b'Coping with ambiguityFeatureCoping with ambiguity in geophysical dataJames E. HannesonFigure2 shows aeromagnetic contours for a 9.5 x 6.4 km area Adelaide Mining Geophysics Pty Ltdcovering what I call the Dubawnt Lake anomaly. It is on the Adelaide, Australiaboundary of map sheets 75P/1 and 75P/8 of a c. 1978 version of jim.hanneson@bigpond.com the Geological Survey of Canadas Aeromagnetic Series. Figure3 shows hand drawn contours from the original processing of a Introduction more detailed aeromagnetic survey, and Figure 4 shows the gravity stations and contours for the same area.All geophysical datasets are to some degree ambiguous.In 1978, I was asked to estimate the depth to the top of this Figure 1 is hardly necessary for geophysical readers; however,feature to determine if drill testing was feasible. I have used geological readers might find it informative. It illustrates thathere the original diagrams, partly to save work and partly to hint the size of a buried dense or magnetic spherical body cannot beat what internal technical reports often looked like and how determined solely from variations in the field values measuredgeoscientists coped with data presentation in the pre-digital age.at the earths surface. From gravity data the depth to the centre and the excess mass (density contrast x volume) can be determined, but the diameter cant be determined without knowledge of the density, and vice versa. Consequently, the depth to the top of such a feature, needed to optimize drillhole design, would remain unknown.The smaller sphere has a density of 6.4 gm/cc, a magnetic susceptibility of 0.64 SI and a radius of 50 m, while these values for the larger one are 0.1 gm/cc, 0.01 SI and 200 m, respectively. The earths magnetic field is typical for central South Australia. The responses for both spheres are precisely the sameas though the causes of the anomalies are concentrated at the sphere centres; furthermore, this is approximately true a short distance above an irregular but equant-shaped body where length, width and height are comparable. This problem, which includes the magnetisation-volume product in magnetics and the conductivity-thickness product for dikes in electromagnetics (EM; not shown), justifies calling geophysical data ambiguous.Avoiding this ambiguity would seem to be a forlorn hope.Figure 2.Aeromagnetic map. Sensor height 305 m, Map area is 9.5 x 6.4 km and the contour interval is variable.However, if a target body has more than one physical property contrast, the ambiguity can be resolved (or at least reduced) by the collection and analysis of an additional (independent) dataset, as the following example illustrates.The Dubawnt Lake GabbroThe feature of interest is a bulls eye magnetic anomaly near longitude 103.1 W, latitude 63.0 N in the Canadian Arctic. Figure 1.Profiles of the calculated gravity (red) and magnetic (blue) responses along a north-south cross-section through two spheres with the sameFigure 3.Detailed aeromagnetic map. Sensor height of 46 m. Map area 850 x excess mass and susceptibility-volume product. 975 m. Contour interval 500 nT.45 PREVIEW DECEMBER 2022'