GEOMAGNETISM

F.E.M. (Ted) Lilley

Participation in the Australian Society of Exploration Geophysicists Convention in Adelaide in February provided an opportunity to present recent RSES results in geomagnetism to the Australian community, and to make contact with collaborators in Adelaide. The ASEG meeting in Adelaide was immediately followed there by an international workshop, the third on 3D Electromagnetic Induction, which provided further opportunities for discussion both nationally and internationally. Collaboration also continued with colleagues in Canada, and Professor J.T. Weaver visited RSES for some ten days following his participation in the Adelaide meetings.

Canadian student Ms Karen Weitemeyer worked as a summer research scholar on various projects at RSES from January to April. She attended the ASEG meeting in Adelaide, and made major contributions to a paper presented there.

Dr C.E. Barton, formerly of Geoscience Australia, was welcomed as a Visiting Fellow in October. Recently elected to a four-year term as president of the International Association of Geomagnetism and Aeronomy, he returned to RSES where he was a research student some quarter of a century earlier.

Research Results

The sedimentary basins beneath the Timor Sea have very high hydrocarbon potential and have already become major exploration areas. Basic structural information on sedimentary basins and salt structures that are potential hydrocarbon traps has been obtained by conventional seismic methods. These methods are good at locating the top of salt structures but have poor sensitivity to lower salt structures due to reverberation and losses of acoustic energy. Magnetotelluric (MT) methods, in which natural electromagnetic variations signals are measured at the seabed, have recently been found to be highly successful in similar marginal seas at locating not only the top but also the bottom of the salt diapirs and the depth to basement, imaging them in terms of electrical conductivity.

This experiment represents a pilot study and some twenty separate MT deployments were made, the majority for just several days. The MT instruments were deployed in transects, with spacing of 2-3 km, along a previous seismic line near the Tern Well in Bonaparte Gulf where there are salt diapiric structures. The purpose is to develop an electrical conductivity model for the area which complements and enhances geological understanding of the salt structures.

The magnetotelluric (MT) instruments free-fall to the sea floor and record magnetic and electric field fluctuations until they are accoustically commanded to re-surface. The instruments weigh approximately 250 kg, and deployment and recovery is by light crane over the side of the vessel.

Four long-period instruments were deployed at the start of the cruise, and recovered at the end. For the short-period instruments, 17 deployments were made, and 16 successful recoveries (there was one loss, thought to be due to interference by a fishing vessel).

In addition to the electromagnetic signals originating external to Earth and causing electromagnetic induction in the crustal rocks, oceanographic signals are also present in the observed data, caused by tidal motion and ocean wave signals.

SEA-SURFACE OBSERVATIONS OF THE MAGNETIC SIGNALS OF OCEAN SWELLS
F. E. M. Lilley, Adrian P. Hitchman, Peter R. Milligan (Geoscience Australia) and Tina Pedersen

Ocean swells have a magnetic signal, caused by the motional induction of sea water moving in the steady main magnetic field of Earth. To check the character of such signals at the sea surface, several years ago a magnetometer was set free from a ship to float unrestricted on the surface of the ocean for periods of several days. The path of the floating magnetometer was tracked by satellite; this procedure enabled also the eventual recovery of the magnetometer by the ship.

Superimposed upon a background of slow change of magnetic field, as the magnetometer drifted across different patterns of crustal magnetisation, are high-frequency signals generated by the strong ocean swell present at the time. These wave-generated signals are typically up to 5 nT trough-to-peak, consistent with theory for their generation by ocean swells several metres trough-to-peak in height.

The power spectra of the magnetic signals show a consistent peak at period 13 s, appropriate for the known characteristics of ocean swell in the area. The power spectra then exhibit a strong (-7 power) fall-off as period decreases below 13 s. This strong fall-off is consistent with oceanographic observations of the spectra of surface swell, combined with motional induction theory.

APPARENT AEROMAGNETIC WAVELENGTHS OF THE MAGNETIC SIGNALS OF OCEAN SWELL
F.E.M. Lilley and Karen A. Weitemeyer

The magnetic signals of ocean swells, caused by the motional induction of sea water moving in the steady main magnetic field of Earth, may be sensed by a low-flying aircraft, carrying out aeromagnetic measurements over the ocean. The apparent spatial wavelength which such signals will have, when observed data are plotted out for geological purposes, can vary greatly. It will depend particularly on the relative speeds and directions of travel of the observing aircraft and the ocean swells. The apparent wavelength of the ocean-swell magnetic signal cannot be less than the actual ocean-swell wavelength. Generally it is greater, and it can range up to infinity in value. For observations over continental shelves the situation is complicated by the dependence of the swell phase-velocity on water depth, so that the swell speed generally slows as land is approached.

MAGNETOTELLURIC THEORY: THE RELATIONSHIP BETWEEN THE MAGNETOTELLURIC TENSOR INVARIANTS AND THE PHASE TENSOR OF CALDWELL, BIBBY AND BROWN
J.T. Weaver (University of Victoria, Canada), A.K. Agarwal (University of Victoria, Canada), F.E.M. Lilley

We examine the relationship between the seven invariants of the complex MT tensor, which we previously proposed as a vehicle for testing the dimensionality of the regional conductivity structure prior to an analysis of MT data, and the three invariants of the real 'phase tensor', recently introduced as an innovative aid in the treatment of MT data. It is found that the relevant invariants, and the necessary conditions on them for galvanically distorted data to be consistent with 1D, 2D or 3D structures, agree in almost every detail for the two approaches. The new method does lead, however, to an improved normalisation of the eighth (dependent) invariant previously introduced. It is shown that the phase tensor can be expressed as a sum of three simple matrices, clearly associated with 1D, 2D and 3D conductivity structures respectively. It is further shown that it can be depicted graphically as a single Mohr circle that retains the principal properties of the separate real and imaginary Mohr circles associated with the MT tensor. The simplicity and elegance of the phase tensor method is achieved by dispensing with the capability of distinguishing between galvanically distorted and undistorted data in 1D and 2D regions, a distinction that is ultimately unimportant and unnecessary with real data. The paper concludes with a simple illustrative example of the theory applied to a real MT dataset from NE Australia. A shallow 1D regional conductivity structure associated with a sedimentary basin is revealed, and a 2D anomaly with a calculated strike angle is also identified.

TWO-DIMENSIONAL MAGNETOTELLURIC RESPONSES OF THREE-DIMENSIONAL BODIES
K. Broxholme (University of Adelaide), G. Heinson (University of Adelaide), S. Busuttil (University of Adelaide), F.E.M. Lilley

Magnetotelluric (MT) tensors have significantly different forms depending on whether the subsurface is one-dimensional (1D), two-dimensional (2D) or three-dimensional (3D). In subsurface geological structures that are not 1D, two-dimensionality is often assumed, as inversion routines for 2D earth models are computationally more tractable than those for full 3D media. In 2D, the MT tensor decouples into two independent modes, the transverse electric (TE) mode and the transverse magnetic (TM) mode. Often only one of these modes is acquired during commercial operations.

Field data were collected with the Mt Isa Mines Distributed Acquisition System (MIMDAS) in the Deep Well prospect of the Curnamona Province in South Australia. The target for the survey was an elongate magnetic anomaly of a type that would normally be approximated as 2D but which has a finite strike length and is therefore a 3D body. With this in mind, the applicability of interpreting data defined as TE and TM were assessed using (a) Mohr circles galvanic distortion analyses, (b) determination of strike of local and regional geology, and (c) comparison of 2D inversion techniques. We show that the TM mode accurately delineates boundaries and that since boundary-charges are included in the inversion formulation, it also provides accurate values of apparent resistivity. The TE mode provides poor boundary delineation and underestimates the resistivity of the 3D body. Joint inversions provide only a small improvement upon TM-only inversions, but determination of dimensionality, strike and detection of galvanic distortion mean that collection of both data modes is still preferable.

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