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

  • 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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Many engineering applications in fractured crystalline rocks use measured orientations of structures such as rock contact and fractures, and lineated objects such as foliation and rock stress, mapped in boreholes as their foundation. Despite that these measurements are afflicted with uncertainties, very few attempts to quantify their magnitudes and effects on the inferred orientations have been reported. Only relying on the specification of tool imprecision may considerably underestimate the actual uncertainty space. The present work identifies nine sources of uncertainties, develops inference models of their magnitudes, and points out possible implications for the inference on orientation models and thereby effects on downstream models. The uncertainty analysis in this work builds on a unique data set from site investigations, performed by the Swedish Nuclear Fuel and Waste Management Co. (SKB). During these investigations, more than 70 boreholes with a maximum depth of 1 km were drilled in crystalline rock with a cumulative length of more than 34 km including almost 200,000 single fracture intercepts. The work presented, hence, relies on orientation of fractures. However, the techniques to infer the magnitude of orientation uncertainty may be applied to all types of structures and lineated objects in boreholes. The uncertainties are not solely detrimental, but can be valuable, provided that the reason for their presence is properly understood and the magnitudes correctly inferred. The main findings of this work are as follows: (1) knowledge of the orientation uncertainty is crucial in order to be able to infer correct orientation model and parameters coupled to the fracture sets; (2) it is important to perform multiple measurements to be able to infer the actual uncertainty instead of relying on the theoretical uncertainty provided by the manufacturers; (3) it is important to use the most appropriate tool for the prevailing circumstances; and (4) the single most important parameter to decrease the uncertainty space is to avoid drilling steeper than about −80°.
... Software like Wellcad (Crow et al., 2013;Chatfield 2015aChatfield , 2015b) and StereoCore PhotoLog system (Orpen, 2014) is specifically designed to compare different oriented core data sets. Numerical methods of calculating orientational uncertainty were developed by Stigsson and Munier (2013). ...
Structural data is vital for the understanding of the geometry and evolution of a deposit and feeds into geologic, structural, resource, and geotechnical models. Accurate models are critical for targeting, resource estimation, and geotechnical design and, if rapidly available, support real-time decisions on drilling and grade control. Structural drill core data add a high-resolution data set to traditional data from mapping or the structural interpretation of remote sensing and geophysical data and, therefore, add indispensable information to any integrated model. In this paper we propose standardized workflows for data collection, review technological advances and quality control processes accelerating structural data collection from both oriented drill core and televiewer techniques , and provide an overview of structures that may be observed in drill core and discuss their significance to record for the geometry of the deposit. Critical to the data collection process is an interpretative process that recognizes and identifies domain-based structures that ultimately are fundamental to developing 3-D structural models.
... The orientation of a fracture measured in a borehole can be calculated using four angles; the bearing and inclination of the borehole together with two angles of the intercept in the local borehole coordinate system, α and β (Stigsson and Munier, 2013). These angles are used to calculate the orientation of the fracture in the global coordinate system using ...
Conference Paper
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Many engineering applications in crystalline rocks use fracture intercepts mapped in boreholes as foundation. From this mapping the distributions of intensity, spatiality and orientation can be inferred. These three distributions combined with the size distribution steer the connectivity of the fracture network and hence the nature of groundwater flow and transport of solutes. This study, however, only focuses on the impact that the orientation uncertainty has on the connectivity. The orientation of a fracture intersecting a borehole can be calculated using four angles, each afflicted with uncertainty. These uncertainties are used to distinguish between natural variability and uncertainty using a χ2 test of a contingency table of fracture poles. Two DFN models are developed, the rock mass fracture and the “measured” model, and the differences in connectivity between the models are analyzed. The rock mass fracture model have 30% more connected fractures, 60% more connected fracture area and is more elongated than the “measured” model at the connectivity level when the first fracture of the two clusters hits any boundary of the modelling cube.
... Thus drill core is the preeminent source of structural data, particularly since each plane can also be identified as a joint, bedding surface, etc., and fully describedinformation that cannot be obtained from televiewer scans. Hence orienting core should be the norm for all drilling contractswhy then is this not the case [2]? ...
Conference Paper
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Whilst drilling techniques are continually improving to obtain continuous, solid core from ever more difficult ground, logging systems remain largely manual with low auditability – such that much of the data used for mapping the sub-surface is entered on trust. Additionally matching core logs with downhole geophysics is often difficult, particularly comparing structures with those measured in televiewer scans, leading to significant uncertainties in modelling the ground sampled. As a result the full value of most drilling projects is seldom attained. In the main these shortcomings are due to inadequate monitoring of the drilling process to ensure the accuracy and reliability of three critical aspects, being: (i) depth registration of the core, to properly locate the position of every feature logged along the borehole path, (ii) core orientation, to precisely define the intersection of the geographic vertical plane with the core segments so that the dip and dip direction of structures intersected in the core can be reliably determined, and (iii) borehole surveying to correctly define the borehole path from collar to end-of-hole, enabling well-constructed models of the logged data to be built. Procedures are put forward to address these issues, following which an image based logging system is presented for optimizing the logging process. Digital photographs are used to create virtual 3-D models of the core on which contacts and structures are picked to record their depth, measure alpha/beta angles and enter full geotechnical and lithological descriptions, using templates customized to conform to the client’s existing database codes. Thus the data is immediately available for audit, much of which can be done offsite using the annotated images. Depth registration is also adjustable, and at any stage, to facilitate alignment and meaningful comparison with downhole geophysics data, which can then also be used with confidence to augment gaps in the core logs as well as to correct sections of poor or non-existent core orientation. Such quality assured data is essential for reliable determination of fracture patterns and their distribution, and to match the joint sets with those mapped in outcrop to define their size and termination for realistic geomechanical simulation of the ground investigated.
In the field of biomechanics which aims to understand the mechanics of living systems, the main difficulty is to provide the experimental data reflecting the multiphysical behavior of the systems of interest. Uncertainties on the experimental available data exist as human variability, measuring protocols and numerical processing. This chapter describes the fundamental and conceptual aspects of the data uncertainty modeling in biomechanics. Different biomechanics data types and related parameters such as physiological, morphological, mechanical, and kinematics and kinetics properties and their uncertainty sources (e.g. experimental and numerical) are identified and introduced. The chapter also presents modeling approaches based on the types and representations of uncertainty. Finally it presents an example of the propagation of data uncertainty and decision-making through a numerical model.
Core orientation data are crucial in many types of geologic or reservoir engineering studies. The paleomagnetic plug sampling technique and the multishot camera technique are the main alternative procedures currently available for orienting cores. In most ways the paleomagnetic technique is simpler than the multishot technique since fewer people and less rig equipment and time are involved. For these reasons the potential for uncorrectable error in orientation is also reduced. Paleomagnetic orientation is more difficult if several components of magnetization are present in each plug sample. Also it is not subject to a wide variety of operational difficulties that can compromise multishot data. High bottom hole temperatures work to the advantage of the paleomagnetic technique. Poor recovery will not affect paleomagnetic orientation accuracy if the rock recovered is at least competent enough to assemble a continuous interval.
Coring, the cutting of a representative cylinder of rock during drilling, is perhaps the most important technique available for gaining first-hand knowledge of the subsurface. The goal of coring is to recover an undisturbed sample of the formation being penetrated. Cores can be used to supply orientation information for evaluating any directional characteristics of a formation. Orientation of core with respect to geographic north is necessary to determine sediment transport directions, downhole stress fields, permeability or porosity directions, or fracture directions. Directional data can play a major role in evaluating field economics, often affecting decisions relating to well spacing, location, and number. The authors have studied the problems associated with core orientation and believe the following discussion can assist in reducing the risk and cost of core orienting operations.
Error analysis of oriented-core data indicates that: (1) in the error in the coring and surveying procedure is +/- 5°; (2) the error inherent to the mechanical goniometer is +/- 2 3/4°; (3) the reproducibility error in using the goniometer is +/- 2° for one analyst and +/- 4° for multiple analysts; and (4) the uncertainty in interpolating and manipulating sets of orientation data may be quite large. Overall error in oriented core procedures should be considered +/- 11°. A simple set of guidelines can be used to determine the most accurately oriented zones within the core. The guidelines are based on the physical character of the core, orientation scribes, and patterns displayed by plotting the azimuth of the orientation grove versus depth. Approximately 26% of oriented core analyzed fits the guidelines of most accurately oriented core. 15 references, 7 figures, 6 tables.
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