Measurement of floating oil layer thicknesses at the Santa Barbara seeps in California to support interpretation of satellite imagery

The offshore natural oil seeps along the California coast near Santa Barbara are a natural testing site for the calibration of remote sensing systems aimed at the detection of oil spills. The main difference between these seeps and other permanent sources of floating oil (natural and unnatural seeps in the Gulf of Mexico) is the petroleum composition. Moreover, while it has been documented that most natural seeps worldwide change their rate of oil discharge over time, the Santa Barbara seeps have maintained a high rate, frequently forming thick layers of floating oil in recent years. This allowed us to perform multiple experiments developing floating oil layer thickness measurement techniques from sea-level instruments. These measurements were then used in validation of airborne and satellite remote sensors.

At the Santa Barbara seeps, we have tested our previously developed method of measuring oil thickness with a crystal tube sampling mechanism that extracts an undisturbed floating oil profile at the sea surface. Samples are then post-processed to quantify the volume of oil captured. Our newer system consists of a submerged spectrophotometer that measures the ultraviolet (UV) and infrared (IR) light attenuation of the floating oil from a fixed UV-IR light source above the water. Both methods have been used for cross validation. The sampling tube is more accurate and precise for thicknesses below 50 um (from silver-rainbow sheens to metallic). Both systems work consistently on thicknesses ranging from >50 um to 350um (the latter was the thickest sample of oil measured at the seep sites). However, the advantage of the submerged spectrophotometer is the real time interpretation of the data. The maximum thickness measured in the laboratory for the submerged spectrophotometer was 2.5mm, while the maximum thickness measured from the sampling tube was 7cm of oil.

These thickness measuring instruments have been used to validate thermal and multispectral sensors mounted on an Unmanned Aerial System (UAS). By overlaying the thickness measurements collected in the field with synchronous data collected from the UAS sensors we can relate the thermal reflective radiation and multispectral signatures from different oil thicknesses. Maps with oil thickness classifications generated from the UAS data are then used to correlate with quasi-synchronous high resolution satellite images obtained by WorldView2-3, Planet, ALOS-2, and RADARSAT-2, all of which are hosted and viewable on the NOAA-Environmental Response Management Application (ERMA).  Further field expeditions scheduled for 2021 will include the UAVSAR sensor, an L-band airborne synthetic aperture radar operated by the NASA Airborne Science Program. This NASA microwave sensor operates at the same frequency as one of the sensors on the upcoming NASA-ISRO SAR (NISAR) mission scheduled to launch in 2022 and data acquired will be used to both improve thickness algorithm development and simulate the expected performance of the NISAR instrument for oil slick detection and characterization. We will prepare these methods to move to operational use as this new resource comes online adding a significant response asset to oil spill characterization and response.