GB2153071A - Method and apparatus for detecting hydrocarbons on the surface of water - Google Patents

Abstract

A method of detecting the presence of hydrocarbons on the surface of a body of water utilizes the fact that, at the Brewster angle, reflections are largely polarized. One part of the radiation received is passed through a filter with its polarizing axis horizontal, and another part is passed through a filter with its polarizing axis vertical, so as to eliminate the surface reflection. The two parts are then combined, to enhance the image with respect to reflected radiation. An apparatus is provided comprising two receivers, e.g. television cameras, 7,8 and associated filters and means 10 for combining the output of the two receivers, preferably by subtracting, 10, a signal generated by the camera with the vertically-polarising filter, from the output of the camera with the horizontally- polarising filter. <IMAGE>

Description

SPECIFICATION Method and apparatus for detecting hydrocarbons on the surface of water This invention refers to methods for the high sensitivity detection of surface films of oil on the ocean and the mapping of both major and minor seepages of gas and oil from natural and other sources. Whereas the invention is mainly directed toward the application in oil and gas exploration offshore, it is also useful for the detection of marine pipeline leakages and the detection of fish oil slicks associated with fish feeding activities.

Considering applications to oil and gas exploration, it has been found that most oil and gas fields exhibit trace amounts of leakage of hydrocarbons to the surface due to the presence of microfractures, faulting and jointing in the overlying rocks. Experience has shown that this phenomena is essentially ubiquitous and the detection of oil and gas leakage can be a useful adjunct to other techniques for oil exploration including both geophysical and geological methods.

Offshore gas fields are particularly well known as having an associated gas leakage to the surface. Such leakage is easy to understand due to the fact that light hydrocarbon gases are particularly mobile in the earth environment and trace quantities migrate upwards quite readily through vertical or steeply inclined systems of fracturing in the earth's crust. Thus, offshore gas fields are seen to be associated with seepages of gas bubbles which are sometimes so strong that they can generate characteristic disturbances in the sea floor muds. Such disturbances include both conical depressions and small mounds on the sea floor, their form being a function of the rate of seepage and the character of the sea floor sediments.

A high proportion of oil fields are likewise found to be associated with gas leakage since they also contain hydrocarbon gases either as a gas cap on top of the oil or as gases dissolved under pressure within the liquid oil phase. Typically, as much as 1 ,000 cubic feet or more of gas may be dissolved in each barrel of oil within the field and this gas tends to escape continuously in trace amounts through fracture systems overlying such oil fields. Furthermore, liquid oil itself will also tend to migrate upwards through both open fractures associated with faulting and also through microfractures. The escape of bubbles or gas at the sea floor is therefore frequently accompanied by liquid hydrocarbon seepage.

In many cases it would appear that gas migrating upwards carries within it the vapor of the heavier oil components and there is also a possibility that bubble streams may form in the subsurface and carry traces of oil within these bubbles-thereby transporting whole oil to the surface. Whatever the mechanism, the gas escaping from fractures overlying oil fields and bubbling through the water columns lying over the field tend to carry sufficient contents of heavy hydrocarbons to form a film of oil at the water's surface. Such oil films can be of considerable significance from the oil exploration point of view, particularly if the associated presence of gas bubbles can be recognized, thereby establishing the slicks as being of natural seepage origin.Methods of identifying separately and together both gas bubble seeps and the presence of oil films or slicks is therefore of considerable importance. However, such natural oil slicks can be extremely variable in thickness covering the range of 10 - 4 to 1 0 meters or even less. The sensitivity of the detection methods used, therefore, is of critical importance in order to be able to identify the presence of both large and small scale seepages of oil and gas.

A number of physical phenomena are important in providing the background to the present invention. These include the behavior of light reflected from dielectric surfaces such as water at the so-called Brewster angle. At this angle all light reflected from the surface is horizontally polarized and, in fact, for all angles within about plus or minus 12" from the Brewster angle, the light reflected from the surface is at least 80 percent horizontally polarized. The Brewster angle varies for different dielectric materials and is at 37 depression from the horizontal in the case of water.

In the case of oil, the Brewster angle is only slightly modified to 34 so that light reflected from the surface of water at the Brewster angle is also substantially horizontally polarized when reflected from an oil film on the surface.

An important optical difference between the characteristics of crude oil and water is that of refractive index. The refractive indeces of crude oil at the blue end of the visible spectrum lie typically in the range of 1.42 to 1.60, whereas the refractive index of water is 1.36. The fact that the refractive index of oil is greater than that of water increases the surface reflectance of an oil layer in the ultraviolet, visible, and infrared spectral regions.

A still further important optical difference between water and crude oil is that in the case of oil the transmission of light in the ultraviolet and blue ends of the optical spectrum is much lower than the transmission of these wave lengths through water. As a corollary of this the variation of refractive index of oil with the wave length is considerably greater than the variation of the refractive index of water through the same range of wave lengths. For this reason, a small patch of oil film on water, when viewed obliquely, can appear relatively brighter than the surrounding water in the ultraviolet and blue portions of the spectrum as compared with the red end of the spectrum.

Some oils can be quite transparent in the visible spectrum, although remaining highly absorptive in the shorter ultraviolet, whereas other oils are dark to the eye and are quite absorptive in the visible spectrum as well as in the ultraviolet spectrum. Therefore, the differential appearance of oil films on the surface of water, when comparing the blue and red responses, is variable according to the type of crude oil involved.

A still further parameter of importance when considering the physical characteristics of oil films is the interfacial damping effect of oil films on waves generated in water. Of particular importance are the so-cailed capillary waves which are generated at the surface of bodies of water by the boundary layer drag of wind passing over the surface of the water.

Such winds cause small waves whose amplitude is damped significantly by the presence of even quite thin oil films. This affects the textural patterns seen on the natural water surface which is particularly apparent when viewed at an oblique angle. Ilf we now consider the application of optically polarizing filters to the study of water or other dielectric surfaces, we will find that if the dielectric surface is viewed obliquely through a plain polarizing filter with its polarizing axis vertical, a high proportion of the reflection from the surface of the water or dielectric will be rejected. This is due to the fact that near the Brewster angle the light is essentially 100 percent horizontally polarized on reflection.

However, any light penetrating the surface will be scattered by the dielectric molecules beneath the surface or by any turbidity or also by any light scattering objects that are present. Such scattered light will be substantially unpolarized and approximately 50 percent of this light will therefore pass through the polarizing filter. Thus, surface reflections are largely eliminated when viewing through a vertically polarized filter and it becomes possible to see through the surface and identify subsurface scattering objects with considerable clarity.

On the other hand, if the surface of the water or other dielectric is viewed through a polarizing filter with its plane of polarization orientated horizontally, then essentially all of the light reflected at the Brewster angle from the water or dielectric surface will pass through the filter and approximately 50% of the light scattered from the subsurface will pass through. This means that there is an approximate doubling in the contrast of reflections from the surface. However, it should be pointed out that there is certainly not a complete rejection of light scattered from subsurface molecules, objects or turbidity.Thus, we do not have a reciprocal situation where a vertically polarizing filter when used at the Brewster angle is able to see a pure image derived from light scattered from subsurface objects without any contamination from surface reflections whilst by contrast a horizontally polarizing filter sees a pure image reflected from the surface without any subsurface scattered contribution. Such a reciprocal situation, if it could be achieved, would enable us to obtain exceptionally high sensitivity to surface effects, and therefore, would be useful for identifying the presence of oil films on the water surfaces.

The present invention achieves this objective of being highly sensitive to surface effects by making use of two images acquired at approximately the Brewster angle, one through a vertically polarizing filter and the other through a horizontally polarizing filter.

Electronic means are then employed to obtain the differential between these two images such that one image derived purely from light scattered from the subsurface is subtracted from the second image that contains both surface and subsurface components. This procedure, when properly implemented, is capable of virtually eliminating the subsurface scattered light in the residual image leaving only the surface reflections.

In a relatively simple embodiment of the invention the surface of the water is viewed at the Brewster angle through two identical color television cameras mounted side by side.

They employ matched solid state two-dimensional arrays of photo sensors typically of the photo diode type. Identical lenses are used in the two television cameras and the photo sensor arrays are scanned in a television type raster slaved to a single raster generator which thereby provides synchronous images that are matched in time and space.

One of the television cameras views the water through a polarizing filter having a vertical axis and the other camera views the water through a polarizing filter having a horizontal axis. Subtractive circuitry is employed such that the common mode component in both images, namely the scattered light from the subsurface, is cancelled, leaving only the surface image. This residual image can be recorded with standard color video recording techniques and it can be displayed on a color video monitor.

One of the important advantages of using a three color system is that changes of surface texture and reflectance caused by effects other than those associated with oil films tend to behave differentially at each of the colors as compared with the effects associated with oil films. As an example, an oil film slick appears noticeably brighter at the blue end of the spectrum than it does at the red end of the spectrum, whereas this is not necessarily the case with effects associated with the wakes of ships where various physical phenomena will cause scarring of the water surface. Furthermore, the patterns associated with rising bub ble plumes appear white on the color television monitor, whereas floating patches of oil have a different color on the monitor. Thus, color aids in the discrimination of bubble plumes as compared with using a single wavelength monochromatic display.

This invention has been described in terms of conventional three color television imagery.

However, the sensitivity can be further enhanced by using ultraviolet enhanced photo diodes in the two dimensional arrays. In this case, television monitors, instead of employing the three primary colors of red, green, and blue, can operate over a different range of wavelengths such as red, blue and ultraviolet.

If employing the long ultraviolet in the region of 360 to 400 nanometers, it is possible to use more or less conventional optics; whereas, if the shorter ultraviolet that occurs in the solar spectrum is employed between 300 nanometers and 400 nanometers, it is necessary to use special lenses fabricated out of quartz.

The differential response between the short ultraviolet and the red end of the visible spectrum is very substantial and the discrimination for oil slicks versus natural slicks arising from other sources is high. However, the problems of optical scattering by atmospheric haze are substantially increased in the ultraviolet spectrum and the appropriate operational weather conditions for the invention become somewhat narrowed, particularly if heavy reliance is placed upon the shorter ultraviolet.

The invention can achieve extremely high sensitivity for submicron oil films due to the elimination of interfering noise associated with subsurface optical scattering, which under clear ocean water conditions can arise from as deep as 11 meters. The advantages to be obtained with the invention in terms of sensitivity, therefore, are very significant in its application for hydrocarbon exploration. Furthermore, the relatively good spatial resolution of video cameras that can be employed is a valuable feature of the system in that it allows bubble plumes to be clearly identified, thus providing for the joint interpretation of bubble seep and oil film data.

In a more advanced embodiment of the invention, it is possible to look at the water surface at the Brewster angle through a pair of single line array photo sensors that image a narrow strip or line on the sea surface rather than a rectangular area. The differential between the narrow strips viewed through the orthogonal polarizers can be acquired in exactly the same way as described for the normal television camera approach, and an image can be created by printing on hard copy the output of the line arrays or their differential output on a continuous strip of film or paper whose motion is proportional to the rate of forward motion of the platform.

Roll stabilization of the platform carrying the sensors has to be employed--such a platform typically being an aircraft. Severe distortion of the image formed progressively along the strip is thereby eliminated. An important advantage of this line imaging embodiment is the fact that optical conditions with regard to atmospheric scattering are approximately matched along a horizontal line image at the Brewster angle on both the vertical and horizontal polarizers. Thus, when generating a differential line image, substantial cancellation of haze scattering effects can be achieved. On the other hand, when looking at a rectangular image rather than a single line image, the conditions of haze scattering vary in a vertical angular fashion along the direction of flight differently through each polarizer.For example, if the upper part of the image seen through each polarizer is at a depression angle of 25 from the horizontal and the lower part of each image seen through each polarizer is at a depression angle of 50 from the horizontal, there will be differences between the two polarizers with regard to atmospheric scattering seen at the 25 depression angle as compared with the scattering seen through the two polarizers at the 50 depression angle. Thus, when differential or ratioed images are derived from the rectangular picture there will be gradations from top to bottom of the picture which are a function of this differential scattering effect.The reason for this is that there is a partial polarization of light scattered by the atmosphere and the amount of polarization and the angle of this polarization is related to the angle of incidence of the solar illumination. Thus, different viewing angles will provide different degrees of transmission through orthogonally placed polarizers. This angular effect can also be seen horizontally across a single line image, particularly if wide angle lenses are employed, but it is far less prominent than the effect seen from top to bottom of a rectangular image where major changes in range and optical path length occur. The ability to achieve matching between the two images seen through the vertical and horizontal polarizers provides the limitation on ultimate sensitivity that can be achieved in detecting oil films.

Since considerably better matching of atmospheric scattering conditions can be achieved at a single narrow viewing angle depressed by 37 from the horizontal, much higher sensitivity is obtained for oil film detection using a line array imaging system that progressively builds up its two dimensional image as the aircraft traverses as compared with a one shot two dimensional imaging system.

In a further alternative embodiment of the invention, it is possible to use two identical film cameras viewing the ocean's surface at the Brewster angle through horizontally and vertically polarized filters. Photographs may be taken on film at periodic intervals such as one second, differential images being subsequently derived by photo electrically digitizing the film images and thereafter computer generating a differential image. This technique is precisely analogous to the use of two teievision cameras, but uses intermediate film storage followed by a separate post-acquisition processing stage to develop the required differential image. The photographic procedure can also be used with continuous strip film cameras of the type that are commonly employed in geophysical aerial survey work.

These cameras use a continuously moving 35mm film with a slit in front of the film. The passage of the film is adjusted to the velocity of the image which is, in turn, a function of the flying height and speed of the aircraft.

When the motion of the film is appropriately adjusted, the image formed is a continuous strip picture of the ground underlying the aircraft. Exposure is controlled either by the width of the slit or by the aperture of the iris diaphragm in the camera. Two such cameras can be mounted side by side and pointed at the surface of the ocean at the Brewster angle and the two continuous strip images thereby generated can be photoelectrically digitized and the differential image generated. As before, this approach is capable of giving better quality than the single frame approach because of the greater uniformity of atmospheric scattering conditions when capturing an image only at one specific angle.It should be noted, however, that film imaging is inferior to direct electrical digitization of the images formed by the lenses, since the dynamic range of film is quite limited and, furthermore, the film recording technique tends to be nonlinear.

The invention may be better understood by providing a more detailed description of the apparatus. The surface of the ocean is viewed by a camera at the Brewster angle, as shown in figure 1. This angle 1 in the case of water is 37O and in the case of oil is 340 degrees.

The camera employs in front of its lens a polarizing filter 2. When the orientation of the polarizing filter is horizontal there is enhancement of the surface reflection with respect to the subsurface light that penetrates the water surface and is returned from the subsurface by scattering from below. This is due to the fact that at the Brewster angle light reflected from the surface is 100 percent horizontally polarized whereas light scattered from the subsurface is substantially unpolarized.

When the water surface is viewed through a vertically polarizing filter, there is essentially total rejection at the Brewster angle of the surface reflection and the image seen is derived entirely from subsurface optical scattering.

Figure 2 is a perspective view of the invention in which two television cameras, 3 and 4, of identical design with identical lenses and matched two dimensional photo sensor arrays, 5 and 6, are used to produce matched images of the water surface.

Figure 3 illustrates in block diagram form, the electronic arrangement of the apparatus of the invention, in which the television cameras 7 and 8 are driven from a single raster generator, 9 which ensures fully synchronized images, and the video outputs of the cameras 7 and 8 are connected to a differential amplifier, 10 which provides a differential output in which the common mode component of the two images, namely the images derived from the subsurface light scattering, are cancelled, leaving a residual image formed by only the light reflected from the surface of the water.

The output of this differential amplifier 10 carrying the image is connected to an image enhancement device 11 which enhances the high frequency spatial components of the image to provide edge sharpening around the slick occurrences on the ocean surface as well as around the edges of small patches associated with bubble plumes. The image enhancer 11 is connected to a video monitor 1 2 and to a video recorder 1 3 so that the surface of the ocean can be viewed from a suitable mobile platform such as a helicopter or fixed wing aircraft in real time on a monitor and the image can also be recorded for more detailed post acquisition analysis.

For the sake of simplicity, the figures illustrate electronic blocks associated with a single optical channel. In the case of the use of three color television, employing independent sensors for each color, it will be necessary to employ this circuitry in triplicate, one set being for the blue, another set for the green, and the third set for the red portion of the spectrum.

In the preferred embodiment of the invention, a differential subtraction technique is employed in order to eliminate from the final image the effects of upwelling light generated by molecular and particulate scattering from beneath the water surface and to present an image which is solely relating to light reflected from the surface. There are, however, alternative presentations which provide simpler methods of accentuating surface effects by using a modified discrimination technique for separating surface effects from upwelling light. One procedure is to take the output of one of the color channels, such as the blue channel, and present the vertically polarized camera to the blue electron gun with a color video monitor and the corresponding output of the horizontally polarized camera to the red electron gun of the color video monitor. In this situation, all the components of the two images that are identical will give an identical mixed color output or hue on the monitor screen, whereas in areas where the two images differ due to the presence of surface reflections, the mixed color output will be different. In other words, the surface reflections form different colors on the screen from the common mode subsurface scattering image.

Whereas this is a simple approach, it is far less comprehensive than using the three color differential method which identifies the surface reflected light simultaneously in three portions of the spectrum, thereby aiding in interpretation.

A still further alternative embodiment is to generate a voltage proportional to the ratio of the outputs of each color channel on each of the color television cameras. Thus, one voltage is generated by suitable circuitry which is a ratio between the blue output on the vertically polarized camera and the blue output on the horizontally polarized camera, another is the ratio of the green outputs for the two cameras, and the third is the ratio of the red outputs for the two cameras. Each of these ratio voltages is connected to the corresponding color electron guns of the color video monitor so that a three color image is presented with the intensities of each color representing the ratios between the two cameras for each color. This approach will tend to have less contrast than the differential approach but, nevertheless, it reaches essentially the same objectives by alternate means.

In operational usage, the system is flown or otherwise traversed on a systematic grid of parallel lines where it can be operated at relatively high altitudes such as 5000 feet for initial reconnaissance surveys or at low altitudes such as 1000 feet for more detailed surveys. By using an acceptance angle for the lenses in the order of 45 , a swath width of 1.375 times the flying height is obtained and, in fact, the acceptance angle of the lens can be chosen to allow the swath width of coverage of the television cameras to provide for complete coverage of the underlying sea surface at the flying height and traverse interval chosen. With regard to flying under cloudy conditions, satisfactory operation can be obtained regardless of the flight direction. However, there are special considerations if the sky is cloudless since solar reflections will occur off the surface of the sea which can cause objectionable sun glint in the images unless appropriate flight directions are chosen. One of the most suitable flight directions is when the camera is pointing 180 away from the sun since the water surface is then uniformly lit and sun glint is virtually eliminated.

A further important factor for consideration with regard to weather conditions is that of wind velocities. When the wind exceeds approximately 1 2 knots, whitecaps will start to be developed on the surface of the water and these provide noise and make it difficult to recognize bubble plumes carried by gas seeps.

As a general rule, therefore, it is desirable to operate the invention only in low winds below 1 2 knots.

Claims (5)

1. A method of examining the surface of a body of water to detect the presence of hydrocarbons, the method comprising the steps of: (i) receiving electromagnetic radiation from the body of water at an angle substantially equivalent to the Brewster angle of the water and/or the Brewster angle of the hydrocarbon(s); (ii) passing a first part of the received radiation through a polarizina filter with its polarizing axis vertical, to eliminate surface reflections; (iii) passing a second part of the received radiation through a polarizing filter with its polarizing axis arranged horizontal, to partially eliminate radiation from below the surface; and (iv) combining the first and second filtered parts of the received radiation, to enhance that portion of the radiation which was reflected from the surface, and examining an image formed by that enhanced portion of radiation to detect the presence of hydrocarbon(s) on the surface of the water.

2. A method as claimed in Claim 1, wherein the radiation is received at an angle in the range 25 to 46 relative to the horizontal.

3. An apparatus, for detecting the presence of hydrocarbons on the surface of a body of water, the apparatus comprising: first and second receivers; first and second polarizing filters, the first polarizing filter being fitted to the first receiver with its axis arranged horizontally and the second polarizing filter being fitted to the second receiver with its axis arranged vertically; and a differential combining unit, for combining the outputs of the two receivers to provide an enhanced image of reflections from the surface of the body of water.

4. A method of examining the surface of a body of water substantially as herein described with reference to the accompanying drawings.

5. Apparatus for detecting the presence of hydrocarbons on the surface of a body of water substantially as herein described with reference to and as shown in the accompanying drawings.