The OCTek Seismic project aims to investigate the structure and tectonics of deepwater rifted margins by applying quantitative geodynamic analysis to interpretations of ION BasinSPAN seismic data. ION Geophysical have provided both the seismic data and the interpretations for the project.

Along the lines of each BasinSPAN OCTek Seismic produces a montage of profiles showing:
• Moho depth and crustal-basement thickness
• Crustal thinning factor and beta factor
• Crustal structure of the Ocean-Continent-Transition (OCT)
• Magmatic-type at the outer margin
• Variations in crustal-basement density and seismic velocity

OCTek Seismic Methodology

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.

OCTek Gravity Inversion

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.

Analysis of Residual Depth Anomalies (RDAs)

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

Backstripping & Subsidence Analysis

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

Joint Inversion of Seismic & Gravity Data

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.

OCTek Seismic Project invested the following geographic areas and SPAN surveys:
• The Black Sea - Black SeaSPAN
• NW shelf of Australia - WestraliaSPAN

The results will be available for purchase by anyone who has licenced the appropriate SPAN-survey or SPAN-lines and will be available on a line-by-line basis for those who have not purchase the whole BasinSPAN.

We believe the results from OCTek Seismic will be of assistance to:
• New Ventures exploration strategy, helping to discriminate continental and oceanic areas, together with crustal and magmatic type, prior to any propriety data acquisition.
•Petroleum Systems modelling, where crustal thickness estimates constrain radiogenic heat input and beta-factors constrain the time-dependent thermal anomaly resulting from rifting and breakup.

If you have an interest in the analysis of any BasinSPAN survey, or in the quantitative analysis of any deepwater seismic information, then please contact us