Geophysics and Geohazards – Defining Subsea Engineering Risk

24 February 2011 Joint SUT and ASEG International Seminar, Subsea Australasia Conference 2011,

Perth, Australia

Report

‘Geophysics and Geohazards – Defining Subsea Engineering Risk’, a joint SUT and ASEG one-day seminar, was held in Perth on the 24th February 2011 as part of the Australasian Oil and Gas (AOG) Exhibition and Conference. An almost full house ensured this second conference, which first took place in March 2010, was truly successful.

After the usual formalities including the thanks to Fugro as the principal sponsor, Dan McConnell, VP Marine Consulting, AOA Geophysics, gave the keynote presentation, ‘On Marine Seep Hunting for Oil and Gas Exploration’. The use of a combination of multibeam echo sounder bathymetry and backscatter data with accurately placed piston cores at the seep sites to directly sample a petroleum system was described. An astounding 46% of the targets showed unambiguous thermogenic gas and 13% showed evidence of migrated liquid petroleum.

Stephen Wardlaw, Business Development Consultant, Fugro Geoconsulting Pte Ltd, then described how autonomous underwater vehicles (AUVs) are now routinely utilised in deepwater site investigations for oilfield developments. Drawing on examples, Stephen’s paper reviewed the contribution from the various AUV instruments, and discussed techniques currently used in AUV data integration in the assessment of engineering geohazards.

The use of marine resistivity surveys on dredging projects, and cable and pipe route surveys to assist in port design and alluvial mining projects was described by Dr Ir Peteralv Brabers, General Manager, Demco NV (Belgium). Dr Ir Brabers provided an insight into the theory of resistivity principles and a brief comparison between acoustic methods and marine geo-electric methods.

After the morning tea and coffee break, Dr Ouzani Bachir, Geoscientist, Offshore Geo-Survey Sdn Bhd, discussed deepwater geohazards characterisation. Unlike the continental shelf where geohazards are well documented and site investigation methods are established, deepwater environments are unfamiliar and lack well logs and borehole data. The use of seismic inversion and amplitude versus offset (AVO) analysis for shallow water flows and gas hydrates assessments was discussed.

Dan McConnell returned to the podium to describe his involvement in the DOE-Chevron Joint Industry Gas Hydrate Project in the Gulf of Mexico, 2009 (Leg II). The three-week, $11.5m expedition drilled seven logging-while-drilling (LWD) holes at three sites that tested a variety of geologic/geophysical models for the occurrence of gas hydrate in sand reservoirs in the deepwater Gulf of Mexico.

The lunch break and an opportunity to visit the exhibition was followed by a description by Bachir Ouzani on how the iCUBE 3D volumes converted from 2D seismic lines shot at 50–500m spacing provide increased data density and allowed improved lateral geological continuity and definition of geological structures.

Dr Steve Tyler, Principal Geophysicist, Fugro Survey Pty Ltd, then described how data from a geohazard and environmental baseline survey within the Bonaparte Basin confirmed a barrier reef complex extended from the Sahul Platform through to the Ashmore reef area approximately 18 000 years ago. The geophysical data highlighted the structural complexity of this area with a significant number of pockmarks identified around the flanks of these outcropping palaeo-reefs.

A system of faults extending along the west coast and up to the North West Shelf was described by James Hengesh, Research Fellow, University of Western Australia (UWA). A magnitude 7.3 earthquake, the largest in Australia’s history, occurred along one of these faults, and other faults have the potential to produce similar large magnitude events. Work is underway at UWA to better understand the hazard posed by these faults and to incorporate them in future seismic hazard assessments for major infrastructure projects in Western Australia.

Dr Douglas Bergersen, CEO/Geophysicist, Acoustic Imaging, described the oil and gas industry desire to standardise the presentation of baseline marine survey data for better long- term management of information across offshore development areas. One such template for the storage of information is the Seabed Survey Data Model (SSDM) based around the ArcGIS geodatabase structure. Dr Bergersen’s paper presented an overview of the integration to date and discussed how software tools may be used to assist and optimise the identification, characterisation and quantification of marine geohazards. The seamless transfer of information to the SSDM was illustrated with a case example from the North West Shelf of Australia.