An automatic program for the interpretation of two-dimensional gravity and magnetic anomalies

The expressions for the spectra of both gravity and magnetic anomalies due to a two‐dimensional structure consist of (except for a factor) sums of exponentials. The exponents of these exponentials are functions of frequency and the locations of the corners of the polygonal cross‐section of the structure. Two computationally feasible methods for determining the exponents from a given spectrum are described in this paper; they are essentially based on the generation of a system of linear equations. The unknown coefficients in this system of equations are functions of the corner locations. The first method requires expansion of the exponentials in the expressions for the spectra in the form of a series and works reliably when the amplitudes of low frequencies are analyzed. The unknown parameters are determined fairly accurately with this method by suitable combinations of the spectra of the observed anomaly and its moments. The second method utilizes an exponential approximation technique for producing the system of linear equations. If only the spectrum of the anomaly is used, the system of equations becomes ill‐conditioned in most cases resulting in grossly inaccurate solutions. However, particular combinations of the spectra of the anomaly and its first and second order moments are found to improve significantly the behavior of the system of equations and thus the quality of results. It has also been found that the mean values of corner locations can be calculated fairly accurately by taking the ratios of the spectra of the anomaly and its moments. Once the corner locations are found, computation of the density contrast in the case of a gravity anomaly and the magnetization contrast for a magnetic anomaly is straightforward.

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Offshore extensions of the Avalon Zone of Newfoundland

Canadian Journal of Earth Sciences ◽

10.1139/e85-119 ◽

1985 ◽

Vol 22 (8) ◽

pp. 1163-1170 ◽

Cited By ~ 7

Author(s):

H. G. Miller ◽

A. K. Goodacre ◽

R. V. Cooper ◽

D. Halliday

Keyword(s):

Strong Correlation ◽

Sedimentary Basin ◽

Magnetic Data ◽

Two Dimensional ◽

Gravity And Magnetic ◽

Magnetic Modelling ◽

Dimensional Gravity ◽

Gravity And Magnetic Data ◽

Avalon Peninsula ◽

Block Faulting

Gravity and magnetic data from the nearshore region of the Avalon Zone of Newfoundland provide evidence of the nature of a sedimentary basin immediately east of the Avalon Peninsula. The data also suggest a strong correlation between the arcuate magnetic patterns of the offshore portion of the Avalon Zone and the Precambrian mafic volcanics mapped onshore. The offshore basin is interpreted to have been formed by Precambrian block faulting and subsequently filled by sediments derived from surrounding paleotopographic highs. Two-dimensional gravity and magnetic modelling results suggest the presence of mafic material beneath the modelled 9+ km of sediment in the basin.

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A geophysical interpretation of the geology of Conception Bay, Newfoundland

Canadian Journal of Earth Sciences ◽

10.1139/e83-128 ◽

1983 ◽

Vol 20 (9) ◽

pp. 1421-1433 ◽

Cited By ~ 5

Author(s):

H. G. Miller

Keyword(s):

Magnetic Susceptibility ◽

Volcanic Rocks ◽

Magnetic Anomalies ◽

Vertical Movement ◽

Geophysical Data ◽

Two Dimensional ◽

Geophysical Interpretation ◽

Gravity And Magnetic ◽

Conception Bay ◽

Avalon Peninsula

Geophysical data from Conception Bay and the adjacent peninsulas of the Avalon Peninsula, Newfoundland are presented and quantitatively interpreted using two-dimensional models to interpret the geology beneath the bay. The portion of the bay underlain by mafic volcanic rocks is determined and the maximum extent of the Cambro-Ordovician rocks containing the Wabana hematite deposit is delineated. All gravity and magnetic anomalies in the area are explained in terms of density and magnetic susceptibility variations confined to the upper 12 km of the crust. The geophysical models indicate that mafic volcanics underlie a significant portion of the study area and are more extensive than indicated by the surface outcrop on land. The models also indicate significant vertical movement on the Topsail Fault and on the extension of a fault passing out into the bay near Holyrood. The Cambro-Ordovician sediments are confined to the southern portion of the block bounded by these faults. The geophysical data are unable to detect the presence of the mafic volcanics east of the Topsail Fault in the study area.

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