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Orientation uncertainty goes bananas: An algorithm to visualise the uncertainty sample space on stereonets for oriented objects measured in boreholes

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  • Terra Mobile Consultants

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Measurements of structure orientations are afflicted with uncertainties which arise from many sources. Commonly, such uncertainties involve instrument imprecision, external disturbances and human factors. The aggregated uncertainty depends on the uncertainty of each of the sources. The orientation of an object measured in a borehole (e.g. a fracture) is calculated using four parameters: the bearing and inclination of the borehole and two relative angles of the measured object to the borehole. Each parameter may be a result of one or several measurements. The aim of this paper is to develop a method to both calculate and visualize the aggregated uncertainty resulting from the uncertainty in each of the four geometrical constituents. Numerical methods were used to develop a VBA-application in Microsoft Excel to calculate the aggregated uncertainty. The code calculates two different representations of the aggregated uncertainty: a 1-parameter uncertainty, the ‘minimum dihedral angle’, denoted by Ω; and, a non-parametric visual representation of the uncertainty, denoted by χ. The simple 1-parameter uncertainty algorithm calculates the minimum dihedral angle accurately, but overestimates the probability space that plots as an ellipsoid on a lower hemisphere stereonet. The non-parametric representation plots the uncertainty probability space accurately, usually as a sector of an annulus for steeply inclined boreholes, but is difficult to express numerically. The 1-parameter uncertainty can be used for evaluating statistics of large datasets whilst the non-parametric representation is useful when scrutinizing single or a few objects.
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... Except for the initial work carried out at the Swedish Nuclear Fuel and Waste Management Co., SKB (Stigsson et al 2014;Stigsson andMunier 2012, 2013;Munier and Stigsson 2007), no attempts to comprehensively evaluate the uncertainty space of objects measured in cored boreholes are available in the literature. The present work, hence, aims to fill this gap by: (1) defining the different sources that may affect the orientation uncertainty; (2) proposing evaluation methods and deriving equations for the inference of the magnitudes of the uncertainties; and (3) briefly pointing out problems arising due to the uncertainties but also noting potential benefits when one more fully understands and quantifies the uncertainty space. ...
... More detailed explanations are found in, e.g. Stigsson (2015). As the fractures are supposed to be self-affine, rather than self-similar, they need both an amplitude measure and a fractal dimension to be fully constrained. ...
... As the fractures are supposed to be self-affine, rather than self-similar, they need both an amplitude measure and a fractal dimension to be fully constrained. One such method is the standard deviation of the correlation function (RMS-COR) method that has been successfully used by Renard et al (2006), Candela et al. (2009) andStigsson (2015). The outcome of the method is a scaling relationship described by Eq. (7). ...
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Preliminary site description Laxemar subarea—version 1.2. SKB R-06-10, Svensk Kärnbränslehantering AB Available at: 〈http://www.skb.se/upload
SKB, 2006. Preliminary site description Laxemar subarea—version 1.2. SKB R-06-10, Svensk Kärnbränslehantering AB. Available at: 〈http://www.skb.se/upload/pub lications/pdf/R-06-10.pdf〉.
Preliminary site description Forsmark area-version 1.2. SKB R-05-18
SKB, 2005. Preliminary site description Forsmark area-version 1.2. SKB R-05-18, Svensk Kärnbränslehantering AB. Available at: 〈http://www.skb.se/upload/pub lications/pdf/R-05-18.pdf〉.
Preliminary site description Laxemar subarea-version 1.2. SKB R-06-10
SKB, 2006. Preliminary site description Laxemar subarea-version 1.2. SKB R-06-10, Svensk Kärnbränslehantering AB. Available at: 〈http://www.skb.se/upload/pub lications/pdf/R-06-10.pdf〉.