The sabbatical visit during the year of Associate Professor Antony White of Flinders University allowed the review of an ongoing series of seafloor electromagnetic experiments around the continental edges of the Australian continent. His visit was also marked by the development of plans to seek a southern extension of the South Australian Eyre Peninsula electrical conductivity structure in Antartica. Accepted Gondwanaland reconstructions suggest an Antarctic limb of the conductivity structure might occur in the vicinity of Commonwealth Bay, Antarctica.
Participation in the Australian Geological Convention in Adelaide in July provided an opportunity to discuss recent RSES results in geomagnetism with the geological community, and to make contact with collaborators in Adelaide. Collaboration also continued internationally with colleagues in Canada.
Following her successful tenure of a 2000/2001 Summer Research Scholarship at RSES, Ms Kate Procko returned to Canberra from the University of Adelaide to hold a 2001/2002 Summer Research Scholarship, again working on seafloor magnetic data. Ms Procko was then awarded an A.L.Hales scholarship, and continued at ANU as an honours scholar.
The Carpentaria Conductivity Anomaly is a major element in the electrical conductivity structure of the Australian continent. The magnetotelluric results on shore define a good conductor within the crust beneath the sediments of the Eromanga Basin. The conductor extends over a depth range of tens of kilometres. Seismic tomographic results show a major gradient in seismic wave-speed in the region. The case history supports the hypothesis that the major conductivity anomalies of the geomagnetic deep-sounding method mark continental sutures, of fundamental significance in recording the creation of continents.
It is therefore of major significance, still being fully assessed, that evidence was found of the conductivity structure continuing north into the Gulf of Carpentaria. A report of the study was presented by A. White at the 16-th Workshop on Electromagnetic Induction in the Earth, held in Santa Fe, New Mexico, in June 2002.
In the Southern Ocean magnetometer experiment, four instruments were deployed on the floor of the Southern Ocean in April 1996. To provide simultaneous magnetic records for reference purposes, a land station was operated at Kingston, Tasmania. This site is to the north of the seafloor sites. Also the magnetic observatory at Macquarie Island, to the southeast of the seafloor sites, provided simultaneous reference data. Examples of data recorded simultaneously at two seafloor sites and at two land reference sites are shown in Figure 1
Figure 1: Examples of simultaneous data from two seafloor sites, Rossel and Girardin, and two land reference stations, Kingston and Macquarie Is, for one day (22 September 1996 UT), in the three geographic components of variation. Note the different scales used for the X, Y and Z plots, the ranges of which are 1300 nT, 200 nT and 600 nT respectively.
The intentions of the experiment were several:
(i) to establish the feasibility of deployment and recovery of the
magnetometer package in the hostile environment of the Southern Ocean,
(ii) to make initial seafloor measurements in the latitude of the
Southern auroral zone,
(iii) to analyse the fluctuation data for ocean-floor conductivity
structure in the vicinity of the Antarctic-Australia Spreading Ridge, and
(iv) to examine the data for evidence of a magnetic signal caused by the
motional induction of the Antarctic Circumpolar Current, especially in
the context of a major experiment in physical oceanography taking place
there at that time.
The seafloor observations were planned to coincide with a major oceanographic experiment, the Sub-Antarctic Flux and Dynamics Experiment (SAFDE) of Luther and colleagues. This experiment, part of the larger World Ocean Circulation Experiment (WOCE), thoroughly instrumented the ACC south of Tasmania for two years, 1995 - 1996. The results of the SAFDE experiment provide valuable information for assessing the results of the present magnetometer experiment.
Of the four magnetometers deployed, two were recovered a year later in 1997, and two in 1998. Two failed to record any data, and one of the instruments which did record data suffered intermittent faults. Within these limitations, the present work addresses particularly the third and fourth of the objectives listed above (the first and second regarded as having been achieved).
A critical matter is that of seafloor conductance. Basic theory for seafloor magnetic fields generated by ocean currents shows that a seafloor signal is close to a linear function of various quantities. One of these quantities is the Sanford velocity (approximately the ocean current velocity integrated down through the water column); another quantity is the electrical conductance of the seafloor. The importance of these two quantities is shown in Figure 2, which for a site on the floor of the Southern Ocean plots change in seafloor magnetic field against change in Sanford velocity, for different values of seafloor conductance.
Figure 2: Change in seafloor magnetic signal (nT) in terms of change in the Sanford velocity (cm/s), for different values of seafloor conductance (marked in siemens, S).
Low seafloor conductance under the Antarctic Circumpolar Current, due to the relative youth of the seafloor and the scouring effects of a strong current, place an extra stringency on the observation of a magnetic signal from the ACC, especially at latitudes where auroral effects are strong (as shown in Figure Geomag_1). Thus further efforts may be assisted if observation sites are chosen particularly for high seafloor conductance, and improved magnetometer performance allows remote-reference methods to be employed to remove auroral ionospheric effects.
Previous work concerning invariants has this year been linked to a new approach by Caldwell, Bibby and Brown, who develop a real "phase tensor". The general complex tensor has seven invariants of rotation, and the real phase tensor has three invariants of rotation. Conclusions drawn regarding dimensionality of geologic structure on the basis of the two methods agree in almost every detail.
Comments on the maintenance of these frames to Ray Martin:
ray@rses.anu.edu.au