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| Titel |
Application of airborne LiDAR to the detailed geological mapping of mineralised terrain: the Troodos ophiolite, Cyprus |
| VerfasserIn |
S. Grebby, D. Cunningham, J. Naden, K. Tansey |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250028510
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| Zusammenfassung |
| The identification of mineral prospects is highly dependent upon the acquisition and synthesis
of a wide variety of geological information, e.g., lithological, structural, geophysical and
geochemical data. Conventionally, the majority of this information is acquired through
field-based surveys. However, the quality of data collected in this manner is often affected by
subjectivity and lack of detail due to coarse sampling over vast areas or inaccessible terrain.
Both multi- and hyperspectral satellite remote sensing and the interpretation of aerial
photography are typically used to help try and overcome some of the limitations associated
with field-based surveys. However, the use of these approaches for the extraction of
exploration data can be hindered by spatial and spectral limitations and by dense forest
cover.
A relatively new active remote sensing technology—known as airborne Light Detection
And Ranging (LiDAR)—offers the possibility of acquiring accurate and high-resolution
(ca. 1–4 m) topographic data through dense forest cover. The ability of LiDAR
systems to detect multiple returns from the emission of a single laser pulse can be
utilised to generate a high-resolution digital elevation model (DEM) of the ground
beneath the forest canopy. Airborne LiDAR is an important tool for geoscience
research, with a wide spectrum of applications including the mapping of landslides
and faults to help inform hazard assessment studies. A LiDAR system can also
provide an insight into the spectral and textural properties of surface materials using
intensity data—a ratio of the reflected laser energy to the emitted laser energy. Where
rocks outcrop, these properties are linked to the surface mineralogy and weathering
at the LiDAR footprint scale. The ability to acquire two high-resolution datasets
simultaneously from a single survey makes airborne LiDAR an attractive tool for the
extraction of detailed geological information in terrain with either sparse or dense forest
cover.
To examine the efficacy of LiDAR in mineral exploration, an airborne survey was flown
over approximately 375 km2 of the Troodos ophiolite, Cyprus—a region noted
for its volcanogenic massive sulphide (VMS)-style mineralisation. Although most
commonly found at the Lower Pillow Lava–Upper Pillow Lava interface, sulphide
mineralisation occurs throughout the pillow lava sequence. Therefore, accurate
identification of geological contacts is a key parameter for VMS exploration in
the Troodos complex. However, the existing geological maps, produced using a
combination of conventional field mapping and aerial photograph interpretation, have
significant differences and do not adequately represent the geological complexity in high
detail.
In this study, we present a semi-automated algorithm for the detailed lithological mapping
of a 16 km2 study area using high-resolution (4 m) airborne LiDAR topographic data in
which non-ground features such as trees and buildings have been removed (i.e.,
bare-earth). Differences in the geomorphological characteristics of each major lithological
unit result in each unit having a distinctive topographic signature in the bare-earth
LiDAR DEM. Thematic maps (slope, curvature and surface roughness) are derived
from the LiDAR DEM in order to quantify the topographic signatures associated
with each lithological unit. With the thematic maps as the input layers, Kohonen’s
Self-Organising Map is used as a supervised artificial neural network to assign
each pixel to a lithology to produce a geological map. The algorithm successfully
identifies the major lithological units—Basal Group (> 50 % dykes and < 50 %
pillow lavas), pillow lavas, alluvium and Lefkara Formation (chalks and marls)—in
excellent detail and highlights geological features to a 20 m resolution. Although the
ability to distinguish between lithologies in some areas is affected by anthropogenic
activity (e.g., farming), the resultant lithological map easily surpasses the quality and
detail of the existing geological maps for the area. As well as providing a qualitative
description of lithology, this method also provides a quantitative perspective of the
terrain.
The results of this study demonstrate the significant potential of airborne LiDAR
elevation data to: (i) quickly furnish high-resolution lithological discrimination and detailed
geological maps over large areas of either forested or non-forested terrain and (ii) provide
valuable baseline information and follow-up targets for ground-based mineral exploration
campaigns. |
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