Multi-Sensor image interpretation applications in Sabah
F. G. Bercha, D.H. Currie, J. A. Dechka, P. Fuenning
Bercha International Inc.
P. Jordan
Sun Malysia Petroleum Company
Abstract
In conjunction with the conventional analog and digital geologic interpretation of Synthetic Aperture Radar (SAR) imagery being applied in the exploration of Sabah, Malaysia, certain multi-band digital applications were required in order to extract information on subtle geologic features, cultural features, and land use. This information was required primarily in support for the planning of exploration program logistics, specifically, a data set consisting of digital SAR and SPOT data, which were co-registered and interpreted for the requisite information. Digital techniques applied to the SAR and multi-spectral SPOT images included filtering, radiometric enhancements, and statistical analysis. Results of a ground truth program were used to corroborate some of the findings. Generally the information produced on vegetation and terrain was of high accuracy for the purposes of logistic planning. Detailed information on the culture and transportation infrastructure: including roads, rivers, villages, and other settlements was found to be of high accuracy. The multi-sensor data set was found to provide an order of magnitude more cultural information than the SAR alone. The techniques utilized form a good basis for the generation of strategic and resource mapping information in accessible tropical terrain.
Introduction
Persistent cloud cover and inaccessibility are often associated with tropical environments. Sabah, Malaysia is no exception. A request was made by Sun Malaysia Petroleum Co. (SMPC) to extract cultural information and geological features from SAR and SPOT data for the defined area of interest. SMPC felt that the use of remotely sensed data could assist with the location of potential anomalous
Structures indicative of geologic closures. Also these data were assessed in the planning of exploration program logistics for the generation of strategic and resource maps for the area as well as accessibility routes. This paper focuses on the cultural interpretation conducted using these data. Analysis of SAR data for the purpose of geological interpretation can be Bercha et at. (1989).
Study Area
The study area is located in Sabah, on the island of Borneo, and covers approximately 11, 300 square kilometers. This area of Borneo is well drained, densely vegetated and mountainous with forestry and resource recovery providing the main industry. A geological description can be found in Bercha et. al. (1989). There are numerous settlements and towns scattered throughout this region.
Data
The cultural and geological interpretation of the Sabah area is based on the SAR data acquired in early 1989. These data were acquired in the X-band with 6m resolutions. In all, 13 flight lines were acquired, providing stereoscopic coverage of the area. Data products supplied were in the form of 1:50,000 and 1:100,000 analog strips and computer compatible tapes.
An image produced by the system Pour 1 "Observation de la Terre (SPOT) satellite was also acquired. This multispectral image was recorded on April 17, 1988 with 20 m resolution. The image quality is good and contains scattered cloud cover of less than 20 percent the entire scene.
Ancillary data included one general geological map covering the whole state of Sabah (1:500, 000) and 14 topographic maps at a scale of 1:50, 000 covering half of the SARR flight lines. These maps had been annotated by field crews with observed dips at various accessible locations. In addition, photographs and a video tape, providing partial visuals coverage of the location was provided to acquaint the interpreters with the area.]
Methodology
- SAR Data
The SAR digital data were loaded into the PCI EASI/ACE Image Analysis System. These data were then mosaiced using both image to image and map to image registration to provide a 1:100,000 image product. The area was divided into a north and south section simply because the images exceeded the maximum size constraints permitted by the digital scanner for hard copy output. Each mosaic was intepretedfor cultural features using manual proceduress.
- SPOT and SAR Data
When the SPOT image was displayed on the image analysis system it was decided that an intensity reduction for the visible bands should be conductedto reduce the effects of the high humidity haze in the tropics. A training area of water was created in the infrared band and compared to the visible bands. The difference in the means of the IR-band and the visible band data wasw determined to reduce the intensity of the visible bands.
Once this was complet, the SPOT and SAR data wer registered in the areas of overlap. The SAR scene was reduced from 6metre to 20 metre resolution to facilitate the registration of the two data sets.
When these data were merged, various enhancement and transformmms were initiated in order to display the topogaphic featuresssexhibited by the SAR data and to maintain the multispectral nature of the the SPOT imagery. The best result was obtained using the intensity hue and saturation conversation (IHS) (Drury, 1986). The intensuty was represented by the radar data and the hue and saturation were represented by components one and two respectively. Once this procedure was completed a look-up table was created to enhance the subtle features displayed on the imagery such as the roads, settlements and topographical features.
