Planar Structural Measurements

June 2nd, 2016

Introduction

The use of oriented drill core, if successful, provides a lot of detailed structural information about the geology of a project. Drillers mark the side of the drill core that either points down or up by using a special orientation device. When the drill core pieces are puzzled together, and the markings match, a continuous reference line is drawn to connect the markings.

Structural features, such as fractures, veins, and foliation, can be measured relative to the orientation line by measuring the so-called alpha and beta angles. The alpha angle represents the “sharpness” of a feature, where 0° follows the drill core lengthwise and 90° is perpendicular to the drill core axis. The beta angle measures the clockwise rotation around the drill core between 0° and 360°. With these two angles any planar feature can be described relative to the drill core’s orientation.

By correcting the alpha and beta angles for the known azimuth and dip of the drill hole, the strike and dip of the structural features is calculated. These can be joined with measurements taken on outcrops.

There are major issues with these measurements, which are a tedious and time-consuming task. First of all, the risk for erroneous and inconsequent measurements is large, as even the most experienced geologist can manage to do it exactly wrong. Secondly, despite keeping protocol, it is very difficult to do a proper quality check or to verify how a measurement was taken due to restricted documentation.

New development: Planar structural measurements

Before analysing drill cores with XRF, Minalyzer CS prescans the drill core tray with a 3D profiling laser line scan. This procedure provides a highly detailed overview of the geometry and structure of the drill core tray (Figure 1).

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Figure 1 – A three-dimensional digital drill core tray, which contains valuable information about the geometry and structure of the drill core.

Alpha and beta angles of planar structural features can be measured quickly and accurately directly from this digital version of the drill core using software developed by Minalyze. The drill cores do not even need to be lifted from the tray. This method is faster and less labour-intensive than conventional methods, while also significantly reducing the risk of incorrect measurements. The method works for all sizes of drill core and even cut cores, such as half core, can be measured.

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Figure 2 – Planar features, such as fractures, dykes or veins, are easily oriented using software developed by Minalyze.

Measurements are stored digitally in direct relation to the drill core, eliminating the risks of erroneous recording of measurements. Adjustments can be made to the measurements if necessary, long after the cores have been sampled and sent to the archive without needing to physically retrieve the core trays. Senior geologists can easily perform a quality check of the measurements by visually verifying them in the software. Geotechnical consultants can inspect the project remotely on a computer screen and can be more engaged throughout the project.

Method validation

The alpha and beta angles are calculated through the purely mathematical relations between the planar structure, the orientation line, and the drill core itself. There is no need to do any calibrations, no zeroing of sensors, and no specification of drill core diameters. Figure 3 shows a comparison between measured angles and their true values. This shows that the method is accurate and reliable.

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Figure 3 – Measurements are accurate.