Feature

Magnetic Meanderings

Studying disturbances in the Earth's geomagnetic field can tell geophysicists about the crust beneath New Zealand.

J. D. McKnight

We live immersed in the Earth's magnetic field, but most of us are unaware that it is constantly changing, on time-scales ranging from fractions of a second to millions of years.

The short-term changes originate in the conducting upper layers of the Earth's atmosphere. The long-term changes -- and the largest part of the field -- originate in the Earth's outer core, a fluid layer of molten nickel-iron alloy 2,200-kilometres thick, deep within the planet.

These changes are a nuisance to some, such as those wanting to predict the magnetic declination for use in navigation and those involved in mineral exploration, who might confuse time-related changes in the field with magnetic anomalies associated with mineral deposits. But they can be a source of information to others, such as those investigating the inner structure of the Earth or the nature of the upper atmosphere and magnetosphere.

New Zealand researchers have monitored the Earth's magnetic, or geomagnetic, field since 1902. At Eyrewell Geomagnetic Observatory, currently operated by the Institute of Geological and Nuclear Sciences, a magnetometer measures the geomagnetic field every 20 seconds. It records the strength of the three "orthogonal" geomagnetic field components which are at right angles to each other, corresponding to up-down, north-south and east-west.

In addition to these, observations are made every two to three years at sites throughout New Zealand and its offshore islands. These observations track regional variations in the behaviour of the core field.

The observatory data and other geomagnetic observations are used within New Zealand for a number of purposes -- to produce regional field models from which navigational information is obtained, to correct magnetic surveys and for geomagnetic research, to name a few. They are also supplied to international data centres and to overseas researchers as part of New Zealand's contribution to the global coverage of geomagnetic observations. The global data is used in the production of geomagnetic field models, to monitor levels of geomagnetic disturbance, to investigate the origin of the geomagnetic field and many other studies.

Geomagnetic Survey

Systematic observation of regional variations in the external magnetic field has previously received little attention in New Zealand. Traditionally, these have been seen as being of less importance than the long-term changes, and harder to record. However, in 1991, Australian researchers loaned 32 magnetometers to their New Zealand counterparts, giving them a chance to record simultaneously the three components of the geomagnetic field at one-minute intervals over a period of several months.

The aim was to quantify the behaviour of the external field throughout New Zealand relative to the field observed at Eyrewell Observatory, and to interpret the external field in terms of its source and its interaction with the Earth. The magnetometers were set up in November 1991 at sites spread from Kaitaia to Invercargill. By shifting some instruments during the experiment, records were obtained from 39 sites in all before the instruments were removed.

Once calibrated and "cleaned-up" by removing stray effects such as those from vehicles, the data from the array offers a range of possibilities for analysis. The predominant feature of the records is the effect of disturbances in the external field, which are due to discharges from the Sun that interact with the Earth's magnetosphere, producing broad-scale magnetic effects at the Earth's surface. These are mainly seen in the horizontal components and tend to be relatively uniform, or at least smoothly varying, over New Zealand.

The feature that was picked out for immediate analysis was the interaction between the variations in the external field and the Earth's crust. Variations in the external field induce electric currents in the outer part of the Earth, and these currents produce magnetic fields that are observed at the surface of the Earth. The magnetic fields due to induced currents are most clearly seen in the vertical component, and variations in the vertical component can indicate variations in the electrical properties of the Earth's crust. Knowledge of the electrical properties of the crust is useful in understanding the composition and physical state of the crust in the New Zealand region.

Coastal Currents

An analysis of the data was done on regular variations in the field occurring at periods of 14 and 85 minutes. These variations are related to the bulk electrical properties of the Earth's crust beneath.

Magnetic disturbances within the crust affect the horizontal and vertical components of the field. One such disturbance that was recorded with the array shows similarities in the horizontal components from various sites around the country. This suggests that the disturbance involved a common source field. However, the vertical component, which is strongly influenced by induced currents, varies a great deal from place to place. Some inland sites, such as Lake Tekapo in the central South Island, show little or no effect, while others, such as Blenheim and Invercargill, show strong effects but of opposite sign, or polarity.

These regional differences highlight a major problem in analysing magnetic array data in New Zealand -- the ocean or coast effect. This effect occurs because the electrical conductivity of seawater is typically far greater than that of rock, resulting in stronger electric currents being induced offshore than are induced onshore. These currents are channelled around landmasses, leading to relatively strong vertical disturbance fields at coastal sites. The change in polarity comes about as a result of changes in the apparent direction of current flow. The effect decreases as you move inland.

The geomagnetic coast effect is of lesser interest than the effect that it obscures, that due to varying electrical conductivity of the underlying landmass. It is this latter data which contains information on the nature of the Earth's crust.

Two approaches are being taken towards removing the "noise" of the coast effect from geomagnetic data. The first uses a computer model which takes into account the varying depth of the oceans surrounding New Zealand, their electrical properties and those of the underlying seafloor. This approach is relatively straightforward but it is difficult to include the desired level of detail while retaining the necessary spatial coverage.

The second approach involves creating an analogue model of the region, the land and ocean being represented by materials with suitably scaled electrical properties. The external magnetic fields are simulated within the model, then the response is measured. This approach enables sufficient detail to be incorporated, but it is rather inflexible, requiring a great deal of work to change the model. Both approaches have so far produced encouraging results and continue to be pursued.

Plotting Induction

A convenient method for displaying results from an induction analysis is by plotting induction arrows. For a particular period of disturbance, these arrows indicate the correlation between the horizontal (source) field and the vertical (induced) field. They point towards regions of enhanced electrical conductivity; their length indicates the degree of conductivity contrast.

By subtracting the coast effect, the arrows can be used to provide an estimate of the induction effect arising from onshore conductivity contrasts. This, in turn, can provide information on the composition and structure of the underlying material.

When looking at such an induction arrow "map" of New Zealand covering a particular period, for example 14 minutes, some systematic patterns are apparent. In the North Island, the arrows tend to be relatively long and to point eastward. This may simply be due to problems with the numerical model used to remove the coast effect. Long arrows pointing out from the top of the South Island suggest that the effect of electric current channelling through Cook Strait may not have been adequately removed for some sites.

Two sites near Christchurch, the easternmost of which is the Eyrewell Geomagnetic Observatory, stand out from other central South Island sites as having relatively strong, consistent induction arrows. It is possible that these arrows are associated with a system of faults in North Canterbury.

Perhaps surprisingly, there appears to be no significant effect associated with the Alpine Fault in the South Island. This fault is a major structural feature marking the boundary between two zones of quite different character, and it is reasonable to expect a contrast in bulk electrical properties across it. It may be that the periods of disturbance considered here are not the right ones for detecting such a contrast.

The analysis so far has been rather limited and further work is under way. To gain a better understanding of the nature of the external fields themselves, it is desirable to study them on as broad a spatial scale as possible. Accordingly, it is planned to integrate the Australian and New Zealand array data sets. Although not obtained simultaneously, the fact that observatories ran in both countries at the time of the experiments enables a direct link between the data sets to be established. Such a broad-scale approach befits the subject, as geomagnetism is, by its very nature, a global study.

Don McKnight is with the Institute of Geological and Nuclear Sciences.