Marine surveying

The data types and samples collected for marine geoscientific purposes vary greatly according to requirements. This reflects the diverse nature of investigations undertaken around the world's coastlines. The survey environment varies from oceanographic studies in the water column to investigating sediment and geochemical processes on the seafloor and imaging the sub-seafloor rocks. This sub-seafloor mapping can range from a few metres to many kilometres through oceanic and continental crust. Surveys are carried over Australia's entire marine jurisdiction, from coastal estuaries and bays, across the continental shelf and slope, to the deep abyssal plains.

Survey design

At sea, data and samples are collected usually from a designated research ship, seismic vessel or suitable charter vessel. Remotely sensed data are collected continuously on computer driven digital data storage systems for archive and processing. Much of these data are then displayed as:

• vertical cross sections
• 3D maps and
• overlays with other spatial data.

Usually measurements are taken every few seconds but can be taken up to several times a second. Typically, continuous data are collected by a range of survey equipment including multi-channel and single channel seismic systems, sub-bottom profilers, sidescan sonars, magnetometers, gravity meters, single beam and multibeam sonar bathymetry systems. Oceanographic equipment, fluorometers and gas detection equipment are also sometimes deployed. The variety of survey methods and tools can be broadly classified into two types, profiling techniques and sampling techniques.

Ships allow for longer surveys in remote areas using a multi-instrument approach. However aircraft and remotely controlled drones can also carry geophysical equipment and by flying regular survey patterns at low altitude can collect aeromagnetic and radiometric data over vast areas. Satellite technology using radar and multi-spectral systems also has revolutionised our understanding of the Earth. An example of this satellite technology is the use of radar on Geosat to measure ocean heights and, as a result, interpret gravity and bathymetry for the Earth's oceans.

Large areas are mapped, using closely spaced survey lines, which allow the data to be mosaiced into one seamless image. An example is multibeam bathymetry resulting in a 3D view of the seafloor. In other instances the lines are more widely spaced as in a regional seismic survey. Here the lines may be hundreds of kilometres long and the data will be presented as a 'fence' diagram, giving representative 2D vertical seismic cross-sections of the sedimentary basins.

The second fundamental marine technique is sampling the water column, seafloor and sub-surface. These samples are collected using many devices including:

• gravity corers
• vibracorers
• push corers
• rotary drills
• grab samplers
• benthic sleds and
• rock dredges.

In addition, video and still image cameras are used either separately or in conjunction with sampling equipment. In situ measurements of geochemical, nutrient, sediment and oceanographic parameters using benthic chambers and other moorings placed on the seafloor are conducted over a period of days to months.

Information recovered from a survey can be used for more than one purpose. For example, geophysical and geological data obtained to assess areas for future petroleum exploration and acreage releases can be useful for climate and ecological studies or for defining fisheries and maritime boundaries.

This opportunity for expanded use of data is partially the result of the multi-sensor approach to surveying which is possible on a research ship, as well as the need to maximise sea time data acquisition. Australia has a very large marine jurisdiction and the task of mapping it has to be done as efficiently and effectively as possible.

On a typical regional mapping survey, the equipment may include a multibeam sonar, sub-bottom profiler, magnetometer and sidescan sonar. A survey for petroleum exploration might also include multi-channel seismic, gravity meter, profiling fluorometer or continuous gas detection equipment.

Both petroleum and seabed mapping surveys will also have a geological sampling component using the equipment described above. At selected sites grab samples, sediment cores, underwater video, rock dredge and water column sampling would all be deployed to classify the seafloor and ground truth the sonar and geophysical data.

Geoscience Australia has many tools for marine surveys but uses contractors extensively for geophysical data acquisition associated with larger programs. An equipment list can be found at the Field and Engineering Support page.