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These are undertaken in areas where historical contamination from diesel, low-distillate, synthetic and water based drilling fluids have taken place. Surveys can be undertaken as both large vessel-based sampling projects or, during routine inspection or maintenance operations using existing ROVs. For the former, the project is typically undertaken in a similar fashion to that of a baseline survey, although the site might be acoustically surveyed first to delineate the size and extent of seabed disturbance prior to sampling. Also, additional tools such as multi corers, gravity cores and larger box corers are often employed in addition to standard samplers in order to delineate vertical structure within the cuttings pile. Alternatively, ROV based surveys are undertaken using a specialist corer, which can provide very high quality core samples at considerable penetration depths. In both cases, specialist low-level analysis is undertaken on both the surface and vertically sub-sectioned sediments for heavy metals and hydrocarbons to provide levels of contamination, together with identification of type, source and condition of the hydrocarbons found. If combined with volumetric analysis of the cuttings pile from bathymetric data, the quantity of contamination within the sediments can be calculated. Detailed macrofaunal analysis identifies biological impacts within the deposition zone. Physico-chemical variables are correlated statistically to disclose the dominant sediment characteristics affecting biological distribution.
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