A collection of cores was sent by the Zinc Corporation for the purpose of magnetic orientation of bedding planes. The collection consists of seven pieces mostly of short length. The results of the magnetic tests, and recommendations for future testing, are discussed in this report.

The following notes are intended to accompany Map No.1049, which has the same title as that given above. This map was prepared at the request of the Survey Directorate, L.H.Q., Melbourne, Department of the Army, and is primarily for the use of the Director of Survey, Advanced L.H.Q., Brisbane. The map is termed "provisional" since it is based on only a portion of the data available and the reductions have been made by approximate methods. Nevertheless since use has been made of that portion of the data which has been most recently observed it is considered the most reliable for most practical purposes. For some time work has been proceeding on a final map showing the distribution of magnetic inclination over Australia and the surrounding areas, and this map will be available in the near future.

The Sydney Harbour Degaussing Range is situated opposite Bradley Head. The purpose of the survey was twofold. First, to determine the average normal values of the vertical (Z) and Horizontal (H) force components of the Earth's magnetic field in that part of the harbour which would be occupied by a ship passing over the coils. Secondly, to measure the magnetic elements, i.e. the Declination, Inclination and Horizontal Intensity of the Earth's field at a base station conveniently situated for use by naval personnel for the purpose of calibrating Pot Magnetometers, etc. The particulars of this survey work and given in this report.

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Interpretation of magnetic field data is complicated by the presence of remanent magnetization and benefits from palaeomagnetic and rock magnetic studies. Additionally, the planning of palaeomagnetic sampling sites and mapping of the distribution of magnetizations recovered in palaeomagnetic studies benefits from inspection of magnetic field imagery. These inter-relationships between magnetic field interpretation and palaeomagnetic studies are particularly important in Australia, where deep weathering and extensive cover by younger formations commonly lead to the magnetic field data being the most complete and detailed source of information for basement-related geological mapping and mineral exploration, and where there are few opportunities for direct palaeomagnetic sampling. For the assistance of both magnetic field interpreters and palaeomagnetists, we have developed a database tool to document and interrogate this relationship. We have started to populate the database, and are planning the facilities required to make the database available as an interactive, web-based resource. The key objectives are to facilitate interpretation of magnetic field data, increase reliability in developing deep drilling targets from magnetic field interpretation, and to better establish the spatial range of magnetizations related to igneous, metamorphic, thermal, alteration and mineralization events. Our intention is to establish a widely used, interactive, minimally-moderated resource rather than a more definitive but possibly less useful one. We envisage that the data base will grow primarily through public contributions, and will also be the forum for debate and speculation regarding Australian magnetization events. We believe that this model is appropriate for wider, global application.

The presence of remanent magnetization must be correctly addressed in the inversion and interpretation of magnetic field data. Better knowledge and understanding of the influence of remanent magnetization will allow improved design of boreholes to test magnetic targets, thus reducing the risk of expensive misses, and also provide new geological insights regarding the distribution of geological events (including mineralization events) which have left a magnetic signature. To assist in this objective, we have designed a database for specification of magnetic field anomalies due substantially to remanent magnetization, and have initially populated this web-delivered database with just over 200 examples from across Australia. A map-based interface allows the user to determine which if any anomalies in an area are interpreted as due in substantial part to remanent magnetization, to recover the associated estimate of resultant magnetization direction, and in some cases to download models of the distribution of magnetization. We are presently researching the application of automated methods to recover magnetization estimates, which will provide a much greater number of estimates, albeit of lower reliability. We envisage a possible 'wiki'-like development of the database, so that once more fully established, it could grow by web upload of contributions from industry, government and academic geophysicists.

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