Seismic Water Bottom Anomalies Map Gallery

Introduction: 

Since 1997, geoscientists at the Bureau of Ocean Energy Management, Regulation, and Enforcement (BOEMRE) have identified and mapped over 21,000 water bottom (seafloor) seismic amplitude anomalies in the deep water northern Gulf of Mexico (GOM) using 3-D time-migrated seismic surveys. These areas are of anomalously high or low reflectivity response over background response, with most areas having overlapping seismic coverage by two or more surveys. These results cover over 156,000 square miles of seismic data interpretation. The water bottom horizon was completely mapped over these surveys using hand-interpreted seed lines and Geoframe’s automatic picking program to fill between. The seismic amplitude was then displayed in plan view. Anomalously high positive and low positive/negative amplitudes were identified and outlined using polygons. The amplitude maps were cross checked with the vertical seismic profiles to verify correctness in the autopicked interpretation. Blanking and/or visible migration pathways (e.g. faults) are often visible in the subsurface up to the water bottom in vertical seismic profiles beneath most of the amplitude anomalies. BOEMRE, NOAA, and industry contractors have confirmed over 100 of the anomalies as hydrocarbon seeps and carbonate hard grounds through utilization of submersible, ROV, AUV, camera sled surveys, piston cores and trawls. Figure 1 illustrates the total coverage of all three types of anomaly. 

Anomaly Types: 

Areas which show a high positive amplitude response relative to the normal water bottom response have been outlined in red as positive anomalies. Most of these positive anomalies have been found to exhibit a slow to moderate rate of hydrocarbon seepage. The positive amplitude response on the seismic data has been found to be caused by the presence of carbonate hard grounds created by bacteria living off the hydrocarbon in the sediments at these seeps, producing calcium carbonate in the process. Most of the sites which have had direct visual observations have thriving chemosynthetic communities, and, often, hard and soft corals. 

Areas which show an anomalously low positive amplitude response on seismic (and, sometimes, a phase reversal from a peak to a trough) have been outlined in green as negative anomalies. These areas have been observed to have rapid hydrocarbon flux.  These high flux vent sites, which have sediment and brine expulsion with the hydrocarbons, usually build cones, or mud volcanoes, where the pre-existing water bottom topography has a low slope. Where the hydrocarbon, brine, and sediment expulsion occurs on steep slopes, flows of sediment downslope have been observed. Most of these flows have relatively high positive amplitude response due to: 1) acoustically faster sediment type than the surrounding hemipelagic mud, 2) higher hydrocarbon saturations, resulting in bacterial lithification of the sediment, and/or, 3) chemosynthetic clam populations. The rate of flux at the actual expulsion sites is often too rapid for bacterial consumption to convert the hydrocarbon to carbonate hardgrounds, and sessile chemosynthetic organisms and corals that require hard substrates aren’t found at these sites. Just as with the positive anomalies, the negative anomalies were checked on the seismic to confirm they are caused by a seep with active migration. Figure 2 illustrates a plan view of positive and negative anomalies, with Figure 3 showing a vertical seismic profile through those anomalies along the northwest/southeast trending red line in Figure 2. 

	Pockmarks Files
	Positive Anomalies Files
	Negative Anomalies Files