POCKMARKS AND SHALLOW NATURAL GAS: THREATS TO OFFSHORE INFRASTRUCTURE DEVELOPMENT

Pockmarks, or seafloor craters, proliferate in a variety of continental shelf settings including previously glaciated areas such as coastal New England and the Canadian Maritimes. Present in fields of thousands throughout Maine’s muddy embayments, these features exhibit slopes of 20˚ to 30˚, and may be 300 meters in diameter and 40 meters deep. Spatial analysis of swath bathymetry data collected in one embayment in Maine indicate that within a 25 km² area, pockmarks represent over 5 x 10⁶ m³ of erosion. Often underlain by shallow natural gas, these features were formed within the past 12,000 years, and are associated with past catastrophic fluid escape. Although pockmarks are ubiquitous along Maine’s coast, they remain largely unknown by the public and even by some government agencies with jurisdiction over these areas. Questions remain about the magnitude and frequency of pockmark formation events. Data from repeat swath bathymetry surveys indicate that pockmarks may not form by processes that act on a daily or yearly basis, but instead may be controlled by events that punctuate decades or centuries, such as earthquakes.

In addition, the presence of shallow gas in muddy embayments with, or without pockmarks, constrains the types of activities that are achievable in these gassy seafloor regions. A principle physical assumption for a static gas field is that the gas that resides in the shallow seafloor is in equilibrium with the overlying sediment. That is, the weight and impermeable nature of the overlying sediment counterbalances the pressure of the gas that would lead it to escape, and thus the gas does not exit the seafloor because it is confined. Disequilibrium from dredging, construction or operation of infrastructure has the potential to cause an eruption of pore fluid (water and gas) that could induce failure and resuspension of sediment into the water column. To ensure safe and effective development of offshore resources states should be proactive in the delineation of their coastal resources and demarcation of their potential hazards. Mapping the seafloor with geophysical remote sensing tools is a cost-effective, first-order method for identifying areas where the potential hazards of seafloor gas, pockmarks, and other features may exist.