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High-Resolution
Remote Sensing Used to Monitor Natural Gas Pipelines Abstract It is in the interest of any gas company to maintain the value of its pipelines and to protect them against damage caused by third parties. As a result of global progress in high-resolution remote sensing and image-processing technology, it is now possible to design natural gas pipeline monitoring systems with space-borne sensors, and to enter into targeted negotiations with satellite operators concerning sensor application. A concept has been developed by a triumvirate consisting of Ruhrgas Aktiengesellschaft (the technological leader and user), the German Aerospace Center DLR (an applied research institute) and Definiens Imaging GmbH (a developer of image-processing software) for creating a satellite-borne pipeline monitoring system that combines high-resolution data — supplied by various sensors via remote sensing systems — with the context-oriented evaluation of these data using innovative image-processing techniques. Background The rules and regulations governing the operation and monitoring of natural gas transmission pipelines vary from country to country. In some cases, the differences are quite significant. Regardless of the details of the individual requirements, it is in the interest of any pipeline owner to maintain the value of its pipelines and to protect them effectively against damage caused by third parties. The monitoring methods most widely used for European natural gas transmission pipelines include foot patrols along the pipeline route and aerial surveillance by small fixed-wing aircraft or helicopters. These patrols prevent circumstances that could place high-pressure pipelines, their surroundings, or their supply source at risk. While these methods ensure a high level of safety in pipeline operation, the cost is also very high. Global progress in high-resolution remote sensing technology has resulted in the ability to use it for pipeline monitoring tasks. Ruhrgas AG, the German Aerospace Center DLR, and Definiens Imaging GmbH have designed just such a remote sensing-based monitoring system. As presented in this article, these parties have identified the remote sensing technologies that currently appear to be best suited for automating gas transmission pipeline monitoring and making such monitoring more efficient.
Monitoring Tasks of Pipeline Operators Within the integrated European natural gas transmission system, Ruhrgas operates a high-pressure gas pipeline system with a total length of about 11,000 kilometers, one that extends from the North Sea to southern Germany and from east to west across the country (Fig. 1). This pipeline system is monitored by foot and vehicle patrols, and by air patrols carried out using small helicopters. Whereas pipelines can thus be protected against third-party damage, and as potential leaks in open country can mainly be detected from helicopters, the current state of the art allows early detection of leaks beneath sealed road surfaces only when patrolling the pipeline route on foot with gas-leak detectors. Monitoring breaks down into three stages: object and situation detection, gas-leak detection, and monitoring of soil movement. These tasks have to be carried out throughout the year at regular intervals, regardless of weather conditions. Although the monitored areas differ significantly in terms of soil characteristics, vegetation and building density, it is important that remote sensing methods be usable in almost all types of terrain, including pipeline sections where the route is not clearly visible (Fig. 2). Most European natural gas transmission pipelines are under soil cover of about one meter in depth. Along the pipeline routes the following situations have to be detected in a strip of 20 meters lying on both sides of the centerline:
In addition, any work carried out within a 200-meter-wide strip must be reported if there is reason to believe that it could affect the pipeline route at a later time. A gas-leak detection system must be capable of identifying small gas leaks at an early stage that exhibit leakage flow rates of from 0.01 to 10 cubic meters per hour. Any major gas leaks caused by severe damage to a pipeline are detected and reported directly by other systems. The ability to distinguish between methane from natural gas and other biogenic methane is an added advantage. Appropriate Remote Sensing Technologies and Their Present Realization Table 1 gives a qualitative overview of remote sensing technologies that appear to be well suited for pipeline monitoring on the basis of this feasibility study. It is clear from the table that a complete monitoring system for natural gas transmission pipelines will call for a combination of different sensors. LIDAR (Light Detecting and Ranging) LIDAR is a light-based laser optical detection method involving the transmission of laser light in the ultraviolet, visible or infrared range that detects and analyzes the reflected light. LIDAR systems have already been used for many years for the remote sensing of air pollutants and various meteorological parameters. Experimental systems mounted on helicopters for the detection of major leakage from transmission pipelines have been tested in the United States and in Russia. In order to measure trace gas concentrations, the DIAL (Differential Absorption Lidar) method is used. This method is based upon the Beer-Lambert absorption law and also upon the absorption properties of the gas to be detected. In order to exclude atmospheric effects and diffuse reflection from the signal, two wavelengths are used for transmission. The first wavelength ((on) is absorbed by the gas, while the second ((off) is not absorbed and therefore serves as a reference. This method may be used with either a closed or an open measurement path. In the case of an open measurement path (Fig. 3), the pulsed laser light is reflected back by particles or molecules in the atmosphere. Trace gas concentrations can be measured with a certain spatial resolution. The result of the measurement is the product of the gas concentration and the absorption cross section, which is a function of the wavelength selected. With a closed measurement path, the radiation emitted by the laser is reflected back to the optical receiver by a topographic target. Measurements of concentrations at specific distances are only possible to a limited extent. The trace gas concentration is the value for the entire measurement path. In order to detect hydrocarbons such as methane or ethane from natural gas leaks, the laser must be set to a wavelength at which these gases have appropriate absorption lines. For a low measurement threshold, the absorption value of the gas should be as high as possible. However, especially in the case of gases such as methane with low background concentrations (~ 1.7 ppm), it is important to avoid saturation effects. Potential absorption curves for the detection of methane are located in the spectral range from 1.6µm to 4.0µm, with three significant bands at about 1.6µm, 2.3µm, and 3.3µm. The strongest absorption lines are located at about 3.3µm. Ethane also has absorption lines in this range. However, overlapping with the absorption lines of water vapor must also be taken into consideration. The design calculations made in the course of work on this feasibility study indicate that appropriately selected LIDAR systems would be in a position to detect the required very small leaks of below 0.1 cubic meters per hour from altitudes of up to 300 meters. This method therefore appears to be suitable for gas-leak detection during regular aerial patrols with small helicopters. Thermography Thermography relies on imaging detectors that pick up the infrared radiation emitted by a body and convert it into a visible image of that body. The wavebands from 3µm to 5µm and from 8µm to 12µm are normally used for thermography. Thermography has a variety of applications, including industrial quality assurance, the assessment of the thermal insulation of buildings, the location of fires, and for environmental monitoring. Various airborne camera systems are available on the market and can be integrated into monitoring systems. Figure 4 shows a recording system that has been combined with an optical camera and installed on an airplane. In the case of the automated monitoring of natural gas pipelines using radar or photographic systems, a combination of these systems with thermographic methods would allow more precise image evaluation and would also increase the probability of detection and reduce the number of false alarms. High-Resolution Optical Systems High-resolution optical systems are available for any platform. Digital images are normally recorded using linear arrays of photosensitive semiconductors (CCD = Charged Coupled Devices). Up to 12,000 of these semiconductors are required for an image line. The Earth’s surface is then scanned in lines at the speed of travel of the carrier. If semiconductor arrays tuned to different sections of the spectrum are connected in parallel, a multispectral image can be recorded. Images of the Earth’s surface are available commercially. Recent examples are the pictures taken by the IKONOS satellite, operated by Space Imaging, that was launched in September 1999. As IKONOS orbits some 680 kilometers from the Earth, each semiconductor represents an area of one meter by one meter on the Earth’s surface. Eleven thousand semiconductors connected in series (one for each meter on the Earth’s surface) allow an image with a width of 11 kilometers. The information from these electronic components is digitized and can either be stored on the satellite or else transmitted directly to a ground station in the line of sight. For an example of the image quality that can be obtained, see Figures 5 and 6. However, an even higher resolution of about 0.5 meters would be needed for pipeline monitoring. This resolution has already been licensed by U.S. authorities and is planned by Space Imaging for 2004. On October 18, 2001, its competitor Digital Globe (known as Earth Watch until September 2001) successfully launched the QuickBird satellite, which will provide optical images with 0.6-meter resolution from the first months of 2002. Of course, optical images of the Earth’s surface can only be recorded from space over cloudless skies. The use of these techniques for routine pipeline monitoring is therefore only conceivable if several satellite systems could be employed jointly to achieve high repetition rates, thus compensating for any monitoring limitations imposed by poor weather conditions. Even in the unfavorable climatic zone of central Europe, it can be assumed that adequately sized sections of the Earth’s surface would be visible at intervals of several days. Hyperspectral Sensors Hyperspectral sensors measure the degree of reflection of natural and artificial objects with high spectral resolution, allowing a variety of surface types and objects to be identified. Many elements on the Earth’s surface (vegetation pigments, minerals, rock, artificial surfaces) show specific absorption characteristics in defined wavelength bands, thereby allowing a quantitative analysis (Fig. 7). The use of imaging systems makes it possible to identify objects by a combination of their spectral signatures and their three-dimensional characteristics. In this way it should be possible to detect changes in vegetation as caused by natural gas escaping beneath the ground. Although the geometrical resolution of airborne sensors should be adequate for pipeline monitoring purposes, the planned satellite-mounted hyperspectral sensors (such as OrbView 4) only reach a resolution of eight meters. Due to U.S. government security requirements, this resolution may be further reduced to 20 meters before the data are made available. Imaging SAR (Synthetic Aperture Radar) Systems SAR systems provide a holographic image of a radar-scanned area. As a result of the wavelengths selected, radar can even penetrate clouds of water vapor, allowing the Earth’s surface to be monitored without regard to weather conditions. However, image resolution depends upon the wavelength used, and also upon the size of the aperture or antenna. With antenna lengths of a few meters, the radar waves (with a wavelength of a few decimeters) can only detect paths with a length of several kilometers on the Earth’s surface. The antenna is therefore extended synthetically to a length of several kilometers by computer augmentation of the signals that are received at various antenna points. However, even a very small metal reflector gives a strong reflection signal, and objects with metal edges can therefore be detected very effectively. As a result of the complex image-processing functions that involve the wavelength and phase of the active signal, SAR can be used to detect features invisible to the human eye. Interferometry can be used to obtain three-dimensional images. Image resolution depends upon the system parameters and the processing methods used, rated at about six meters for satellite-borne systems. Airborne systems offer resolutions down to 0.5 meters. Significant increases in the image resolution of commercially available satellite systems can be expected over the next three to four years. Radarsat II (planned for 2003) will have a resolution of about three meters, while TerraSAR (launch scheduled for 2005) is expected to reach a resolution of one meter. The technology is largely independent of the time of day and of weather conditions. However, because of system-inherent features, the Earth’s surface is always viewed obliquely. As a result, shading or distortion effects may therefore occur. Interferometric SAR Interferometric SAR uses the phase information contained in the radar waves of two or more SAR images to develop terrain models and detect ground surface movements in the centimeter range. With tandem operation of identical SAR satellites such as the combined flights of the European ERS-1 and ERS-2, and with the planned operations of Radarsat II and Radarsat III, images of the same area can be recorded with very short intervals of one day (with ERS) or even only a few minutes (as planned by Radarsat). As regards pipeline monitoring, this method could conceivably be used for detecting subsidence following water abstraction and the collapse of subterranean hollows, or for monitoring slopes that are subject to slippage. Microwave Radiometers Microwave radiometers use a scanning antenna to detect radiation in the microwave range and allow a vertical view of the Earth’s surface. The received microwave radiation is proportional to the so-called apparent temperature of the surface observed which is, in turn, a function of the emission and radiation properties of the surface. A number of experimental systems have been developed, with promising results. From altitudes between 100 meters and 3000 meters, this method could be used almost irrespective of the time of day and weather conditions to obtain information on objects along the pipeline route. Data Fusion and Image Processing Approaches In order to manage and interpret the extremely large volumes of data contained in high-resolution optical and SAR images, it will be necessary to combine the most advanced information and knowledge systems available with object and signal analysis methods. The objective must be to automatically extract easy-to-handle warnings about pipeline hazards with a very low proportion of false alarms from the available data. Advanced data management procedures include data mining, change detection, and feature detection. Data Mining Data mining is a term that refers to automatic searching for previously specified signals, objects and features contained within large volumes of graphical data. In contrast to databases, the data in an image are not available in a sorted form but only as pixels. Data mining has given rise to the investigation of new data access and distribution methods such as information mining, scene understanding, synergetic decompression and classification, data and information visualization, user adaptation, and the semantic modeling of information extraction. Change Detection and Feature Detection Change and feature detection are the main strategies for detecting changes in the vicinity of a pipeline route. In change detection, image data are compared with the corresponding data from an earlier image, pixel by pixel. Changes in the scenery are reflected by differences between the corresponding pixels. This highly direct method results in problems with natural changes in vegetation, lighting or surface conditions (snow or rain), and the resulting radiometric changes in the pixels. Comprehensive corrections are required to reduce the very high proportion of false alarms generated by automated change detection. In addition, pixel-based change detection calls for comprehensive processes and reference data for geo-encoding. Feature detection is better suited for analyzing changes in complex scenery. This method includes the identification and generation of objects from the original pixel-based files and the establishment of semantic links between these objects and known features, for example, in the form of a feature database. Image features such as vehicles or pits are classified on the basis of radiometric, geometric and other links between the image objects, and consequently placed in relation to neighboring objects and known information from geographic information systems (Fig. 8). Any vehicles detected off-road and in open country near pipeline route coordinates are therefore identified as potential hazards. By assessing the area of view, assumptions can be made regarding a distinction between agricultural vehicles and construction equipment. This combination of object identification and a semantic knowledge network would appear to be an image-processing procedure especially well suited for pipeline monitoring. eCognition, a commercial analysis system, allows data from different sensors (optical, radar, infrared) to be merged and combined with graphic information system data for object identification. System Concepts and Additional Development Steps Any system employed for the monitoring of pipelines on the basis of remote sensing must meet two major operational requirements:
The results of this feasibility study confirm that the present state of the art in remote sensing and evaluation technology makes it possible to implement airborne sensor concepts that feature the following elements:
Moreover, in view of the growing fleet of commercial, high-resolution optical and SAR satellites in space (Fig. 9), it is conceivable that certain pipeline monitoring tasks could be transferred to space-borne sensors, at least for situational and object-detection purposes. Prospects appear good for optical systems with resolutions better than one meter, and for future SAR systems in the one- to three-meter range. The classic task in the processing of remote sensing data is the interpretation of an individual image, and the radiometric information recorded in the various spectral channels. Interpretation can be made significantly more precise by combining information from a number of different sources, e.g., by fusing data from different sensors or data recorded by the same sensor at different times (multitemporal analysis), or else by combining these data with information from geographic information systems. As a fundamental prerequisite for the automated evaluation of image data, it will be essential that the position of natural gas transmission systems be defined with sufficient accuracy in digital form within a GIS. The image-processing programs currently available for automatic object and situation detection will need to be developed still further and adapted to these applications. Finally, the resulting data need to be bound into pipeline operators’ communication systems. The next step will be to draw up detailed specifications for the interaction of different satellite-borne sensor systems, suitable image-processing programs and communication interfaces, and to verify systems interaction using suitable test cases. For this purpose, data from targeted aerial patrol campaigns and commercial satellite images could be linked and evaluated using the software modules referred to above. On January 1, 2002, a consortium comprising seven European gas pipeline operators, two national space agencies, as well as eight European sensor developers and system integrators commenced the remote sensing, pipeline monitoring project PRESENSE (Pipeline Remote Sensing for Safety and the Environment), funded by the European Commission 5th Framework Research and Technology Development Program. PRESENSE is a 30-month program that aims to specify a satellite-supported pipeline monitoring system and to quantify the potential for improving the effectiveness of such a system. Conclusions With the possibilities offered by remote sensing technologies, in-depth correlation of the requirements of pipeline operators will allow targeted practical discussions with satellite operators. In addition the International Space Station (ISS), currently being built, will provide a suitable platform for future testing without requiring the launch of a special satellite. In this manner, it could help develop a focused pipeline monitoring system based upon remote sensing technology that would provide effective, reliable support to pipeline operators throughout the world. Additionally, this process could detect hazards to pipeline systems and the environment at an earlier stage and in a much more comprehensive manner than has been previously experienced. There are good prospects that such a satellite-supported pipeline monitoring system may be ready for operational use within the next ten years, with certain parts of such a system available on an even earlier timetable. Acknowledgements This paper first appeared in unedited form at the 2001 International Gas Research Conference Proceedings (Amsterdam, The Netherlands) in November 2001. The authors wish to thank the remote sensing experts in the specialist departments of the DLR Institutes at Oberpfaffenhofen, the Institute of Photogrammetry and Remote Sensing of the University of Karlsruhe, Aero-Sensing Radarsysteme and Astrium, and the experts of a number of gas companies whose know-how and willingness to engage in discussions have been instrumental in developing this system concept.
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Figure
2. Example of the Ruhrgas pipeline route in a built-up area with poor
visibility. The high-pressure natural gas pipeline (50 bar) runs roughly
parallel and equidistant to the buildings and the row of trees.