Search USGS⌕ Search
Home/Archive/USGS

USGS · 70241579

A comparison of three methods of determining geomagnetic paleointensities

Robert S. Coe·C. Sherman Gromme
View original source ↗Export metadata ↓

Abstract

Paleointensity studies by the methods of the Thelliers, Wilson, and van Zijl were compared for adjacent specimens from each of five basaltic lava flows. For three of the flows, the actual paleointensity is independently known within ±6%. All specimens had Curie temperatures above 500°C, and those from four of the flows had lower Curie temperatures as well, indicating they probably underwent varying degrees of high-temperature oxidation and disproportionation during initial cooling. These four suffered irreversible changes in their blocking temperature spectra when heated in air above 400 to 500°C. Nonetheless, the paleointensities derived by the Thelliers' method below these temperatures agree with the actual values within the uncertainties. The values obtained by the other methods are not as accurate or reliable. In four out of five lavas, A. F. cleaning of 100 to 200 oe prior to each measurement of remanence in the Thelliers' method degraded the results for determination of paleointensity.

Explore related subjects

Journal of Geomagnetism & GeoelectricityArticle

Keep this discovery

Explore connections, maps & timelines

BibTeXRIS

Robert S. Coe, C. Sherman Gromme. 1973. A comparison of three methods of determining geomagnetic paleointensities. https://doi.org/10.5636/jgg.25.415

Cite the original work for its findings. Save a collection to share your selection of sources.

KEEP EXPLORING

Related USGS reports

USGS1992

Quiet geomagnetic field representation for all days and latitudes

This paper describes a technique for obtaining the quiet-time geomagnetic field variation expected for all days of the year and distribution of latitudes from a limited set of selected quiet days within a year at a discrete set of locations. We used a data set of observatories operated by Indian and USSR scientists in 1976 and 1977 near 75°E longitude as illustration. Our method relies upon spatial smoothing of the decomposed spectral components. An evaluation of the fidelity of the resulting model shows correlation coefficients usually above 0.9 at the lower latitudes and near 0.7 at the higher latitudes with variations identified as dependent upon season and field element.

Journal of Geomagnetism & Geoelectricity↗
USGS1989

An attempt to obtain a detailed declination chart from the United States magnetic anomaly map

Modern declination charts of the United States show almost no details. Greater detail may be of value to surveyors trying to follow old land deed descriptions, or to pilots of small planes or small pleasure boats operating in inland waterways. It would be extremely expensive to make adequate declination measurements needed for such a chart. It was hoped that declination details could be derived from the information contained in the existing magnetic anomaly map of the United States. This could be realized only if all of the survey data were corrected to a common epoch, at which time a main-field vector model was known, before the anomaly values were computed. Because this was not done, accurate declination values cannot be determined. In spite of this conclusion, declination values were computed using a common main-field model for the entire United States to see how well they compared with observed values. The provisional geomagnetic reference field for 1978.5 was used as the main-field model. The computed detailed declination values were found to compare less favorably with observed values of declination than declination values computed from the IGRF 1985 model itself. This result indicates that the computed anomaly elements or their combination with main-field values cannot be used as accurate anomaly values, but they may be used as an indication of where anomalies probably occur.

Journal of Geomagnetism & Geoelectricity↗
USGS1988

New trend- trigonometric model for interpolation and prediction of the geomagnetic field utilizing the new DGRF models

At the IUGG Assembly at Vancouver during August 1987 new definitive geomagnetic reference field (DGRF) models to degree 10 for 1945, 1950, 1955, and 1960 were adopted by IAGA. Before these new DGRF models were accepted, the author developed a trend and trigonometric model (old trig model) based on the models IGRF 1945, IGRF 1950, IGRF 1955, IGRF 1960, DGRF 1965, DGRF 1970, DGRF 1975, DGRF 1980, and IGRF 1985, which were all approved by IAGA in Prague in August 1985. The old trig model consists of 720 trend and trigonometric coefficients for the calculation of spherical harmonic coefficients (SHC) only to degree eight because the early IGRF models were truncated there. These trend and Fourier sine coefficients can replace the equal number of SHC contained in the 9 DGRF-IGRF models.

Journal of Geomagnetism & Geoelectricity↗