AERIAL OBSERVATIONS OF OIL AT SEA

HAZMAT Report 96-7

April 1996

Modeling and Simulation Studies Branch

Hazardous Materials Response and Assessment Division

Office of Ocean Resources Conservation and Assessment

National Oceanic and Atmospheric Administration

Seattle, Washington
HAZMAT Report 96-7

Disclaimer

The National Oceanic and Atmospheric Administration (NOAA) has released this

report. However, such release does not signify that the contents of this report

necessarily represent the official position of NOAA or of the Government of the

United States, nor does mention of trade names or commercial products constitute

endorsement or recommendation for their use.
HAZMAT Report 96-7

Contents

Introduction ...................................................................... 1

Oil Spill Terminology ...................................................... 1

Preparing for an Overflight ............................................ 6

References ....................................................................... 10

Tables

Table 1. General Glossary of Terms to be Used in

Oil Spill Observations........................................ 2

Table 2. Oil Spill Observation Glossary....................... 3

Figures

Figure 1. Oil Spill Observation Form............................ 8
HAZMAT Report 96-7
INTRODUCTION

During the initial phases of an oil spill response, information about the release is often

extremely sketchy. Although various types of remote sensing techniques are available for

detecting and mapping oil distribution, the most reliable technique is visual observations

from aircraft.1 These observations are used by the response team to forecast subsequent oil

movement, implement appropriate oil spill countermeasures, and inform the wider response

community of the present status of pollution distribution.

Although overflights can be a valuable tool in the response effort, reports from different

observers can vary widely. This problem is particularly apparent during major spills, when

many observers report oil-position data from overflights. Many of these observers are

untrained and have little experience identifying and quantifying oil floating on the sea. As

the spill progresses, a surprising number of false positive sightings may be reported. Ice,

internal waves, kelp beds, natural organics, pollen, plankton blooms, cloud shadows,

jellyfish, algae, and guano washing off rocks have all been reported as oil by untrained

observers. These false reports obscure knowledge of the actual location and description of

the spill. Some of these problems can be minimized if observers use a common reporting

standard (Pavia and Payton 1983, McFarland et al. 1993).

OIL SPILL TERMINOLOGY

For many years the Hazardous Materials Response and Assessment Divisionof the National

Oceanic and Atmospheric Administration (NOAA) has tracked and forecasted the movement

of oil spills. Through these experiences, we have developed in-house guidelines for aerial

observations of oil slicks. The intent of this section is to provide:

✰ a uniform terminology for describing oil sightings,

✰ techniques for planning observational overflights, and

✰ a data format for reporting spill observations.

 

1 A number of remote sensing systems are available (e.g., side-looking airborne radar, laser

fluorescence, microwave radiometer, infrared-ultraviolet line scanner, and LANDSAT satellite

systems). However, problems associated with each of these systems preclude their exclusive use

during oil spills.
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The terminology used to describe oil spilled in the marine environment is often inconsistent.

Although attempts have been made to standardize terms (ITOPF 1981; Pavia and Payton

1983; Gillot 1988), there is no universally accepted terminology. As a result of our oil-pill

experiences, NOAA has developed a general glossary of terms to describe the appearance of

oil floating on the water (Table 1). To understand the significance of each of these terms,

some general knowledge of the physical processes controlling the movement and fate of the

oil is helpful.

Table 1. General glossary of terms to be used in oil spill observations.

Light Sheen A light, almost transparent layer of oil. Sometime confused

with windrows and natural sheen resulting from biological

processes. Sometimes referred as transparent sheen.

Silver Sheen A slightly thicker layer of oil that appears silvery or shimmers.

Occasionally called gray sheen.

Rainbow Sheen Sheen that reflects colors

Brown Oil Typically a 0.1- to 1.0-mm thick layer of water-in-oil

emulsion. (thickness can vary widely depending on wind and

current conditions). Maybe referred as heavy or dull colored

sheens.

Mousse Water-in-oil emulsion often formed as oil weathers: colors can

range from orange or tan to dark brown.

Black Oil Area of black colored oil sometimes appearing with a latex

texture. Often confused with kelp beds and other natural

phenomenon.

Streamers Oil or sheen oriented in lines, windrows or streaks. Brown oil

and mousse can be easily confused with algae scum collecting

in convergence lines, algae patches, or mats of kelp or fucus.

Sometimes called streaks, stringers or fingers.

Tarballs Weathered oil that has formed a pliable ball. Size may vary

from pinhead to about 30 cm. Sheen may or may not be

present.

Tarmats Non-floating mats of oily debris (usually sediment and/or plant

matter) that are found on beaches or in shallow water just

offshore.

Pancakes An isolated patch of oil shaped in a mostly circular fashion,

pancakes can range in size from a few meters across to

hundreds of meters in diameter. Sheen may or may not be

present.

During the first few hours of a major release, the primary process affecting the movement of

oil is spreading (Payne and McNabb 1984). The oil is still relatively fluid at the start of the

spill and, particularly for lighter products, will spread rapidly into a thin film that is typically

a few microns thick. Since the thickness of the film is on the order of the wavelength of

visible light, the reflection of sunlight off the thin oil film and the resulting colors can give
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some indication of oil thickness. Under laboratory conditions using artificial light, standards

have been developed that give approximate oil thickness based on the color or appearance of

the oil film on water (Table 2) (Fingas 1979, Schriel 1987, Gillot 1988).

Although it is tempting to try to estimate the volume of oil observed on an overflight based

on oil color and the surface area of the floating oil, several complicating factors make this

technique unreliable. For example, very light crudes and light refined products (e.g.,

gasoline and diesel) will quickly spread into very thin films when spilled. Under ideal

lighting conditions, these films will appear as bright colors or rainbow sheens.

2 As shown in

Table 1, a bright color sheen is approximately 0.3 microns thick. In reality, the appearance

and color of the oil sheen varies with the amount of available sunlight, sea surface state, and

viewing angle. Because lighting conditions are highly variable during an actual spill, oil

thickness observations based on the color of the slick are generally not reliable. Glare due to

very low sun angles and sunlight directly overhead can make observations particularly

difficult due to poor contrast between the oil sheen and water. Additionally, observations of

the oil slick can be hampered by viewing in an up-sun direction, wearing sun glasses or face

shields, or looking through Plexiglas windows.

Table 2. Oil Spill Observation Glossary*

Approximate Layer-Thickness Approximate Volume per Acrea

millimeters inches liters/km^2 gallons/nm^2

barely visible 0.00004 0.000002 50 40

silver sheen 0.00007 0.000003 100 75

first color trace 0.0001 0.000004 200 150

bright colors 0.0003 0.00001 400 300

dull colors 0.001 0.00004 1,200 1,000

dark colors 0.003 0.0001 3,600 3,000

*Reproduced from the "Oil Spill Slide Rule," ©1985 Government Publishing Office The

Hague/The Netherlands

In our example, sheens could easily be interpreted as a silver sheen under low light

conditions which, according to the table, is significantly thinner then a sheen containing

bright colors. An estimate of quantity of oil using this method would therefore, be under

 

2 Italicized terms are defined in Table 1.
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estimated. Moreover, the major amounts of oil will usually be in the black or brown portion

of the slick with thicknesses that can range from 0.1 millimeter to several millimeters without

a change in color.

Other factors that make this technique unreliable are the on-scene weather and difficulties

inherent in estimating area of coverage. Waves will increase natural dispersion during the

early parts of the spill, break the surface tension that causes the oil to look "slick," and mix

some of the oil into the surface layer temporarily. Under calm sea conditions, an observer

will probably view most of the floating oil. A few hours later, if the wind has increased and

breaking waves have developed, it is not unusual for an observer to report significantly less

oil due to overwashing of the oil by waves even though the amount of floating oil probably

has not changed. Observers should note that as the wind speed increases, the observers'

ability to detect the oil decreases (Ministry of Transport 1992).

As the spill progresses, a combination of wind drift and surface currents (advection), rather

than the initial spreading process, will dominate the thickness distribution of the oil. When

the wind begins to blow over the surface of the water, wind-generated waves are formed.

Energy and momentum from the smaller (capillary) waves are absorbed by the oil film

causing the oil to appear "slick." Wave action will continue to stretch and recompress the oil

film and, over time, the film will eventually tear apart into smaller patches. These patches

often appear as streaks of oil or streamers. The thicker patches of oil within the streamers

absorb the wave energy and momentum better than the thinner sheens, hence, heavier

concentrations of oil will sometimes appear to be "leading" and sheens "trailing." Once the

oil films are torn apart and compressed into streamers, thickness distribution of the oil

becomes even more patchy and difficult to estimate.

After oil spends even a short time floating on the ocean surface, it starts to change its

physical characteristics due to various physical, biological, and chemical processes. These

processes are collectively referred to as “oil weathering.” Initially, the more volatile

components of the oil evaporate and, to a lesser extent, some fraction of the oil will dissolve

into the water column. This process will continue and, for very light crudes and light refined

products, it may eventually account for the loss of a major fraction of the pollutant.

For heavier crude oils, the weathered oil is more likely to reach a stage where it can form a

water-in-oil emulsion or mousse. As mousse forms, the viscosity of the mixture increases

rapidly and the color changes from black to a range of colors from dark brown to tan or
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orange. Depending on its viscosity and age, the mousse may or may not have an associated

sheen. In addition, mousse formation may lead to a water-in-oil mixture that is up to about

75 percent water. This effectively quadruples the size of the spill and, since this mixture may

be as viscous as peanut butter, the problems of cleanup and recovery are magnified.

Eventually, the weathered oil will fragment into smaller patches of oil that can vary in

diameter from less than one meter, referred to as tarballs, to hundreds of meters, called

pancakes. At this point, visually defining the leading edge of the spill becomes very

difficult as little or no visible sheen is associated with the weathered oil. If the pancakes or

tarballs are overwashed by breaking waves, observations become even more difficult. As the

oil continues to weather, the pancakes and tarballs may continue to break down into smaller

and smaller patches (approximately coin size) that become even more widely dispersed.

Usually, these tarballs are still buoyant and, therefore, constrained to the water surface. If

there is a line of convergence of surface water, as often happens where large rivers flow into

the ocean or where there is a sharp change in the bathymetry, the oil can be concentrated

along this line. This makes it possible for oil slicks to literally reconstitute themselves as a

threat even after they have dispersed to very low concentration levels. On larger spills, this

can happen hundreds of kilometers from the original sources. Because these convergence

lines are natural collection points for oil and oily debris, observers should carefully note their

locations. The fact that oil can collect at these zones is important for oil skimming

operations, particularly if the convergence zone is persistent. These zones can be identified

by changes in the water color or by debris, seaweed, or foam that has collected at the surface.

In general, any oil is subject to the processes outlined above but there are significant

variations for particular types of refined products and some crude oils. Light refined

products such as gasoline, jet fuel, and diesel typically have very high evaporation rates and

do not create persistent slicks. These types of spills typically create rainbow and silver

sheens. If they reach a coastline within a few hours, a slight staining, or soot-like bathtub

ring (in the case of diesel) is common.

Heavy refined products such as intermediate fuel oils (IFO) and bunkers are, in some ways,

the opposite of the lighter oils. The refining process has removed the lighter components and

left them somewhat pre-weathered. As a result, their properties don't change as much as

lighter oils' as they age and so may result in quite persistent floating-pollutant problems.

These oils can form a mousse, but usually only slowly, and after a period of days. They may

not spread into thin films; rather, they often break up into smaller pancakes and then tarballs.
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It is also common for these oils to have lost enough of their light ends so that they do not

rapidly form sheens. The resulting tarball fields are very difficult to observe using either

visual or remote sensing techniques (Pearlman et al. 1992). This, combined with the

persistence of the tarballs, makes these kinds of spills quite likely to cause long-range, and

occasionally unexpected, beach impacts.

There are a few crude oils and some heavier fuel oils that are heavier then water and thus

don't float. These sinking oils are rare and documented observations of them are even more

rare but, from the few instances where oil has sunk, what typically appears on the water

surface is a light sheen near the source of the spill. Visual observation of submerged oil is

extremely difficult unless the water is very clear and shallow.

Spill characteristics appear differently under low light conditions and under strong winds

conditions. Observations in an up-sun direction are typically difficult to interpret. New

observers should be trained by experienced observers in order to make the proper

distinctions between oil types and to properly identify natural phenomenon.

PREPARING FOR AN OVERFLIGHT

An important key to aerial observations is preflight planning. This is particularly crucial

during major spills or Spills of National Significance (SONS) where many people require

access to overflights. For logistical purposes, people with varying interests may be asked to

share overflights. It then becomes critical to identify the mission objectives so that the

overflight is successful for everyone onboard. At the beginning of the spill, the objective

may be as straightforward as verifying that a release has occurred. This involves flying

directly to the incident site, and if there is a release, recording critical information as shown

in Figure 1.

As the spill progresses, a number of different objectives may evolve. They may be mapping

the distribution and appearance of the oil, verifying modeled forecasts of the oil movement,

providing responders with an overview of the incident, or directing cleanup operations. The

type and availability of aircraft, frequency of flights, fuel capacity, number of passengers,

amount of equipment, and landing needs will vary depending on the reason for the flight. By

first clearly defining the mission objectives, aircraft logistics can schedule aircraft best-suited

to meet those objectives.
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For this paper, we will assume that the mission is to map the distribution and coverage of the

floating oil. As discussed in Pavia and Payton (1983), the type of platform selected for oil

mapping is often limited by on-scene logistics. At a minimum, the aircraft must have space

for two observers (excluding the pilot), visibility from both sides, pilot-observer

communications, and adequate navigational aids with which to follow a proposed flight path.

Oil that is known to be close to the coastline is best viewed from a helicopter. Ideally, a door

or window is removed so that the main observer can view the oil looking straight down,

without the finer details of the oil slick being obscured by Plexiglas. For oil further offshore,

a multi-engine aircraft may provide a wider margin of safety, longer range, and higher

speeds.

Prior to take-off, the observer should become familiar with the spill area. The detail at which

this is accomplished depends on the mission objective. For example, if the observer is flying

directly to the release site with the objective of seeing the the source of the spill, then

familiarization with the area is not critical and a quick review of a map (preferably a nautical

chart) is adequate. However, if the observer is planning to map the distribution of the oil

then environmental conditions, such as on-scene winds, visibility, sea state, time of day, and

surface currents are important considerations.
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Oil Spill Observation Form

OBSERVER INFORMATION

Date: Start and End Time of Overflight:

Platform (H-65, C-130 etc.):

Viewing Altitude:

Filled out by: Unit: Phone #:

DESCRIPTION OF SPILL/INCIDENT

Location of source (latitude, longitude):

Source (platform, vessel, etc.) leaking (Y/N):

Fire (Y/N):

Product spilled: Maximum spill potential:

Orientation of slick: Slick dimensions:

Colors of the slick: Estimated percent coverage of oil:

Current Response Activities (on-scene skimmers, boom deployed etc.):

ON-SCENE WEATHER CONDITIONS

Surface wind speed and direction:

Sea state:

Visibility: Precipitation or fog:

ON-SCENE SURFACE CURRENTS

Surface current direction:

Stage of tide (flooding or ebbing):

Figure 1. Oil observation form.
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The on-scene weather important to note during the overflight includes visibility, surface wind

speed and direction, and sea state. The pilot can best estimate the surface wind speed and

direction or, using the Beaufort Scale, an observer can estimate the wind speed from the sea

state. This information is important for oil movement forecasting as well as determining

whether the conditions for viewing oil were optimum. For example, restricted visibility due

to fog would indicate poor viewing conditions so that the observer may not have seen the

entire spill. Strong winds would suggest that a particular crude oil is likely emulsifying or

for a fuel oil No. 6 spill, much of the oil may be difficult to view due to overwashing.

Without using current probes3 or dye markers4, estimating the speed of the surface current is

extremely difficult from an aircraft. However, the observer can estimate the direction of the

current by comparing the water movement in relation to a stationary object such as a moored

buoy, navigation light or, even better, kelp or sea grass beds. By noting the time of the

overflight, the stage of the tide can be estimated from the National Ocean Service (NOS)

tidal current and height tables.

Based on the mission objectives, platform, and environmental conditions, an actual flight

path should be planned for the area of interest. Although any kind of chart that covers the

spill area will work, the most useful will be a nautical chart of an appropriate scale. This

type of chart provides information, such as navigational aids, which an observer can use as a

reference during the flight for plotting the location of the oil.

There is no general consensus on the best way to overfly an oil slick however, an overflight

of a large spill would probably first entail an overview of the slick, perhaps from an altitude

of 1,000 to 2,000 feet. At these altitudes, determining the general orientation and dimensions

of the slick is much easier than flying at a lower altitude. Estimating the oil coverage and the

color of the oil is probably best made by dropping down to an altitude of 200 to 500 feet or

less.

 

3 A current probe is a preassembled set of dye markers, two of which are attached to a weighted

timer. As the probe enters the water, a styrofoam plug at the top of the probe is compressed and a dye

marker is immediately released. The rest of the probe sinks to the bottom and, after a known amount

of time (depending on how the timer was set), another dye marker is released. Assuming that the

second dye marker comes to the surface at approximately the same location as the first, the current

speed can be estimated by measuring the distance between the dyes.

4 Dye markers can be deployed near a fixed reference point. The current speed can be estimated by

timing the movement of the dye from the time of deployement. By flying directly over the dye, the

direction of the current can be determined from the aircraft's compass.
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Similarly, there is no consensus on the best time to observe an oil slick but here are some

factors to consider. In the early morning or early evening there is often not enough contrast

to see some oils clearly. In the middle of the day, the sun may glare off the water surface,

making it hard to distinguish oil. The flight track should be set up to minimize the glare with

the optimum schedule for mapping oil distribution depending on the angle of the sun (e.g., in

mid-latitudes the middle of the morning or afternoon is usually a good viewing time).

REFERENCES

Fingas, M.. 1979. Remote sensing of oil spills, Spill Technology Newsletter 4(3), pp. 191 -

192.

Galt, J. A. 1994. Trajectory Analysis for Oil Spills, J. Adv. Mar. Tech. Conf. 11, pp. 91 - 126.

Gillot, A., G. H. R. Aston, P. Bonanzinga, Y. le Gal la Salle, M J. Mason, M. J. O'Neill, J. K.

Rudd, D. I. Stonor. 1988. A Field Guide to the Application of Dispersants to Oil Spills.

Report No. 2/88. The Hague, The Netherlands: CONCAWE. 64 pp

ITOPF. 1981. Aerial Observations of Oil at Sea. Technical Information Paper No. 1, The

international Tanker Owners Pollution Federation LTD. pp. 6

McFarland, B., Murphy, J. and D. Simecek-Beatty. 1993. Conventions for reporting and

displaying overflight observations, Proceedings from the 1993 Oil Spill Conference,

(Prevention, Behavior, Control, Cleanup), Tampa, Florida, March 29-April 1, 1993,, pp.

789-790.

Ministry of Transport. 1992. Visibility Limits of Oil Discharges: Investigation into the

Visibility Limits of Operational Oil Discharges from Ships, Ministry of Transport, Public

Works and Water Management, The Netherlands, pp. 43.

Payne, J. and D. McNabb, Jr. 1984. Weathering of petroleum in the marine environment,

Marine Technical Society Journal, 18(3), pp. 24-42.
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Pavia, R. and D. Payton. 1983. An approach to observing oil at sea, Proceedings from the

1983 Oil Spill Conference, (Prevention, Behavior, Control, Cleanup), San Antonio, Texas,

February 28-March 3, 1983, pp. 345-348.

Schriel, R. 1987. Operational air surveillance and experiences in the Netherlands,

Proceedings from the 1987 Oil Spill Conference, Tenth Biennial, (Prevention, Behavior,

Control, Cleanup), Baltimore, Maryland, April 6-9, 1987, pp. 129-136.