Evolution of Seismic Acquisition Techniques in Papua New Guinea
Seismic imaging in the often karstified limestone of the Highlands of Papua New Guinea is notoriously difficult. Remoteness, rugose terrain and unpredictable weather make operations difficult and at times dangerous. The reliance on helicopters also makes acquisition costly. Imaging is complicated by the complex sub-surface structure, highly karstified surface limestone and often extreme topography. There have been four key operational advances in recent years: 1. Utilisation of Lidar surface imaging as a survey / line design tool; 2. Changing back to man-portable shot hole drilling; 3. Use of polymer plugs for shot hole tamping; and; 4. Adoption of autonomous nodal systems replacing cable systems. In this paper we review the impact of each of these advances on seismic image quality. Line positioning and source design (such as shot hole depth / charge) can be optimised through the digital analysis of airborne LiDAR (Light Detection and Ranging) data. LiDAR is typically available at the time of seismic survey planning as it is typically acquired early on a licences exploration cycle. Skilled interpretation of the Lidar data can help identify the intensity of surface karstification, provided detailed bare earth models in support of detailed line routing to avoid or bypass the worst karst areas. Shots can be deeper in more karstified areas so that larger charges can be selectively used. Safely and cost effectively reducing shot point spacing has been a considerable challenge. In PNG, through the 1980’s and 1990’s, it was standard practice to undertake man portable shot hole drilling, but the equipment was heavy and shot depth typically shallow. In the 2000’s helicopter- portable heavy drilling rigs were introduced, enabling the drilling of deeper holes that could be loaded with larger charges, however helicopter time is expensive with the cost limiting the number of drilled shot holes. The development of efficient light weight man-portable drilling has enabled more efficient drilling and loading of deeper shots. This results in the cost-effective delivery of a denser shot point spacing providing subsurface image points and therefore higher fold and better image quality. Source coupling is a common problem in limestone areas as shot hole drilling produces little fine-grained natural tamping material suitable for tamping. Good tamping minimises blowouts, directing source energy and therefore maximising the opportunity for the downwards propagation of induced energy. Flying-in dedicated tamping materials is prohibitively expensive. Using limestone rubble results in higher numbers of blow-outs, with the lost energy degrading shot and therefore image quality. Polymer based two-part expandable chemical shot hole plugs are a viable alternative. They are readily transported (small volume, light weight, nontoxic and biodegradable) and work by expanding down hole forming a solid seal, fixing the dynamite in place. Finally, to maximise fold the replacement of cable-based recording systems with light weight nodal technology is transforming our ability to cost effectively minimise receiver point spacing. Our cable system was retired three years ago after comparative testing gave us the confidence that nodal recording systems work under the jungle canopy. The key advantage of nodal systems is (1) they are compact, lightweight and easily deployed, reducing operational cost and risk, and (2) receiver coverage is improved as we can position nodes up to the edge of obstacles such as cliffs and rivers. Their key disadvantage is real time noise recording, as with weak signal any noise can have a significant detrimental impact on data quality, however operational methodologies have been developed for monitoring and managing noise. The rapid evolution nodal technology continues with integrated node/geophone units now being field tested. We will present results of experiments with the latest generation of lightweight stand-alone nodes which would allow an even denser receiver spacing in the future.
AAPG Datapages/Search and Discovery Article #90371 © 2020 AAPG Asia Pacific Region, The 1st AAPG/EAGE PNG Geosciences Conference, PNG’s Oil and Gas Industry: Maturing Through Exploration and Production, Port Moresby, Papua New Guinea, February 25-27, 2020