of the McKenzie subsidence model, which makes allowance for the isostatic effects of new volcanic crust produced during rifting and breakup. The results of the subsidence analysis are compared with the gravity inversion results which make a similar correction.
The combined results of the subsidence analysis and the gravity inversion provide input to petroleum-systems modelling and heat-flow studies
The joint inversion method compares the predicted Moho from the gravity inversion with the interpreted seismic Moho, in both depth and time. The seismic and gravity Moho are brought into coincidence by considering the lateral variations in density and velocity required to achieve this. This:
1) Identifies possible lateral variations in basement type and character,
2) Acts as a quality control on the validity of the interpreted seismic Moho.
The combined results of OCTek Seismic analysis allow us to produce crustal cross-sections which illustrate OCT structure, predict the presence of volcanic additions to the crust and locate the continent-ocean-boundary.