L134 Otway Basin seismic survey, VIC, 1991 Stacked seismic data for lines BMR91-T2, BMR91-T3, BMR91-T4, and BMR91-T5

Brief description

Geoscience Australia conducted a deep seismic reflection test survey in the onshore portion of the Otway Basin in the southeastern part of South Australia and southwestern part of Victoria, from late October to early November 1991. The seismic test survey was undertaken to test the feasibility of recording deep seismic reflection data prior to commencing a major deep seismic reflection survey in the Otway Basin. A planned major deep seismic reflection survey by AGSO formed part of a National Geoscience Mapping Accord (NGMA) project to study the early development of the Otway Basin. The seismic reflection test survey operated for a period of three weeks, including mobilisation and demobilisation to the survey area, with five test sites occupied during the test survey. The seismic test sites were positioned to examine the feasibility of recording deep seismic reflections in areas with different outcropping rock types along the proposed main seismic lines. eMF seismic profiles were recorded along all five test lines, with uphole shoots, shot depth and charge size comparison tests performed at several of the sites. Good quality deep seismic reflection events were recorded at several of the seismic test sites. The seismic test survey was important in highlighting areas with difficult shothole drilling conditions, especially the palaeo-sand dunes along the SA-Victoria border, and the Tertiary shelly sands with shallow water table near Lake Bonney. A major seismic survey would require all shothole drilling rigs to be equipped with portable mudpits to enable shotholes to be drilled in the palaeo-sand dunes and shelly sands.

Raw data for this survey are available on request from clientservices@ga.gov.au - Quote eCat# 74948

Lineage

Maintenance and Update Frequency: unknown

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 NGMA project partners included AGSO, South Australian Department of Mines and Energy, Geological Survey of Victoria and various universities. The objectives for conducting a 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 3 seconds two-way travel time (ie. to 5 km depth). The lack of resolution of seismic reflection events at greater depths is 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 3 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 leI Powergel in an attempt to input greater seismic energy into the earth to resolve seismic reflections from sediments greater than 5 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 planned a seismic test survey using a reduced seismic crew over a short period of three weeks. 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.