Pseudo-Outcrop Visualization of Borehole Images and Core Scans

A pseudo-outcrop visualization is demonstrated for borehole and full-diameter rock core images to augment the ubiquitous unwrapped cylinder view and thereby to assist non-specialist interpreters. The pseudo-outcrop visualization is equivalent to a nonlinear projection of the image from borehole to earth frame of reference that creates a solid volume sliced longitudinally to reveal two or more faces in which the orientations of geological features indicate what is observed in the subsurface. A proxy for grain size is used to modulate the external dimensions of the plot to mimic profiles seen in real outcrops. The volume is created from a mixture of geological boundary elements and texture, the latter being the residue after the sum of boundary elements is subtracted from the original data. In the case of measurements from wireline microresistivity tools, whose circumferential coverage is substantially less than 100%, the missing circumferential data is first inpainted using multiscale directional transforms, which decompose the image into its elemental building structures, before reconstructing the full image. The pseudo-outcrop view enables direct observation of the angular relationships between features and aids visual comparison between borehole and core images, especially for the interested non-specialist.

💡 Research Summary

The paper introduces a novel “pseudo‑outcrop” visualization technique for borehole wall images and full‑diameter core scans, aiming to overcome the limitations of the conventional unwrapped‑cylinder view that is difficult for non‑specialists to interpret. The core idea is to map the circumferential image from the borehole coordinate system into the Earth‑frame of reference through a nonlinear projection, thereby constructing a solid three‑dimensional volume that can be sliced longitudinally to expose two or more faces. In these faces, planar geological boundaries appear as true planes rather than sinusoidal traces, allowing direct observation of dip and azimuth relationships without additional calculations.

A major practical obstacle is that wireline micro‑resistivity tools often provide less than 100 % circumferential coverage, leaving gaps in the image. The authors address this by applying a multiscale directional transform‑based inpainting method rooted in compressed sensing. The image is decomposed into a sparse set of transform coefficients that capture both texture and structural elements; missing coefficients are estimated, and the full image is reconstructed with minimal error, preserving curvilinear features such as bedding, fractures, and clasts.

Once a complete 2‑D image is available, the authors perform a hierarchical planar decomposition. The log is divided into overlapping depth windows; within each window the dominant set of planar or sub‑planar features is identified and subtracted (Level 1). The residual image is then processed again to extract the next dominant planar set (Level 2), and this recursion continues until the remaining residual consists mainly of non‑planar texture. For each level, the optimal plane family is defined by a dip angle φ (angle with the vertical) and an azimuth ψ (direction of the normal’s projection on the horizontal plane). These angles are obtained by minimizing the variance of the residual after subtracting the planar approximation, using a Nelder‑Mead simplex optimizer with multiple random starts to ensure global convergence. The optimal plane’s value is taken as the mean of all points belonging to that slice, and this constant is extended throughout the constructed volume.

The volume is then modulated in its external dimensions by a proxy for grain size, effectively giving the synthetic outcrop a realistic “topography” that mimics the appearance of real outcrops where coarser grains produce larger bulges. Residual texture, obtained after all planar components have been removed, is propagated through the volume unchanged, preserving the fine‑scale heterogeneity of the formation.

Visualization is achieved by making two longitudinal cuts through the oriented volume, exposing a wedge of material. In these cuts, the extracted planar features appear as straight, correctly oriented surfaces, and the angular relationships between them can be read directly. Because the same processing can be applied to both borehole images and core scans, the method enables side‑by‑side comparison in a common Earth‑frame, facilitating the correlation of core observations with log data.

The authors demonstrate the workflow on three real borehole wall images, showing the original data, successive planar extractions (up to four levels), and the final composite that includes residual texture. They present contour maps of the variance function V(φ,ψ) to illustrate the identification of global and local minima corresponding to dominant dip sets. The optimization is generally robust when the depth window exceeds the borehole circumference; however, when the aspect ratio approaches unity, the objective landscape becomes noisier and additional smoothing is required.

In conclusion, the paper delivers a complete pipeline: (1) gap‑filling via multiscale directional inpainting, (2) hierarchical planar decomposition to isolate geological boundaries, (3) construction of a grain‑size‑scaled 3‑D volume, and (4) multi‑face slicing for intuitive pseudo‑outcrop visualization. This approach enhances the interpretability of high‑resolution borehole logs, supports more accurate geological modeling, and provides a valuable educational tool for non‑specialist audiences. Future extensions could integrate other high‑resolution logs (e.g., micro‑CT), real‑time interactive rendering, and quantitative analysis of the extracted planes for automated structural interpretation.