The New Zealand Regional Geomagnetic Field Mapping, Modelling, and Interpretation

Abstract

Advances in regional surveying, particularly the use of satellites, have enabled the mapping of the internal geomagnetic field at wavelengths intermediate between those due to core sources and those due to shallow crustal sources. The Magsat satellite provides a three-component data set a mean altitude of 404 km which shows these so-called intermediate-wavelength anomalies to range in amplitude up to 15 nT in total force. Averaging the Magsat residuals after correcting for the effects of external fields and subtracting a normal field reveals positive anomalies to the north-west and south of New Zealand which are separated by an east-west trending negative anomaly. Altitude related errors in the anomaly map are eliminated by modeling the observed field by a grip of equivalent dipole sources at the Earth's surface which are aligned with the normal field. Assuming an origin for the anomalies by induction in the crust, the magnetization contrast at the intermediate spatial scale must be in the range 0-1.5 A M-1. Marine total force observations provide regional surface coverage. Correcting for core and external fields and filtering along-track to attenuate short-wavelength fields reveals anomalies in the intermediate-wavelength bandpass of 400-4000 km with amplitudes ranging up to 150 nT. The spatial distribution of the intermediate-wavelength field derived from the marine data set is consistent with that of field derived from the satellite data set. Aeromagnetic and satellite vector data sets are used to derive coefficients for a three dimensional New Zealand regional field model with a resolution of 1000 km using the method of spherical cap harmonic analysis. These coefficients are then modified using a linearized least squares process which fits the model to the aeromagnetic and satellite vector data sets and the marine total force data set. The model reproduces the main features of the observed satellite and marine intermediate-wavelength anomaly fields. Estimates of the error in the International Geomagnetic Reference Field over the region are 38 nT near the Earth's surface and 55 nT at Magsat altitudes. Estimates of the strength of the intermediate-wavelength field over the region are 39 nT near the Earth's surface and 36 nT at Magsat altitudes. The intermediate-wavelength field is thought to have a lithospheric origin, with the acquisition of viscous remanent magnetization by deep crustal rocks being the most likely primary source. Low-magnetization schists which from the basement of the eastern South Island and of the continental crust to the east appear to be the magnetization highs over the over the central and southern Campbell Plateau and the Lord Howe Rise/Norfolk Ridge area are due to a once-continuous source body which has been offset along the line of the Alpine Fault. A simple forward model of the continental crust in the region indicates that the removal of the long-wavelength component of the crustal field along with the normal field has only a small effect on the intermediate-wavelength anomaly pattern.