Airborne Thermal IR Remote Sensing of Lushan Geothermal Field
Chi-nan Wu and Ta-Ko Chen
Manager, Researcher, Remote Sensing Lab
Energy & Resources Labs, ITRI
Bldg 24, 195-6 Sec4, CHUNG HSING RD.
CHUTUNG HSINCHU 310,
Tel : (886)-3-5915465 Fax : (886)-3-5820038
E-mail : 690398@itri.org.tw
TAIWAN
Keywords
Airborne MSS, Thermal IR, Brightness Temperature
Abstract
Airborne DS-1260 MSS thermal infrared (TIR) data were used to analyze the surface temperature distribution of Lushan Geothermal Field. The main purpose is focused on detecting the geothermal outcrops for assisting the route planning of Central Cross-island Expressway, as it's deemed that a geothermal outcrop should be 500 meters away from the Expressway. TIR data were acquired in the early morning with pixel size about 9.5 meter, which were pre-processed to remove S-bend and aspect ratio effects, and GCPs were adopted to geometrically correct the image using 3'rd order polynomial equation. Vector data such as the existing roads and the planned route were also digitized to overlay with the image for the convenience of image interpretation and field check. The data quality of TIR data was evaluated by S/N and NEDT, and brightness temperature of each pixel was calibrated using plank's equation and blackbody temperatures. The resulting thermal imagery were ground-checked and interpreted, and it's found that those hot spots ( =22°C) were all irrelevant to geothermal activity, instead, they were owing to metal roofed houses, residential buildings or bare rocks located on the sunshine slope. On the other hand, the temperature of geothermal outcrops was between 19 to 21°C. It's because of the relatively coarse pixel size, small scale hot spring seepage and atmospheric absorption that the sensed geothermal temperature was lower than the actual temperature, consequently, a hot spring outcrop is not necessary appearing as higher temperature in a thermal image. Some thermal anomaly areas were located, while owing to the view angle effect or canopy shaded effect, a few small scale geothermal seepage were ignored by the TIR image, Fortunately, they are all located at a distance more than 500 meters from the planned Expressway.
1. Introduction
Airborne thermal Infrared (IR) data were adopted to analyze the surface temperature distribution of Lushan Geothermal Field from Wushe to Yunhai in central Taiwan. The main purpose is focused on detecting the hot spring activities, which will be helpful for the planning of Central Cross-island Expressway. As the hot spring areas are usually geological weak zone, the expressway should be 500 meters away from those areas. Besides, the acquired imagery could be used as ancillary data for the investigation of landslides and geology of the area. This work is a part of geological investigation during the planning stage of the Expressway, integrated by United Geotech Inc. and sponsored by National Highway Construction Bureau.
2. Data Acquisition
2.1 Scanner System
A Daedalus DS-1260 Multispectral Scanner (MSS) was used to acquire the desired remotely sensed data. DS-1260 MSS can record eleven channels of image data Simultaneously, including a thermal IR channel using a MCT or InSb detector. Thermal IR channel receives the spontaneous radiation from ground surface and it is good for temperature analysis, while the other channels receive reflected energy from the Sun, which are helpful for surface imaging. During the airborne mission, 12 channels of imagery data are recorded with a High Density Digital Tape (HDDT), the 12'nd channel also records the same thermal IR data as the 1 1'st channel but with different signal gain.
2.2 Mission Planning
(a) Target area : Lushan Area, from Yunhai to Wushe. Coordinates, TM 2 (262000, 2658000) to (275000, 2661000)
(b) Detectors : 10-channel DE 160 Spectrometer and DE 237 MCT detector. Table 1 Shows the spectral channels of the detectors.
(c) Scan line : 716 pixels, 1 byte/pixel.
(d) Field of view : 85.92°, i. e., 2.1 milli-radian/pixel.
(e) Ground resolution : within 10 meters.
(f)
Flight altitude : < 5762 meter for the elevation of 1000m as at Lushan.
(g) Scanning time : predawn or early morning in order to avoid the heating effect of sunlight.
| Channel #
| Wavelength ( um ) | Channel
# | wavelength
( um ) |
| 1
| 0.38 -
0.42 | 6
| 0.60 -
0.65 |
| 2
| 0.42 -
0.45 | 7
| 0.65 -
0.69 |
| 3
| 0.45 -
0.50 | 8
| 0.70 -
0.75 |
| 4
| 0.50 -
0.55 | 9
| 0.80 -
0.89 |
| 5
| 0.55 -
0.60 | 10
| 0.92 -
1.10 |
| 11
| 8.0 -
14.0 |
Table 1. Spectral channels of DS-1260 MSS
2.3 Data Acquisition
The airborne imagery of the target area were acquired at AM 7:35 Aug. 19, 1996 with a scan rate of 25 scans/sec. During the flight, the temperature of 2 reference blackbodies was set at 10.9 and 26.4°C, respectively. The flight altitude was 18,000 ft and flight direction was 265°. As a result, 3640 scan lines were recorded, and each line contains 736 bytes of data including 20 bytes of ancillary data such as blackbody temperatures and their corresponding digital numbers (DN), gains, etc., for calibration.
3. Data Processing
3.1 Pre-processing
(a) Reformatting
The acquired imagery data in HDDT were transferred to hard disk and their data format were transformed from band interleaved (IL) to band sequential (BSQ), which are ready for the following consecutive processing.
(b) S-bend Correction
According to the imaging geometry, the panoramic effect in the image was removed. After this operation the pixel number per line is changed from 716 to 888 pixels/line.
(c) Aspect Ratio Correction
Three parameters i. e., Scan rate, ground resolution and flight speed were used to estimate the aspect ratio of the acquired imagery. For the elevation of 1000 meters as at Lushan, the computed aspect ratio is 2.77, consequently, integer 2 was adopted to minimize the aspect ratio effect. In other words, an image line is extracted from every two lines of original image.
3.2 Geometric Correction
Refer to 1:10,000 photographic base map, ground control points (GCP) were selected and their conjugate pixels were also identified in the image. A derived three order polynomial equation was then used to perform geometric correction of the acquired image, trying to fit the image as close as possible to the map.
3.3 Vector Data Digitizing
The existing roads in Lushan area and the planned route of Central Cross-island Expressway were digitized, those vector data were then overlaid on the thermal anomaly image which would be helpful for locating positions during field check and image interpretation.
