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Analytic continuation; a tool for aeromagnetic data interpretation

Jeffrey B. Thurston and Bengt Fornberg
Analytic continuation; a tool for aeromagnetic data interpretation (in Gravity, electrical, and magnetic methods, Irina Filina (prefacer), Maurizio Fedi (prefacer), Jiajia Sun (prefacer) and Alan Morgan (prefacer))
Leading Edge (Tulsa, OK) (April 2024) 43 (4): 228-234


Extending potential-field data to vertical complex-plane slices provides opportunities, unique to analytic functions, for subsurface imaging. We begin with an existing numerical method for differentiation by integration using Cauchy's integral formula. An example from data over northern Ontario, Canada, illustrates better conditioning compared to derivatives computed using finite differences. Next, these quantities are used for analytic continuation using rational complex-series expansion (Pade approximation). This method provides stable downward continuation to distances that are significantly greater than are achievable by Taylor series and spectral methods. A synthetic test confirms that the rational approximant faithfully reproduces the magnetic response of a thin sheet. A sign reversal of the total field, coincident with the origin of the sheet, features prominently on this example. We also provide synthetic results for sheets with finite depth extent, point sources, thin slabs, and contacts. These also exhibit polarity flips near the source location. Further, these provide a template for interpreting the source geometry using the shape of the approximating function. Additional synthetic tests illustrate an automatic method for locating closely spaced random assemblages of isolated poles. We apply our methods to a deposit-scale heliborne survey over a recently discovered volcanogenic massive sulphide ore deposit in a Canadian greenstone belt. The analytically continued profile over the deposit suggests the presence of a subvertical thin sheet of unknown depth extent. This interpretation coincides with a sulphide ore body that has been previously delimited by several drill holes. Some known prospects near this deposit are also imaged on maps and profiles of the Pade approximant.

ISSN: 1070-485X
EISSN: 1938-3789
Serial Title: Leading Edge (Tulsa, OK)
Serial Volume: 43
Serial Issue: 4
Title: Analytic continuation; a tool for aeromagnetic data interpretation
Title: Gravity, electrical, and magnetic methods
Author(s): Thurston, Jeffrey B.Fornberg, Bengt
Author(s): Filina, Irinaprefacer
Author(s): Fedi, Maurizioprefacer
Author(s): Sun, Jiajiaprefacer
Author(s): Morgan, Alanprefacer
Affiliation: Consultant, Calgary, AB, Canada
Affiliation: University of Nebraska-Lincoln, Lincoln, NE, United States
Pages: 228-234
Published: 202404
Text Language: English
Publisher: Society of Exploration Geophysicists, Tulsa, OK, United States
References: 21
Accession Number: 2024-032270
Categories: Applied geophysicsEconomic geology, geology of ore deposits
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. sects.
N42°00'00" - N57°00'00", W95°00'00" - W74°00'00"
Secondary Affiliation: University of Colorado Boulder, USA, United States
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2024, American Geosciences Institute. Reference includes data from GeoScienceWorld, Alexandria, VA, United States. Reference includes data supplied by Society of Exploration Geophysicists, Tulsa, OK, United States
Update Code: 2024
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