THE RELATIONSHIP BETWEEN POCKMARKS, GAS-ENHANCED REFLECTORS AND ACOUSTIC WIPEOUT IN AN ACTIVE ESTUARINE POCKMARK FIELD, PENOBSCOT BAY, MAINE

Sidescan sonar and bathymetric surveys of Penobscot Bay, Maine have revealed the presence of pockmarks. Nearly 6000 pockmarks ranging in size from a few meters in diameter to >100 m diameter and 30 m relief have been mapped in eight fields with field densities approaching 1400/km2. Pockmarks are the result of pressurized pore fluids escaping from the subsurface (i.e., methane and water). Interpretations of repeat sidescan sonar surveys from Belfast Bay indicate that the pockmark field is actively producing more new pockmarks than are lost, with a net gain of 9 pockmarks/yr averaged over a nine-year period. Several reconnaissance and one detailed geophysical survey using sidescan sonar and shallow seismic reflection profiling conducted near the Black Ledges indicate areas of acoustic wipeout (AW) and gas-enhanced reflectors (GER). The detailed survey was used to correlate pockmarks and activity zones with the various indicators of shallow gas. Forty-six vibracores were collected to correlate seismic stratigraphy with physical sedimentology. GER appear to relate to a locally pervasive gravel lag surface and sandy lenses, zones of higher gas permeability. Active regions of the fields typically show areas of AW and/or GER’s. Directly below pockmarks are “windows” through AW revealing the stratigraphy, indicating a depletion of gas below the pockmark. Conversely, inactive areas are devoid of large-scale AW and host only occasional GER’s. Pockmarks in these areas do not appear “crisp” on sidescan sonar images and several seismic lines suggest slumping on the pockmark margins and filled pockmarks. The relationships observed generally suggest that the pockmark fields are actively depleting gas reservoirs from the margins of the field. Erosion of sediments overlying GER’s suggests potential leakage of the reservoir through the intersection of a gas-permeable layer, preventing over-pressurization. Thus, activity is confined to central portions of the field.