ABSTRACT

An important contribution to rock mass characterisation is related to the mapping of discontinuities. Contact-free measuring principles tremendously improve conventional geological mapping due to the ability to take measurements fast and consistently with virtually no access and time restrictions, as well as to provide objective records of the rock mass. This paper describes an approach for rock face characterisation using scaled three-dimensional images. Two digital photos taken with a zoom-lensed, calibrated off-the-shelf camera serve for a three-dimensional reconstruction of the rock face geometry. The related principles originate from computer vision, allowing highly-flexible picture taking and automatic image processing. The rock face is represented on the computer by a photorealistic spatial representation – a 3D image. From it, measurements are taken by marking visible rock mass features, e.g. spatial orientations of joint surfaces and traces, as well as areas, lengths, or positions. Orientation measurements are depicted in hemispherical plots together with statistics on their spatial distribution and rock mass parameters such as joint spacing are provided. Two commercially available systems have been used for a variety of mapping projects proving their significant support to conventional field work.

1 INTRODUCTION

Geometric information on rock discontinuities forms an integral part of ground investigation and rock construction works. Existing difficulties in conventional geological mapping such as access restrictions, sampling difficulties, human bias, or instrument errors allow for improvements.

The idea using photos for determining geometric fracture parameters is straightforward and not new. Several publications already addressed existing restrictions of conventional mapping, e.g. related to time and access (Linkwitz, 1963, Rengers, 1967, Hagan, 1980, Crosta, 1997) and how an image based approach overcomes those drawbacks. However, it showed that practical efforts for a reasonable economic daily use were too high. Progress in digital camera technology and mobile computing power at reasonable cost opened new possibilities as described more recently (Roberts & Poropat, 2000, Gaich et al., 2004, Haneberg, 2006).

This publication describes a modern approach using digital images for geological mapping based on so-called three-dimensional images.

2 GENERATION OF 3D IMAGES
2.1 Background

A 3D image is a combination of a real (digital) photograph with the geometric information on the objects it shows. In the actual cases the objects are rock faces and walls. The geometry of the exposed rock mass can be reconstructed from digital images using methods of photogrammetry (Slama, 1980; Wolf & Dewitt, 2000). Stereoscopic photogrammetry deals with the measurement of three-dimensional information from two images showing the same object or surface but taken from different angles. This principle is also referred to as Shape from Stereo (see Figure 1).

The geometric relationship between two corresponding points in a stereoscopic image pair is used to determine the position of the original object point in space. This requires accurate information on

  • the image formation process of the camera (interior orientation) and.

  • the camera position and angular orientation when taking the pictures (exterior orientation).

(Figure in full paper)

Historically, the first issue led to purpose-built cameras relying on mechanically accurate imaging.

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