The explosion of the Deepwater Horizon (MC-252) drilling platform on 20 April 2010 led to a long, unprecedented response from the responsible party and the federal government. Previous US spills were confined primarily to the ocean surface and a limited amount of oil was available. However, the vast Macondo oil field provided a huge source of oil to the Deepwater Horizon spill. Additionally, the oil entered the water column 1500m below the ocean surface. The largest concentrations of oil from the wellhead were from spills that rose to the surface and that stayed below 1000m depth. The two primary collection locations of oil required a broad range of measurement techniques to locate and monitor the substance as part of the response effort.
Surface oil was monitored primarily from the air using satellite, aircraft, and surface ship assets. Satellite visible, IR, and synthetic-aperture-radar imagery helped locate the position of oil in the northern Gulf of Mexico and its potential movement away from the spill site. Additionally, daily overflights by National Oceanic and Atmospheric Administration and other aircraft provided higher spatial and temporal resolution data that was assimilated into daily products. Visual reports of sheen by experienced observers on ships and aircraft were also collected. Models that predicted/forecasted the movement of surface oil also benefited from both high-frequency radar installations along the northern Gulf of Mexico (which provided hourly surface-current vectors on the inner continental shelf) and surface drifters deployed by boats and aircraft. Use of these assets resulted in daily products that guided the response effort in nearshore areas and helped direct efforts in deepwater surface waters to collect, burn, or otherwise dispense with the oil. These remote-sensing assets tracked the surface oil, but monitoring subsurface oil required different techniques, because the water column inhibits use of space-based and airborne remote-sensing techniques below the surface.
Subsurface data was collected using aircraft- and ship-expendable instruments, ship casts and tows, and independent gliders and floats. In addition to salinity and temperature profiles to determine the subsurface structure, fluorometry and dissolved-oxygen measurements provided information related to oil and its consumption by micro-organisms. The fluorometer provided an estimate of fluorescence, which was interpreted as related to the presence of oil. The combination of fluorometry and dissolved-oxygen measurements enabled determination of the location of oil within the water column (see Figure 1) and subsequent monitoring and movement tracking of oil away from the wellhead (see Figure 2).
Figure 1. Conductivity, temperature, and depth (CTD) trace showing increases in fluorescence and decreases in dissolved oxygen (DO) at depth indicating activity related to oil. Lat, Long: Latitude, longitude.
Figure 2. Example map showing the path of Deepwater Horizon oil interpreted as DO sags. NAD83: North American Datum of 1983. MLLW: Mean lower low water.
Water samples collected from conductivity, temperature, and depth (usually known as CRD) casts were analyzed on board for polycyclic aromatic hydrocarbons, Rotifer toxicity, methane, dissolved oxygen, as well as benzene, toluene, ethyl benzene, and xylene (better known as BTEX). Some vessels were equipped with gas-chromatograph/mass-spectrometer equipment to provide analytical chemistry of water samples, and some were able to analyze particle-size distributions using laser in situ scattering transmissometry sensors. Water samples were also returned to on-shore laboratories for extensive analyses to determine their chemical and biological constituents.
Instruments available at the time of the incident were not necessarily ‘tuned’ to detect the presence or absence of oil, or provide reliable concentration estimates. It was decided that if micro-organisms were present and processing the oil, a dissolved-oxygen signature would be measureable. Fluorometers manufactured by different vendors used different wavelengths, resulting in different levels of sensitivity. Efforts were made to cross calibrate instruments whenever possible. Monitoring of the wellhead site by low-frequency acoustic methods was another important component of the response effort. As the Deepwater Horizon response evolved, so did the use of various sensors and platforms that were available. Because of the nature of the spill and the variability of the environment, the response had to be adaptive. It was important to measure the oil in the vicinity of the wellhead, but it was also necessary to trace the oil's transport and determine its fate.
The data collected by the responsible party and the federal government during the response to the Deepwater Horizon oil spill was used to help guide response efforts. The Joint Analysis Group and the Operational Science Assessment Team (OSAT) further investigated the data. The response ended on 17 December 2010 with an OSAT report. The recovery and damage-assessment phase of the Deepwater Horizon incident continues, as does the collection of all data into a central archive at the National Oceanographic Data Center.1
National Oceanic and Atmospheric Administration (NOAA)
Stennis Space Center, MS
Richard Crout is the chief data officer at NOAA's National Data Buoy Center. He worked with the Subsurface Monitoring Unit during the Deepwater Horizon incident.