Robert D. Regan

THE EFFECT OF FINITE DATA LENGTH IN THE SPECTRAL ANALYSIS OF IDEAL GRAVITY ANOMALIES

The interpretation of gravity data which has been transformed from the space domain to the frequency domain is subject to limitations imposed by the transformation process and the nature of the data. One potential source of error in the spectral analysis of gravity anomalies is the incomplete definition of the anomaly due to insufficient data length. Comparison of theoretical Fourier transforms and the transforms of finite‐length segments of gravity anomalies due to two‐dimensional vertical slabs and horizontal cylinders indicates that the interpretation error can be held to less than ten percent if the profile length is at least six times the maximum depth to the source of the anomaly. This result was achieved with a rectangular data window in the transformation process. Greater error resulted from the use of the Bartlett, Parzen, and Tukey data windows.

THE USE OF GEOMAGNETIC FIELD MODELS IN MAGNETIC SURVEYS

Global geomagnetic field models, usually computed from spherical harmonic series, are becoming more important in the reduction of magnetic surveys. When used correctly, a numerical model of sufficient complexity, including adequate secular variation correction, provides a suitable representation of the regional field. The best known and most widely used of the available field models is the International Geomagnetic Reference Field (IGRF). However, the IGRF may not be suitable for the reduction of all magnetic survey data because of its imperfect fit to the main field, particularly since 1968.

FOURIER TRANSFORMS OF FINITE LENGTH THEORETICAL GRAVITY ANOMALIES

The mathematical structure of the Fourier transformations of theoretical gravity anomalies of several geometrically simple bodies appears to have distinct advantages in the interpretation of these anomalies. However, the practical application of this technique is dependent upon the transformation of an observed gravity anomaly of finite length. Ideally, interpretation methods similar to those for the transformations of the theoretical gravity anomalies should be developed for anomalies of a finite length. However, the mathematical complexity of the convolution integrals in the transform calculations of theoretical anomaly segments indicate that no general closed analytical solution useful for interpretation is available. Thus, in order to utilize the Fourier transform interpretation method, the data must be of sufficient length for the finite transform to closely approximate the theoretical transforms.

Theoretical transforms of the gravity anomalies of two idealized bodies

Odegard and Berg (1965) have shown that the interpretational process can be simplified for several idealized bodies by utilizing the Fourier transform of the resultant theoretical gravity anomalies. Additional studies relating similar conclusions for other idealized bodies have been reported by Gladkii (1963), Roy (1967), Sharma et al (1970), Davis (1971), Eby (1972), and Saha (1975), and a summary of the spatial and frequency domain equations is given in Regan and Hinze (1976, Table 1); however, the transforms of the three‐dimensional prism and vertical line elements, often utilized in interpretation, have not been previously examined in this manner. Although Bhattacharyya and Chen (1977) have developed and utilized the transform of the 3-D prism in their method for determining the distribution of magnetization in a localized region, it is still of value to present the interpretive advantages of the transform equation itself.

The current status of the IGRF and its relation to magnetic surveys

As aeromagnetic surveys covered increasingly larger areas and as the need arose for the compilation of adjacent surveys with significant time differences, there has been an increase in the use of geomagnetic field models for regional field definition. Indeed, few aeromagnetic contracts exist today without some provision for removal of the International Geomagnetic Reference Field (IGRF). Yet, as publications indicate (Regan and Cain, 1975a, 1975b; Reford et al, 1976; Regan, 1977, 1978), the IGRF is far from perfect for this application despite the considered attention of the IAGA (International Association of Geomagnetism and Aeronomy) working group charged with its upkeep and maintenance. One reason for this is that the model is meant to serve all areas of geomagnetism and, as such, is not specifically tailored to the needs of the exploration community. The recent summary report of the IAGA committee (Peddie, 1982) could lead to some confusion because there are seven IGRF models published. A rational que...

Satellite magnetic data: How useful in exploration?

In 1979, Magsat, a satellite specifically designed for magnetic anomaly studies was flown by NASA for the earth science community. For almost a decade, researchers have been evaluating the data gathered from Magsat; the results of these analyses showed that satellite magnetic data offered a new perspective to the exploration picture, and gave more accurate field models for use in the reduction of ground and aeromagnetic surveys. NASA’S original decision to implement the Magsat program was in part based on the favorable response by the academic and commercial communities (including the SEG) to a 1975 survey concerning their interest in such a satellite.

GEIS; a new approach for the integration, interactive analysis and interpretation of geoscience data

The evolution of computer usage—from batch processing to single—purpose systems to networked hardware and software systems—and the wealth of relevant data presently available in digital form (plus the unparalleled amount to accrue from planned studies) warrant the design of an information system specifically tailored to the geosciences.

REVISION OF THE IGRF: A Summary Report on the Zmuda Memorial Conference on Geomagnetic Field Models

On March 24 and 25, 1975, the Zmuda Memorial Field Model Conference cosponsored by the Society of Exploration Geophysicists (SEG) was held at the Broadmoor Hotel, in Colorado Springs, Colorado. This meeting was designed to provide an opportunity for dialogue between those who derive geomagnetic field models and the various users and to serve as an opportunity to discuss the plans for the proposed revision of the International Geomagnetic Reference Field Model (IGRF). The conference included an objective appraisal of the use of field models in magnetic surveys and documented the need for a more accurate reference field. This report summarizes the conference results as they pertain to the members of SEG. A complete summary, including abstracts, is published in the July (1975) issue of EOS, Transactions of the American Geophysical Union.