Merge branch 'main' into patch-6

content/foundations/seeing_underground/foundations_seeing_underground_example.rst

@@ -3,27 +3,27 @@
 Mineral exploration example
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-Large quantities of magnetic field measurements are routinely gathered over mineral and petroleum exploration prospects using airborne techniques. Resulting magnetic anomaly maps can provide information about geological trends because rocks containing higher proportions of the mineral magnetite have a higher magnetic susceptibility, and will affect the local behavior of the earth's magnetic field. Data can also be inverted to reveal three dimensional features of the earth. 
+Large quantities of magnetic field measurements are routinely gathered over mineral and petroleum exploration prospects using airborne techniques. Resulting magnetic anomaly maps can provide information about geological trends because rocks containing higher proportions of the mineral magnetite have a higher magnetic susceptibility and will affect the local behavior of the earth's magnetic field. Data can also be inverted to reveal three-dimensional features of the earth. 
 
 Regional and local magnetic surveys
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-Figure 3 (supplied courtesy of Placer Dome Exploration) provides an example of regional information from an area surrounding the Mt Milligan copper porphyry deposit, located in central British Columbia. Geological trends can be discerned using this type of data, however, exploration for a specific deposit requires more detailed information about local subsurface distributions of rock types. Figure 3b shows anomalous strengths of the earth's magnetic field for a small region of one ore body. Evidently there is a range of different rock types below the surface, but details of location, depth and magnetic susceptibility are difficult to determine directly using conventional methods. 
+Figure 3 (supplied courtesy of Placer Dome Exploration) provides an example of regional information from an area surrounding the Mt Milligan copper porphyry deposit in central British Columbia. Geological trends can be discerned using this type of data; however, exploration for a specific deposit requires more detailed information about local subsurface distributions of rock types. Figure 3b shows anomalous strengths of the earth's magnetic field for a small region of one ore body. Evidently, there is a range of different rock types below the surface, but details of location, depth and magnetic susceptibility are difficult to determine directly using conventional methods. 
 
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 Inversion to obtain 3D details 
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-The goal of inverting this data set was to produce detailed 3D models of magnetic susceptibility to help geologists develop a more complete understanding of the rocks associated with the ore deposit. The first step was to reduce the dense data set from the small region (Figure 3a) to a more manageable 1,029 evenly spaced data points and to divide the model region into 169,000 cells. Then a desirable model type was chosen. In this instance, the process was set up with two criteria; namely to find a model that was (i) as close as possible to a uniform earth with zero susceptibility, and (ii) included structure that was smooth in all three spatial dimensions. 
+The goal of inverting this data set was to produce detailed 3D models of magnetic susceptibility to help geologists develop a more complete understanding of the rocks associated with the ore deposit. The first step was to reduce the dense data set from the small region (Figure 3a) to a more manageable 1,029 evenly spaced data points and to divide the model region into 169,000 cells. Then, a desirable model type was chosen. In this instance, the process was set up with two criteria, namely, to find a model that was (i) as close as possible to a uniform earth with zero susceptibility and (ii) included structure that was smooth in all three spatial dimensions.
 
 In addition, the numerical procedure for finding plausible subsurface models of susceptibility was constrained so that data predicted from the model would match observed field measurements to a degree specified by assuming a noise level (on measurements) of 5%. The resulting model was a 3D volume represented by the 169,000 cells, each with a magnetic susceptibility recovered by the inversion. 
 
 Visualizing results
 ===================
 
-There are several ways to usefully present volumetric information of this kind. Contour plots of horizontal or vertical slices through the volume, as shown in Figure 4, provide quantitative details at any required location. Alternatively, for a more general impression of the model, a 3D isosurfacee image can be created. This is shown in Figure 5, which suggests there is a well-defined volume of magnetically susceptible rocks associated with this deposit. This model correlates well with one of the known principal local rock units (MBX monsonite stock) and with locations of mineralization. 
+There are several ways to usefully present volumetric information of this kind. Contour plots of horizontal or vertical slices through the volume, as shown in Figure 4, provide quantitative details at any required location. Alternatively, for a more general impression of the model, a 3D isosurface image can be created. This is shown in Figure 5, which suggests there is a well-defined volume of magnetically susceptible rocks associated with this deposit. This model correlates well with one of the known principal local rock units (MBX monzonite stock) and with locations of mineralization. 
 
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 Corroboration with independent geophysical results
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-Few geophysical surveys are used alone with no other independent information. At Mt Milligan many types geophysical surveys were performed on the ground, from airborne platforms, and from within boreholes. For example, a similar inversion procedure was used to interpret DC electrical measurements gathered over the same area. The 3D isosurfacee image of Figure 6 shows a model of the distribution of chargeability (the capacity for material to hold an electrical charge), a physical property related essentially to metal or clay content and grain size. The apparent anti-correlation between magnetic susceptibility and chargeability at Mt Milligan is evident only after careful inversion of two unrelated geophysical data sets. This example illustrates that conducting inversions on multiple types of data can provide an enhanced understanding of the surveyed region; in this case it provides insight about subsequent alteration of the rocks that occurred after the initial formation of the mineral deposit.
+Few geophysical surveys are used alone with no other independent information. At Mt Milligan, many types of geophysical surveys were performed on the ground, from airborne platforms, and from within boreholes. For example, a similar inversion procedure was used to interpret DC electrical measurements gathered over the same area. The 3D isosurface image of Figure 6 shows a model of the distribution of chargeability (the capacity for material to hold an electrical charge), a physical property related essentially to metal or clay content and grain size. The apparent anti-correlation between magnetic susceptibility and chargeability at Mt Milligan is evident only after careful inversion of two unrelated geophysical data sets. This example illustrates that conducting inversions on multiple types of data can provide an enhanced understanding of the surveyed region; in this case, it provides insight about subsequent alteration of the rocks that occurred after the initial formation of the mineral deposit.
 
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content/foundations/seeing_underground/foundations_seeing_underground_primer.rst

@@ -7,12 +7,12 @@ Geophysics primer
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-Geophysical surveys are performed when information about the earth's subsurface is desired but direct sampling through expensive and invasive techniques such as drilling or trenching is insufficient, impractical or ill-advised. A survey may be as large as the whole Earth, as small as the top few meters of the subsurface, or anywhere in between. 
+Geophysical surveys are performed when information about the earth's subsurface is desired, but direct sampling through expensive and invasive techniques such as drilling or trenching is insufficient, impractical or ill-advised. A survey may be as large as the whole Earth, as small as the top few meters of the subsurface, or anywhere in between. 
 
 Measuring physical properties
 =============================
 
-During a geophysical survey, energy is put into the earth and responses are recorded at the surface, in the air or in boreholes. Resulting data reveal information about the earth because the behavior of the energy within the ground is controlled by the distribution of the earth's physical properties. For instance, one basic physical property is magnetic susceptibility, which describes a rock's ability to become magnetized. This physical property provides information on rock type and structures because the rock's magnetic susceptibility relates directly to mineral type, to the chemical alteration processes. A second important physical property is electrical conductivity, which quantifies a material's ability to carry electrical current. Figure 2 illustrates one way that a geophysical survey can be carried out to provide information about the subsurface distribution of electrical conductivity.
+During a geophysical survey, energy is put into the earth and responses are recorded at the surface, in the air or in boreholes. Resulting data reveal information about the earth because the behavior of the energy within the ground is controlled by the distribution of the earth's physical properties. For instance, one basic physical property is magnetic susceptibility, which describes a rock's ability to become magnetized. This physical property provides information on rock type and structures because the rock's magnetic susceptibility relates directly to mineral type and the chemical alteration processes. A second important physical property is electrical conductivity, which quantifies a material's ability to carry electrical current. Figure 2 illustrates one way a geophysical survey can be carried out to provide information about the subsurface distribution of electrical conductivity.
 
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 Traditional interpretation
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-Traditionally, useful information was extracted from geophysical field results by examining maps or line profiles of raw or filtered survey data. Such images are useful for estimating locations and quantities of buried materials, and to help choose locations for more invasive (and expensive) techniques such as drilling. For example, large scale maps of magnetic or gravity data often show geologic structure, or identify an anomalous region that might be associated with a desired target. As an example Figure 3 shows the magnetic data acquired at the Bathurst region of New Brunswick. The major features observed are related to geologic structure. 
+Traditionally, useful information was extracted from geophysical field results by examining maps or line profiles of raw or filtered survey data. Such images are useful for estimating locations and quantities of buried materials and to help choose locations for more invasive (and expensive) techniques such as drilling. For example, large-scale maps of magnetic or gravity data often show the geologic structure or identify an anomalous region that might be associated with a desired target. As an example, Figure 3 shows the magnetic data acquired at the Bathurst region of New Brunswick. The major features observed are related to geologic structure. 
 
 
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-The goal of the inverse problem is to find a mathematical model of the earth that produced the field observations. That is "what subsurface physical property distribution caused the data that were observed at the surface?" Earlier inversion solutions involved characterizing the earth by a few prisms or layers and finding geometrical and physical properties of these simplified earth models. 
+The goal of the inverse problem is to find a mathematical model of the earth that produced the field observations. That is, "What subsurface physical property distribution caused the data  observed at the surface?" Earlier inversion solutions involved characterizing the earth by a few prisms or layers and finding geometrical and physical properties of these simplified earth models. 
 
-Due to the earth's extreme complexity, useful models often need to have many parameters, usually more than the number of data. This means that the problem of finding a model (i.e. estimating values for every parameter) is one in which there are more unknowns that data. Such problems do not have unique solutions, and this nonuniqueness is exacerbated when data are noisy or inaccurate. Formal inversion methods address these issues using well defined mathematical techniques. An appendix explains inversion in a little more detail. 
+Due to the earth's extreme complexity, useful models often need many parameters, usually more than the number of data. This means that the problem of finding a model (i.e. estimating values for every parameter) is one in which there are more unknowns than data. Such problems do not have unique solutions, and this nonuniqueness is exacerbated when data are noisy or inaccurate. Formal inversion methods address these issues using well-defined mathematical techniques. An appendix explains inversion in a little more detail. 
 
 Some benefits of applying rigorous inversion can be seen by comparing traditional map and pseudosection plots of the raw data to the information in 3D and 2D models obtained by inversion.