L135 Otway Basin seismic survey, VIC, 1992 Stacked seismic data and images for lines BMR92-OT1, BMR92-OT2, BMR92-OT3, BMR92-OT4, BMR92-OT5, BMR92-OT6, and BMR92-OT7

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Geoscience Australia conducted a deep reflection seismic profiling survey in the onshore portion of the Otway Basin in the southeastern part of South Australia and southwestern part of Victoria, form February to June 1992. The seismic survey formed part of a National Geoscience Mapping Accord (NGMA) project to study the early development of the Otway Basin. The objective of the seismic survey was to acquire a new deep reflection seismic data using explosive seismic energy sources to improve on the knowledge of early sedimentary sequences in the Otway Basin, especially at depths greater than 3 km. The seismic survey obtained 461 km of 5 to 10 fold Common-Middle-Point (CMP) deep reflection seismic data.

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Statement: The Australian Geological Survey Organisation (AGSO), as part of the Australian National Geoscience Accord (NGMA), proposed to conduct a deep seismic reflection profiling survey in the Otway Basin region of SW Victoria and SE South Australia during 1992. The objectives for conducting the seismic survey were aimed at achieving a better understanding of the sub-surface geology of the onshore Otway Basin region, thought to have considerable resource potential, including hydrocarbons, industrial gases and geothermal energy. Previous AGSO deep seismic reflection profiling surveys (using explosives as the seismic energy source) in other areas of Australia to investigate basin sequences and basin formation have provided new perspectives on basin development not always apparent in industry seismic data. Recent industry high CMP fold and higher spatial resolution seismic data using seismic vibrators as the seismic energy source, have produced excellent seismic data in the onshore region of the Otway Basin down to 2 seconds two-way travel time (ie. to 3 km depth). The lack of resolution of seismic reflection events at greater depths was possibly attributable to the restricted capacity of the commercial seismic vibrators to provide enough seismic energy to resolve deeper seismic reflection events at greater than 2 seconds two-way travel time. A proposal was therefore prepared to use explosives as the seismic energy source, with charge sizes in the vicinity of 10 kg of ICI Powergel in an attempt to input greater seismic energy into the earth to resolve seismic reflections from sediments greater than 3 km depth. The AGSO seismic reflection acquisition system is specifically designed for seismic acquisition using an explosive seismic source in conjunction with AGSO seismic shot hole drilling rigs. To test the feasibility of recording deeper reflection events using large explosive charges as the seismic source, AGSO conducted a short test survey during October and November 1991 (Wake-Oyster & others, 1993). The test sites were located in several areas of the Otway Basin with different technical problems for seismic acquistion, including areas with outcropping Tertiary volcanic rocks, near surface limestones and palaeo-sand dunes. The results from the test survey showed that deeper reflections could be recorded using an explosive seismic energy source. The test survey also highlighted operational problems in shot hole drilling. Difficulty in drilling shotholes in palaeo-sand dunes using air and water injection showed that for a major seismic survey, drilling using mud would be needed and that portable mud pits would be required.