The Role of Remote Sensing in Mapping Swelling Soils
P. C. Kariuki 1 , T. Woldai 2 and F. van der Meer 3*
1 Division of Earth System Analysis, International Institute for Geo-Information Science and Earth Observation (ITC), P.O Box 6, 7500AA Enschede, The Netherlands Tel: +31 (0) 53 4874400 Fax: +31 (0) 53 4874336
E-mail: 1 kariuki@itc.nl; 2 woldai@itc.nl; 3 vdmeer@itc.nl
* and Delft University of Technology, Faculty of Civil Engineering and Geosciences, Department of Applied Earth Sciences, Mijnbouwstraat 120, 2600 GA Delft, The Netherlands Tel: +31-15-2787840 Fax: +31-15-2781186
E-mail: f.d.vandermeer@citg.tudelft.nl
Asian Journal of Geoinformatics, Vol. 5, No. 1, September 2004
Abstract
The use of satellite images during the early stages of mineral exploration has been very successful in pointing out the presence of minerals such as smectites and kaolinite important in the identification of hydrothermal alterations. These same minerals are key to the soil swelling properties and their identification from space makes remote sensing a good tool in the characterization of soils in terms of swelling potential. Here several methods used for spectral enhancement of multispectral images are used on an Enhanced Thematic Mapper image (ETM+) in order to detect these minerals on soils, in an area in the central Kenya, where swelling soils are a major problem in the ever-expanding urban centres surrounding the Nairobi city. The techniques were based on separation of the areas based on the presence of iron oxides, hydroxyl bearing minerals and vegetation cover. The imagery was subjected to several data enhancement techniques before interpretations that included; principle component analysis, band rationing and minimum noise fraction. Interpretations were done based on observations made after these manipulations which gave characteristic differences between the heavily vegetated terrain consisting of high iron oxides and thus red soils and the low lying scarcely vegetated grasslands consisting of dark grumosolic soils. Spatially varying micro-topography consisting of gilgai topography and evident in the ETM+ panchromatic band 8 was used to further identify areas with swelling soils. This micro-relief complimented the spectral interpretations. The results were confirmed by field surveys and reveal a new method of integrated image interpretation in terms of spectral and spatial resolutions in identifying soils physical/chemical properties.
5. Conclusions
This study has established that the combined use of spatial and spectral resolution of satellite data could result in the identification and location of soils susceptible to swelling. The spectral information of the Landsat 7 ETM+ was useful in establishing differences in the signature of heavy vegetation and scarce vegetation cover and to some extent, the hydroxyl. The resolving factor of the satellite data was improved further by use of the higher spatial resolution panchromatic in establishing micro topography differences in the swelling soils. This was concluded to show the imagery potential in characterising the soils physicochemical properties, thus providing a tool for establishing the soil properties at a wider scale. The seasonal changes in the soil morphology should be taken into consideration in any future study.
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