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The application of airborne thermal infrared Remote Sensing to the detection
petroleum pipes

Zhou Yanru, Wang Xiaohong
Geological Remote Sensing Center,
Ministry of Geology and Mineral Resources
The People's Republic of China


Abstract
The physical basis of petroleum pipes detection with thermal infrared remote sensing was demonstrated in this paper. The presupposition of pipe detection was discussed according to the field temperature test data and the possible interference was analyzed too. The selections of optimum imaging conditions were discussed from the selection of maging season and time, flight altitude and temperature range. According to the local condition, the first half nights in the middle ten days of December were selected as imaging time and it was successfully. After image processing and interpretation, the interpretation keys were sep up, and the pipe in-corrected. The application results are excellent.

Intoduction
At the beginning of several major oilfields development in China, because of time limitation, there were no design and construction maps for some areas. The underground distribution of pipe nets is not known is some area. All this has held up the oilfield development and some project reforms. So, it is badly in need to make clear the distribution of underground pipe nets for many oilfields, to prevent oil and gas leakage, harming the production and human safety and other bad results from pipe break caused by unsuitable construetion. In the middle ten days of December 1989, in Panjin area of Liaohe oilfield, we carried out the experimental research of petroleum pipe detection with the method of airborne thermal infrared remote sensing with DS-1230 quantitative double-channel infrared scanner and DS-1268 multispectral scanner installed in Twin-otter aircraft, and got gratifying achievements.

The experimental area is locate din the lower reaches of Liaohe River, near the sea inlet and is a part of Liaohe flood plain. Suangtaizi River comes across the area. Most of the area is rice field but the rice reaped, with criss-cross network of irrigation channels. There are marshlands at the south side and north side of the experimental area and along the riverbanks.

The oil-bearing structure in the oilfield is like a spindle in NNE direction. The oil-drilling platforms; are distributed densely around the structure. A lot of oil, gas and water pipes interweave like a network: All underground pipes produce a great impact to the construction of buildings, road, railway and channels, as well as geophysical prospecting for oil and drilling. Any careless would cause the break of oil, and water pipes. The leakage, rising and running of oil gas and water can make the production and construction to be stopped, even more serious damages.

The physical basis and presupposition
All substances on the earth with the absolute temperature above zero (-273°C) can radiate infrared energy; the expression of energy radiation is following [1]:

Fr=edT4kin..............(1)
Fr:physical radiated enery, w/cm2
e:object radiant emittance
d:object radiant constant 5.67x10-7
Tkin: object surface temperature

It is known from expression (1) that the object-radiated energy depends mainly on the object surface temperature and its radient emittance. For the detection of oil, gas and water pipes, the main point is to use the temperature anomaly produced by the thermal conduction of the pipes temperature on the surface of soil. So, the main consideration here is the temperature. The soil surface with the background of farm field does not have great influence on the change of object radiant emittance.

The airborne thermal infrared scanner can do area scanning and can get the ground thermal images with very high resolution of temperature, and can tell the little difference between 0.2-0.5°C. As long as the temperature difference or radiant emittance difference exists among the different objects, the infrared detector can find them. Then the different objects can be distinguished.

The surface temperature anomelies are the physical presupposition for the detection of under ground pipes. The temperature anomalies appear in two ways: (a) the thermal conduction of thermal source objects across the soil covers; (b) after the pipes were buried under the ground the soil texture has changed , the soil moisture content changed also. All these changed will cause temperature anomalies.

The test data analysis of radiant temperature
On the basis of discussing the physical presupposition, in the middle ten days of December of the same year, the existence of the physical presupposition was proved by the ground radiant temperature measurement two days before the flight. The measurement results are: the surface temperature of the uncovered petroleum pipes reached about 40°C~50°C at day and night, (the local temperature - 3°C~-5°C at day time, the ground surface temperature -12°C~- 7.5°C). The surface temperature anomalies caused by underground petroleum pipes are usually -7.5°C~-8.5°C at night, and are 2°C~-4°C higher than the background temperature. (Fig. 1 and Fig. 2).

The temperature for sewage conduits and natural gas pipes mainly dependents on the environmental temperature and as same as ground temperature. No temperature anomalies appeared on the ground temperature testing results.

Considering the flight imaging time, the temperature test was carried out at the first half night for -different ground objects and at different time. The ground radiant temperature curves comparing with different ground objects at 21 and 22 o'clock of night are shown in Fig. 2)


Fig.1 The radiant temperatur anomaly profile of petroleum pipes
21:00,    December 14, 1989.
Fig. 2 The ground radiant temperature curves comparison for different object
1. petroleum pipe;    2-ice surface;
3. asphalt road;    4-concrete road;
5. sand road;     6- vegetable field;
7- bare field     8- reeded land.
21:00-22:00,    December 14, 1989

It can be seen from the urves: (a) The tow temperature curves at 21 and 22 o] clock fitted very well, that means the ground temperature at this period has little change and tends to be stable. (b) The temperature of background objects such as bare field, vegetable field, reeded land and sand and soil road is relatively low; The surface temperature of concreate road and as phalt road is relatively high; The temperature on ice surface is highest among them; The ground temperature anomalies on the top of petroleum pipes are about 2~4° higher than the temperature of the background objects. (c) The radiant temperature of asphalt road, concrete road and the cannel full of water (ice surface) is close to that of the pipe anomaly. So, all that will be the main interference factors in the image interpretation of petroleum pipes.

The selection of imaging condition
The selection of thermal infrared scanning image condition is the key to decide the infrared detection effects. The principle of imaging condition selection is that the detected objects produce maximum temperature difference against background, in addition to the weather condition at that time and the possible interference factors on the ground.

Imaging season: Considering the purpose of this test and the test area characteristics, we selected the middle ten days of December as imaging season. The meteorological data show that is early the middle ten days of December as imaging season. The meteorological data show that is early winter. The weather is gradually becoming cold, and the ground temperature decreasing. The surface soil was not frozen. The thermal conduction caused by underground pipes easily formed temperature anomalies on the ground, and often appeared as great difference of temperature. For the ground surface condition, the rice field was drawn off water, and the crops had been cutted off. The water level on the marshland decreased, and the reed became withered and yellow. SO, the interference factors from surface water and vegetation were almost eliminated or at the minimum level. It was very favourable for imaging.

Imaging time: Imaging at night is the presuposition for the thermal target detection with the surface as background, because the ground temperature-increasing phenomenon caused by sun radiation was removed. The ground test data showed that at the high latitude area, the ground surface temperature tends to be stable at 20o,clock at night to 6 o,clock next morning at this season. The temperature difference between different objects was clear, and the maximum difference of temperature appeared before the dawn. Considering the thick fog caused by the cold air before dawn at this season, and avoid the interference of the strong fog sbsorptivity we selected the period from 20 to 23 o,clock for flight imaging and got good results.

Flight altitude: The flight altitude not only decides the imaging scale, but also directly influence the object resolution of scanning image.

The mathematical expression of the object resolution[2]:
d = b.H -------------------(2)
d: Object resolution(m)
b: The transient view angle for scanning(2.5m rad)
H: imaging flight relative altitude(m)

It is shown in the expression (2) that the object resolution is inversely proportional to the flight altitude, e.i. The lower flight altitude, the higher. Object resolution. The field test data showed the temperature anomaly width appeared on the ground for the petroleum pipe was usually 1-1.5m at night. Considering the safety of aircraft flying at night, we selected 600m as imaging flight altitude. The ground resolution (d) was 1.5m. So, the pipe anomalies usually cannot be missed, and the result was as good as expected.

The selection of normal temperature range: DS-1230 quantitative double-channel infrared scanner with two blackbodies as reference source can control the upper and lower limitation of the received temperature signal. According to the ground test data and the signal in the flight, we selected BB2=-2°C (the upper limit temperature), BB1=-15° (the lower limit temperature) for imaging. In this way, the radiant temperature from different objects on the ground on the ground was in the range. The upper limit of the normal temperature range is a little higher than practical need, in order to stick out the pipe anomalies in the relatively dark background of images.

Image interpretation and test results
The ground radiant temperature signals received by the airborne imaging scanner were recorded on the high-density tapes. The recorded information was transformed by a special image processing system into black-and-white analogic images or digital colour images for the purpose of visual interpretation. The night-scanning infrared images are used for image interpretation, with the reference of colour infrared pictures, topographic maps and the petroleum pipe distribution maps. In the procedure of interpretation, the image interpretation keys were set up in the first place, to eliminate all kinds of object interference factors such as high way, channel, ridges and other linear objects. Then, the interpreted pipes were accurately positioned, and transferred to 1:10000 topographic map. The last products were the interpretation map of petroleum pipe distribution.

The petroleum pipe interpretation keys: The petroleum pipe anomalies have white-tone linear display on thermal infrared black-white analogic images. The geometric characteristics are in straight lines and turned lines, some of them in winding or arc lines. Most of them are in bunching on plane distribution, or in radiant ray shape linked with well sites and with the calculation stations and oil-transferred station as centers. (See page, 6, Fig.3 and Fig.4)

After image interpretation and map compilation, 364 lines of pipe were found in the 108Km2 area. Comparing the image interpretation maps with the pipes distribution maps, there are 8 lines of pipes with different location, and 17 lines of pipes without display. The field test and verify proved that the image interpretation maps are accurate. Most of the 17 undisplayed pipes are abandoned pipes; unused pipes or water supply pipes with normal temperature and natural gas pipes. Gerelly, the test was successful with expected results.

Conclusion
  1. The test successfully proved that the airborne thermal infrared remote was a new efficient technology for petroleum pipe detection, and can be used in production.

  2. The test result showed that the selection of imaging season, imaging time, flight altitude and the normal temperature range for the thermal infrared remote sensing investigation was reasonable. The effectiveness of test was good.

  3. The advantage of airborne thermal infrared remote sensing for petroleum pipe detection are: visualization, easily-positioned and convenience for comparison; It is not limited by access Its cost is lower than the other ground geophysical methods.

    This investigation is a experiment of production. It is believed that the technology will produce great social and economic benefits.
References
  1. F.F. Sabin: the Principle of Remote Sensing and Interpretation the Geological Press, 1981.

  2. Zhou Yanru: Airborne Thermal Infrared Remote Sensing Images Collection, the Geological Press, 1988.,