Remote Sensing Techniques For Detecting Oil Slicks

There are a variety of devices for "seeing" oil on water-sensors that make use of both spatial and spectral dimensions. All of the devices are limited to a greater or lesser degree, but all are also considerably more reliable than the naked human eye. Introduction The Coast Guard, in discharging its responsibility, spends a considerable amount of time and effort monitoring U. S. waters to detect oil leaks or spills. In most instances, oil spills have been detected by visual sightings. However, it is recognized that human vision is not entirely adequate for the job of detecting oil slicks, and the Coast Guard is evaluating other sensors to determine their usefulness for the detection and surveillance of oil. Most of the private and government attention in this area of oil pollution has been devoted to developing regulations, methods, and procedures to reduce the probability of oil spillage. Additional effort is being expended to develop cleanup techniques and equipment to control oil that is accidentally spilled and then reported to the proper authorities. In any event it is reasonable to assume that in spite of new regulations and preventive measures there will still be oil spills-due either to human carelessness or to calamities beyond human control-that will not be reported quickly. To combat unreported incidents, effective techniques are needed for detecting and observing oil slicks remotely and in any weather. This would allow timely countermeasures to be taken to minimize the possible pollution hazards to the coastal and inland waters of the U. S. and to detect violators of U. S. and international laws concerning pollution of the sea by oil. The initial effort in oil detection by the Applied Technology Div. has centered on the question, "Does oil have a unique signature that enables one to differentiate between it and other pollutants?" (a signature being a particular characteristic of the target that distinguishes it from its background or other targets). It is now known that oils do exhibit characteristic signatures. However, their electromagnetic characteristics may vary with time and environmental conditions, a fact that may help or hinder detection, depending on how well the phenomenon is understood. We shall consider here the signature properties of oil slicks that make them amenable to remote sensing. We shall give particular attention to those signature properties of oil slicks that are useful in (1) detecting properties of oil slicks that are useful in (1) detecting oil films on water surfaces, (2) mapping the areal extent of the slicks, (3) measuring the thickness of the slicks, and (4) identifying oil types. Signature Properties of Oil Ultraviolet Region (less than 0.4 um) In the ultraviolet region, the mechanism for detecting an oil slick is based on both the reflective and the fluorescent signatures of oil as indicated in Fig. 1. The reflective properties of oil are characterized by the fact that a repeatable positive response is obtained from thin slicks ( 0.1 um), whereas a variable response is obtained from thicker slicks. This variable response is dependent upon oil type, water quality, and illumination conditions. JPT P. 267