Subsurface oil refers primarily to offshore and deepwater 

oil that was released near the sea floor and is suspended 

in very small droplets below the water’s surface. This 

includes chemically-dispersed oil from subsea injection 

and/or aerial application of dispersants on the water’s surface. These droplets of oil typically are too small to be seen 

and do not sink. These oil droplets stay suspended in the 

water column or rise very slowly because of density differences, physical processes at sea, or because they mix with 

suspended particulates. Subsurface oil has an extremely 

low potential to land on the shore or in marshes.

Overview

The detection and monitoring of subsurface oil is a joint 

effort that includes NOAA, BP, EPA, academics and 

others, and is coordinated by the Subsurface Monitoring Unit (SMU) based in the Unified Area Command. 

Both federal and contracted vessels are currently sampling 

along grids for indicators of the subsurface oil. The data 

collected by onboard scientists are transmitted to shore, 

and the results and modeling are used to plan future areas 

to be sampled. Samples are analyzed on board, and additional water samples are sent to EPA labs for analysis.

Looking for indicators of subsurface oil

One of the most 

useful tools in the 

search for dispersed 

oil underwater is the 

CTD (conductivitytemperature-depth), 

which is attached to 

a rosette containing 

Niskin bottles used 

to collect water. As 

the CTD rosette is 

lowered to depths 

as great as a mile 

or more, it records 

several important 

parameters: temperature, salinity, 

dissolved oxygen, and fluorescence. As it returns to the 

surface, the niskin bottles are programmed to collect 

water samples at certain depths. The water samples are 

sent to onshore labs for analysis of the presence of oil, rate 

of natural degradation and whether it is from the Deepwater Horizon (DWH) wellhead or from another source, 

such as naturally occurring seeps of oil from the ocean 

floor. These take a few weeks to analyze, but other results 

can be used more immediately to determine if oil might 

be present and to guide further sampling.

Deep Sea Subsurface 

Oil Assessment

Things You Should Know

About Deep Sea Oil

1. There is not a “river of oil” under the 

surface.

Presently most of the concentrations of submerged oil being found are less than 1 part 

per billion (ppb)

2. The oil that is still present is a “cloud” 

of microscopic particles that are constantly 

changing.

This submerged oil is dispersed and rapidly 

changing in size and extent due to natural 

processes such as microbial and chemical degradation, diffusion, and dissolution. It is no 

longer present in concentrations that can be 

cleaned or removed at sea.

September 2010
Dissolved oxygen (DO)

Certain species of bacteria are capable of consuming 

the oil and as these populations grow in response to this 

additional food source, they use more DO. Therefore 

one of the signals to look for in the search for dispersed 

subsurface oil is a decrease in DO concentrations. One 

test being used identifies and measures DO in the water 

column. Decreased levels of DO can be indicative of bacterial degradation of oil taking place. This cannot be used 

alone as a marker for subsurface oil, but with other tests it 

can be useful in finding it.

Fluorescence

Various materials emit fluorescence at different wavelengths of light, and this can be measured with 

fluorometers. The fluorometers on this project are tuned 

specifically to wavelengths at which oil fluoresces; the 

more oil encountered the greater the fluorescent response. 

Again, a fluorescent signal by itself cannot be used to indicate the presence of oil, but it is an important clue.

Deep Sea Subsurface Oil Assessment

Presence of oil

When tests for DO and fluorescence are looked at together, they can indicate the presence of subsurface oil and 

biodegradation. When a peak in fluorescence occurs at the same depth as a decrease in DO, this is most likely due to 

oil being eaten by bacteria. The bacteria use up oxygen as they consume the oil, which accounts for the DO decrease, 

and oil that is present will fluoresce. Our monitoring continues based on these DO and fluorescence readings and 

are informed by modeling and remote sensing technology. Remote sensing includes autonomous underwater vehicles 

such as ocean gliders, and they assist scientists in better predicting the complex ocean circulation that can affect the 

movement of subsurface oil.