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Outcrop fracture characterization using terrestrial laser scanners: Deep-water Jackfork sandstone at Big Rock Quarry, Arkansas

¹ Geosciences Department, University of Texas at Dallas, P.O. Box 830688, Mail Station FO21, Richardson, Texas 75083-0688, USA
² Real Earth Models LLC, 4100 McEwen Road, Suite 240, Dallas, Texas 75244-5184, USA

View larger version (43K): [in this window] [in a new window]   Figure 1. Surface classification algorithm applied to synthetic data. (A) Pass 1, the multileader cluster algorithm produces 81 clusters with an average (Navg) of 15 points per cluster (where Nmin is the minimum number of points allowed in each cluster and Nmax is the maximum number of points allowed per cluster). (B) Pass 2, the final partition produced by the k-means cluster algorithm identified three clusters, corresponding to the three orientations of the six faces of the test shape. Parallel faces were assigned to the same cluster since they have the same orientation. (C) The optimum number of three clusters in the final partition is determined by the L-shaped break value of the minimum description length (MDL) criterion. (D) Equal-area stereographic projection of the orientation data. The distribution of poles to the planes defined in the pass 1 partition indicates that there is a strong clustering about the mean for each orientation. Mean vectors for each cluster are plotted as circles.

View larger version (35K): [in this window] [in a new window]   Figure 2. Location map of the outcrop at Big Rock Quarry (star symbol) and tectonic map of the Ouachita salient during the Ouachita orogeny (modified after Whitaker and Engelder, 2006). Foreland basins are shaded; Broken Bow (BBU) and Benton (BU) uplifts are stippled. DRU—Devil's River Uplift; FWB—Fort Worth Basin; AMB—Ardmore/Marietta Basin; AU—Arbuckle Uplift; AB—Arkoma Basin; JM—Jackfork Mountain Fault; MSB—Mississippi Slate Belt; BWB—Black Warrior Basin; OAJ—Ouachita-Appalachian Juncture.

View larger version (29K): [in this window] [in a new window]   Figure 3. Map view of the outcrop. Big Rock Quarry is located along the north bank of the Arkansas River in North Little Rock, Arkansas. The exposure has a horseshoe shape and is almost one kilometer long and 60 m high. Black boxes delimit the areas used in the surface classification and fracture analysis. The areas are labeled with corresponding figure numbers. The ones that are not labeled were analyzed, but they are not detailed in this paper.

View larger version (94K): [in this window] [in a new window]   Figure 4. Three-dimensional model of part of the outcrop at Big Rock Quarry built from laser scanned (east, north, elevation) points. Total outcrop face is 250 m long and 50 m high. (A) Perspective view (looking northwest) of the laser mapping by a low-resolution, robotic laser range-finding system. The 2722 points are decimeter spaced and are accurate to 5 cm. (B) Three-dimensional terrain model of the outcrop from the points in A. Due to the low resolution of the terrain data, the fracture pattern is not evident. (C) High-resolution point cloud of the same part of the outcrop as in A. The model has 613,083 points at about half-centimeter spacing. (D) Three-dimensional terrain model of the outcrop from the points in C. Fractured pattern of the outcrop is clearly visible. White borders delimit the areas used in the surface classification and fracture analysis. (E) Photo realistic model of the outcrop at Big Rock Quarry.

View larger version (56K): [in this window] [in a new window]   Figure 5. Surface classification algorithm applied to the low-density point cloud. (A) Perspective view of the northwest part of the outcrop, looking northwest; total outcrop face is 250 m long and 50 m high. (B) Pass 2 partition identifies four different orientations of the cliff faces. Clusters plotted with the same color have the same orientation. (C) The optimum number of four clusters in the final partition is determined by the L-shaped break value of the minimum description length (MDL) criterion. (D) Equal-area stereographic projection of the orientation data. The distribution of poles to the planes defined in the pass 1 partition indicates that the clusters are oriented NS or NE-SW, which reflect the trend of the outcrop as can be seen in the map view in Figure 4B. Mean vectors for each cluster are plotted as circles.

View larger version (78K): [in this window] [in a new window]   Figure 6. Surface classification and fracture orientation of area 1, Big Rock Quarry. (A) High-resolution image of the outcrop with fracture planes clearly visible. (B) Surface geometry of the outcrop is remotely captured from a dense 3-D point cloud (7212 points) and displayed here as a triangulated mesh. Distinct orientations are shown as color-coded point clouds, where each color represents a particular orientation, red (NE) and green (NW). (C) Seven clusters, corresponding to the identified orientations, are shown as color-coded point clouds. Three primary fracture orientations were observed: NE- (red), NW- (green), and NNW-striking (yellow) fracture sets. The bedding is displayed in black. The green (NW) and red (NE) planes are nearly orthogonal to each other, and both of them are perpendicular to bedding. (D) The optimum number of seven clusters in the final partition is determined by the L-shaped break value of the minimum description length (MDL) criterion. (E) Equal-area stereographic projection of the orientation data. The distribution of poles to the planes defined in the pass 1 partition indicates that there is more variation in strike orientation within each cluster than in dip. Mean vectors for each cluster are plotted as circles.

View larger version (97K): [in this window] [in a new window]   Figure 7. Orientation of the fracture planes in thin sandstone beds in area 2, Big Rock Quarry. The relatively close spacing of the fractures in these thinner (centimeter- to decimeter-thick) sandstone beds is consistent with the relatively small bed thickness. (A) Picture of the outcrop showing fractured sandstone beds. This part of the outcrop is positioned at 20 m below the part analyzed in Figure 6. (B) Virtual model of the part of the outcrop represented in part A. The point cloud mapping the fracture planes oriented NW is shown in green. (C–D). Surface classification analysis identifies the following three orientations: NNW (yellow), NE (red), and NW (green). Due to the high volume of data, this region of the outcrop has been divided in two parts (left [C] and right [D]) for analysis. (E–F) Equal-area stereographic projections of poles to fracture planes (the colors correspond to the identified orientations). The position of the mean vectors is plotted on the stereonet as a circle.

View larger version (76K): [in this window] [in a new window]   Figure 8. Orientation of the fracture planes in thick sandstone beds in area 3, Big Rock Quarry. Amalgamated sandstone reaches a thickness of 2 m. (A) Picture of the outcrop showing fractured sandstone beds. (B) Virtual model of the outcrop displayed in part A. Distinct planar orientations are shown as color-coded point clouds, where each color represents a particular orientation, red (NE), yellow (NNW), and green (NW). The yellow and green faces are more planar than the red faces. (C) Surface classification analysis identifies three orientations: NE-oriented fracture planes (red), NW-oriented planes (green), and NNW-oriented planes (yellow), and the bedding is displayed in black. Fracture orientations are consistent with the orientations in Figures 6 and 7, implying a consistent mechanical behavior of the rocks. (D) Equal-area stereographic projection of poles to fracture planes (the colors correspond to the identified orientations). The positions of mean vectors are plotted on the stereonet as circles.

View larger version (86K): [in this window] [in a new window]   Figure 9. Orientation of the fracture planes in thick sandstone beds in area 4, Big Rock Quarry. Area 4 is 100 m distant from area 1 (Fig. 6), and sandstones reach a thickness of 1 m. (A) Picture of the outcrop displaying thick, continuous sandstone beds. (B) Virtual model of the outcrop in A with the point cloud mapping the NW-oriented fracture planes shown in green. (C) Surface classification analysis identifies three orientations for the fracture planes: NW (green), NNW (yellow), and NE (red). Fracture orientations are consistent with those found at the other locations. The magenta points are debris slopes accumulated on ledges. (D) Equal-area stereographic projection of poles to fracture planes (the colors correspond to the identified orientations). Mean vectors are plotted on the stereonet as circles.

View larger version (20K): [in this window] [in a new window]   Figure 10. Equal-area stereographic projection for the main orientations identified at the outcrop. (A) Fracture orientation measurements indicate that three major sets persist across the outcrop—NW (green), NNW (yellow), and NE (red)—and all of them are nearly perpendicular to bedding (black). (B) Mean values for the three identified orientations are: N43.27E/86.54E (northeast), N46.22W/89.06W (northwest), and N15.11W/88.81W (north-northwest).