Seismic

Seismic data is the principal geophysical method used to image the subsurface in both land and marine environments. Geoscience Australia has recognised the importance of the seismic technique since the late 1940s and is a world leader in integrating seismic data with other Earth imaging and surface geology datasets to understand mineral and petroleum systems. Geoscience Australia routinely acquires seismic data to gather new pre-competitive data in Australia's unexplored onshore and offshore frontier regions as well as obtaining additional regional seismic datasets in already explored regions of Australia known to have petroleum, geothermal, groundwater or mineral potential.

Seismic reflection

Data can be acquired in two configurations, 2D which is acquired along a single continuous line and results in a single cross-section image of the sub-surface; and 3D, which results in a cube of data allowing imaging of the subsurface from any angle. Both configurations are available for both land and marine acquired data, although the details of the acquisition method vary.

Marine seismic acquisition

This data is acquired using a vessel towing an airgun array (the seismic source) and a series of cables (3D), or single cable (2D), behind it. These cables contain receivers (hydrophones) at regularly spaced intervals along the length, thereby enabling a recording of a reflected sound wave to be obtained at many points away from the source. For example the seismic image above was acquired using a cable that was 8100m long and contained 648 receivers along its length.

The airgun array creates an air bubble that sends a sound wave into the seabed and subsurface that reflects off boundaries between different geological units. The reflected sound waves are then measured by the hydrophones in the cable. As the vessel moves forward this process is repeated along a pre-determined course to map out the geologic units of interest; every 37.5m in the case of the seismic above. This allows over one hundred km of seismic to be acquired in a day.

The large volumes of data that are produced from this surveying method are then analysed and processed to produce an image like that above. The data is then interpreted to develop a geological model of the subsurface.

Land seismic acquisition

The seismic source for collecting seismic data on land is usually Vibroseis trucks, which use steel plates to vibrate the ground through a range of frequencies (sweep) to send the sound signal into the subsurface. An array of geophones (receivers) at the surface measures the time for the reflections from the subsurface geology to arrive. The geophones may be configured as a 2D array, usually comprising a cable with geophones at regular intervals up to a distance 20km, or as a grid (3D).

The field crew layout the cables and geophones along the line every few metres. Once the cables and geophones are in place, the Vibroseis trucks drive slowly along the line and generate the seismic signal. The process is controlled from a recording truck, where the signals from the geophones are recorded for analysis and processing to produce an image of the subsurface geology.

Australia's seismic data library

From 2014 to 2016 Geoscience Australia acquired over 4000kms of new 2D offshore pre-competitive seismic data, to open new areas of offshore Australia to both hydrocarbon exploration and CO₂ geosequestration.

Offshore seismic data, acquired by Geoscience Australia, other research institutions and/or industry, is made publicly available through Geoscience Australia's data repository using the NOPIMS. In addition to the ongoing collection of seismic data, Geoscience Australia reprocesses selected lines from the existing seismic reflection data and uses newer and more advanced processing techniques to improve the final seismic image.

Onshore seismic data, including deep crustal studies conducted by Geoscience Australia, can be downloaded directly from the project webpages.

Some recent examples of Geoscience Australia seismic reflection surveys include:

• The 2015 Houtman Sub-basin marine seismic survey (GA349), acquired offshore Western Australia in a largely unexplored area which may have future hydrocarbon potential.
• The 2015 Gippsland Basin marine seismic survey (GA352), which acquired over 900km of 2D seismic data to assess CO₂ sequestration potential.
• The 2015 Boulia land seismic survey (L207), which acquired 700km of deep crustal seismic across key geological provinces onshore Queensland.