GEOLOGIC FLOYD F SABINS Remote Sensing Enterprises Fullerton California Figure 1 Reflectance spectra of vegetation and sedimentary rocks showing spectral ranges of Landsat TM and SPOT systems All figures in Part 1 are from Sabins 1997 self at Chevron from which I retired in 1992 Remote sensing has several applications to geophysical exploration of onshore regions Images are interpreted to produce geologic maps that show structural trends and potential prospects which are in turn used to plan an efficient seismic program Furthermore if an area lacks a reliable base map one can be readily generated from remote sensing data as can maps showing access and trafficability which can also improve the efficiency of field operations When Lee Lawyer a former SEG President was chief geophysicist of Chevron Overseas Petroleum he insisted that any onshore seismic survey be preceded by a remotesensing interpretation Landsat images have also been employed in shallow offshore areas to identify uncharted reefs and other hazards to seismic surveys Remote sensing is defined as the science of acquiring processing and interpreting images from satellites and aircraft that record the interaction between matter and electromagnetic energy Remote sensing images of the earth are acquired in three wavelength intervals or regions of the electromagnetic spectrum The visible region ranges from 0 4 to 0 7 m and is divided into the blue green and red bands The infrared IR region ranges from 0 7 to 30 m and is divided into the reflected IR and ther mal IR portions The reflected IR portion ranges from 0 7 to 3 0 m the energy is predominantly reflected sunlight at wavelengths longer than visible light The Landsat and SPOT satellite systems acquire valuable images in the visible and reflected IR regions The thermal IR portion ranges from 3 0 to 15 0 m the energy is radiant or heat energy Thermal IR images have considerable potential for exploration in arid and semiarid terrains however the method has been underutilized in recent years largely owing to the lack of suitable images Images in the visible reflected IR and thermal IR are acquired in the passive mode by systems that simply record the available energy that is reflected or radiated from the earth The microwave region ranges from 0 1 to 30 cm images for exploration are primarily acquired in the active mode called radar This paper focuses on three systems Landsat Thematic Mapper and SPOT panchromatic images in the visible and reflected IR regions and aircraft radar images Coordinated by M Ray Thomasson and Lee Lawyer T his paper the first of two that summarize remote sensing as applied to petroleum exploration provides an overview of the science and the computer techniques used to process the data The second to be published in TLE s May issue will describe successful oil exploration projects that employed remote sensing technology The examples are from projects by colleagues and my COLUMN Remote sensing for petroleum exploration Part 1 Overview of imaging systems Landsat Thematic Mapper images Landsat is an unmanned satellite that orbits the earth in a sun synchronous pattern at an altitude of 705 km The two second generation satellites carry the thematic mapper TM system TM is a multispectral system which records seven separate Figure 2 Landsat TM bands a 2 green b 4 and c 7 both reflected IR d geologic features near Thermopolis Wyoming as interpreted from Figure 3 APRIL 1998 THE LEADING EDGE 467 images or bands for each scene Figure 1 shows wavelength ranges of the three visible bands 1 2 3 and three reflected IR bands 4 5 7 Band 6 records thermal IR energy but is rarely used for exploration The reflectance spectra of common sedimentary rocks and vegetation in Figure 1 provide insights for selecting the optimum TM bands for interpretation The spectra of the different rocks are very similar in the visible bands but have major distinctions in the reflected IR bands therefore TM bands 4 5 and 7 are especially useful for mapping different rock types TM images are acquired by a cross track scanner with an oscillating mirror that sweeps across the terrain normal to the satellite ground track A spectrometer separates the reflected sunlight into the six spectral bands The image data are telemetered to ground receiving stations via the tracking and data relay satellites TDRSS A TM image covers 170 185 km2 of terrain with a spatial resolution of 30 m The digital data are stored in a raster format An individual band consists of 5667 scan lines each of which contains 6167 picture elements pixels for a total of almost 35 million pixels A pixel represents a 30 30 m ground resolution cell and records the intensity of reflected energy on an eight bit scale ranging from 0 minimum reflectance to 255 maximum reflectance Any three bands may be merged in any combination of blue green and red to produce a color composite image There are 120 possible color combinations but theory and experience show that a small number of combinations are suitable for most regions and applications Figure 2 shows TM bands 2 4 7 for a small subarea that includes the town of Thermopolis in the Bighorn Basin of Wyoming These bands are generally optimum for arid and semiarid terrain such as central Wyoming Figure 3 is a color composite of bands 2 4 7 merged in blue green and red Figure 2d is a map showing major geologic features interpreted from this small scale color image Four anticlines are seen in the image The Little Sand Draw and Gebo anticlines are oil fields that were discovered long before the launch of Landsat Images of existing oil fields are valuable examples for interpreting images of frontier regions Detailed maps at scales as large as 1 50 000 are 468 THE LEADING EDGE APRIL 1998 Figure 3 Color composite image of TM bands 2 4 and 7 combined in blue green and red Thermopolis Wyoming subarea Figure 4 Terrain returns and image signatures for a pulse of radar energy interpreted from larger scale versions of TM images Part 2 of this article will describe how TM images contributed to oil discoveries in the Central Arabian Arch Landsat was launched by NASA and is operated by Space Imaging EOSAT Images in digital or hardcopy format are sold by Space Imaging EOSAT 4300 Forbes Boulevard Lanham Maryland 20706 http origin eosat com and by the USGS EROS Data Center Sioux Falls South Dakota 57198 http edcwww er usgs gov SPOT images The SPOT unmanned satellite orbits the earth in a sun synchronous
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