The Coal Oil Point seep field is among the most active and studied hydrocarbon seep fields in the world. The water column of the Coal Oil Point seep field was acoustically surveyed from 31 August to 14 September 2016 with a 200‐kHz split‐beam echo sounder to map the distribution of natural hydrocarbons in the region. An in situ direct capture device was used to measure the volumetric gas flux of natural hydrocarbons for three localized seep sites while simultaneously collecting acoustic volume backscatter measurements of the hydrocarbons within the water column. The acoustic volume backscatter was calibrated with the measured volumetric gas flux, and the resulting relationship was used to determine flux over the entire seep field. The estimate of integrated volumetric gas flow rate over a survey area of approximately 4.1 km2 was 23,800 m3/day. The estimates of integrated volumetric gas flow rate and volumetric gas flux were compared to measurements reported in previous studies and were 2 to 7 times smaller than results obtained by Hornafius et al. (1999, https://doi.org/10.1029/1999JC900148), which had a total survey area of 18 km2. However, differences between methodologies limit the ability to assess natural variability in the Coal Oil Point seep field.

Inversions of models of broadband acoustic scattering to detect and quantify weakly scattering targets, such as oil droplets in seawater, require precise knowledge of the physical properties that determine scattering. When the characteristic impedance contrast between a target and the surrounding medium is weak, small differences between the true and modeled impedance can cause significant errors in modeled scattering. For crude oil, currently available empirical models of density and sound speed are derived from measurements made at reservoir conditions (high temperature and pressure), which may not be relevant to oceanographic conditions due to phase changes in the oil. Measurements of the density and sound speed, as well as thermal characterization of phase changes via differential scanning calorimetry, of four crude oils at oceanographically relevant temperatures and pressures were made and compared to a commonly used empirical model for sound speed and density. Significant deviations between the measured and modeled values were found and different empirically fit models were developed. A literature review of sound speed data was also performed, and the innovative empirical model shows improvement over the commonly used empirical model for both the data measured here and the measurements in the literature.

Improved in situ quantification of oil in the marine environment is critical for informing models of fate and transport and evaluating the resiliency of marine communities to oil spills. Broadband acoustic backscatter has been used to quantify a variety of targets in the water column; from fish and planktonic organisms to gas bubbles and oceanic microstructure, and shows promise for use in quantifying oil droplets. Quantifying water column targets with broadband acoustic backscatter relies on accurate models of a target's frequency dependent target strength (TS), a function of the target's acoustic impedance, shape, and size. Previous acoustic quantification of oil droplets has assumed that droplets were spheres. In this study, broadband (100.5–422 kHz) acoustic backscatter from individual oil droplets was measured, and the frequency dependent TS compared to a model of acoustic scattering from fluid spheres and two models for more complex shapes. Droplets of three different crude oils, two medium oils, and one heavy oil were quantified and all droplets were oblate spheroids. The impact of the deviation from sphericity on the accuracy of each model was determined. If an inversion of the model for spherical droplets was used to estimate flux from acoustic observations, errors in the predicted volume of a droplet were between 30% and 50%. The heavy oil also showed deviations in predicted volume of 20%–40% when using the two models for more complex shapes.

In 2004 a debris flow generated by Hurricane Ivan toppled an oil production platform in Mississippi Canyon lease block 20 (MC20). Between 2004 and the installation of a containment system in 2019 MC20 became an in situ laboratory for a wide range of hydrocarbon in the sea-related research, including different methods of assessing the volumetric flow rate of hydrocarbons spanning different temporal scales. In 2017 a shipboard acoustic Doppler current profiler (ADCP) and high-frequency (90 to 154 kHz) broadband echosounder were deployed to assess the flow rate of liquid and gas phase hydrocarbons. Measurements of horizontal currents were combined with acoustic mapping to determine the rise velocity of the seep as it moved downstream. Models of the rise velocity for fluid particles were used to predict the size of oil droplets and gas bubbles in the seep. The amplitude and shape of the broadband acoustic backscatter were then used to differentiate between, and determine the flow rate of, hydrocarbons. Oil flow rate in the seep was estimated to be 56 to 86 barrels/day (mean = 71 barrels/day) while the flow rate of gaseous hydrocarbons was estimated to be 98 to 359 m 3 /day (mean = 229 m 3 /day).