Seismic Water Bottom Anomalies Map Gallery

Introduction:

Fig1_GOM anomaliesSince 1997, geoscientists at the Bureau of Ocean Energy Management (BOEM), formerly MMS, have identified and mapped over 28,000 water bottom (seafloor) seismic amplitude anomalies in the deep water northern Gulf of Mexico (GOM) using 3-D time-migrated seismic surveys. The purpose of this mapping program is to understand the distribution of natural hydrocarbon seeps and the related benthic fauna (chemosynthetic and coral communities) in the GOM, and to characterize other seafloor features related to the geological framework of the seafloor. These areas show anomalously high or low amplitude response over the background response, with most areas having overlapping seismic coverage by two or more surveys. These results cover over 230,000 square kilometers of seismic data interpretation. The water bottom horizon was completely mapped over these surveys using hand-interpreted seed lines and the geophysical interpretation software Geoframe’s automatic picking program to fill the gaps 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., vertical gas “chimneys” and faults) are often visible in the subsurface up to the water bottom in vertical seismic profiles beneath most of the amplitude anomalies. BOEM, NOAA, and industry contractors have confirmed over 400 of the anomalies as hydrocarbon seeps and carbonate hard grounds through utilization of submersible, ROV, AUV, camera sled surveys, piston cores, trawls, and multibeam sonar identifying water column gas plumes. Figure 1 illustrates the total coverage of all eight types of anomaly.

Anomaly Types:

The first type of anomaly and most common (numbering 22,087) is called “wb_anom_pos.” These anomalies show a high positive amplitude response relative to the normal water bottom response (and usually show hydrocarbon migration pathways on vertical seismic profiles) and have been outlined in red in Figure 1. Most of these positive anomalies that have been directly investigated by submersible or ROV 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.

Fig2_GOM anomaliesThe second type of anomaly is called “wb_anom_neg” (numbering 2,670). These areas show an anomalously low positive amplitude response on seismic (and, sometimes, show a negative, or trough, resulting from a phase reversal from a peak to a trough) and have been outlined in green in Figure 1. 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. Fig3_GOM anomaliesOf the 2670 high flux seismic anomalies, 60 mud volcanoes have been recognized to date (“wb_anom_mvol”), with 56 being confirmed as such by direct observation (“anom_conf_mvol”). 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 (A-A’) through those anomalies along the northwest/southeast seismic traverse.

Fig4_GOM anomalies The third anomaly type, as noted above, is called “wb_anom_flow” (numbering 496). These relatively high positive amplitude anomalies are interpreted to be flows of sediment out of high flux vent sites and mud volcanoes that either contained hydrocarbons and were subsequently partially lithified, attracted chemosynthetic clams which consumed what hydrocarbon was present, or were made up of acoustically faster sediment (i.e., sand). The flows that have been visited by submersibles (see Figure 2) often are a combination of 2 or more of the above. Figure 2 shows flows (outlined in dark blue) that were partially lithified and had dead clam shells, both of which caused higher positive seismic response than the soft hemi-pelagic mud that dominates the deep water Gulf of Mexico. Figure 4 is a vertical seismic profile (B-B’) showing the high amplitude (relatively a smooth surface) seafloor trending downslope from an active vent site. Many more flows will be identified in the near future and added to this database.

Fig5_GOM anomalies The fourth type of anomaly is called “wb_anom_pock”, or pockmark, outlined with purple polygons in Figure 1 (numbering 3,720). These circular to oval depressions are interpreted to be created by the removal of sediment through rapid, and, possibly, explosive gas expulsion. Some of these have visible active migration pathways on the seismic data, while most appear to be dormant and without discernible active migration. This type of expulsion is interpreted to be gas only (no sediment, brine, or oil has been observed during direct observations), Fig6_GOM anomaliesand they appear to be destructive due to the removal of sediment, unlike the constructive mud volcanoes. Due to a lack of continuing seepage at most of these pockmarks and a lack of hard substrate, chemosynthetic organisms and corals are unlikely to be associated with them. Figure 5 is an amplitude map of an area with numerous pockmarks. Figure 6 is a vertical seismic profile (C-C’) showing the pockmarks on the seafloor.

Fig7_GOM anomalies The fifth type of anomaly is called “wb_anom_Cret” (numbering 212), outlined in blue in Figure 1. Long, linear high positive amplitude anomalies extend along the face of the Florida Escarpment and, from several dives with manned submersibles and ROVs, they have been found to be outcrops of Cretaceous-aged, well indurated carbonate rock that have provided substrate for corals every time they have been visited. Seismic data and paleontological data from well control on the Florida Platform establish the Fig8_GOM anomaliesCretaceous age of these rocks. The only “Cret” polygons in the file are those mapped on 3-D seismic data; though there are 2-D data sets to the south along the Escarpment, they are not amplitude balanced and were not used to develop this shapefile data set. Figure 7 is a bathymetry/amplitude map across the Florida Escarpment with the Cretaceous carbonate outcrops outlined in blue. Figure 8 is a seismic traverse (D-D’) across the Escarpment.

The sixth type of anomaly is called “wb_anom_salt” (numbering 40). These areas in the Gulf of Mexico are where salt outcrops on the seafloor and have a very similar response as high positive seep anomalies. From vertical seismic profiles, it is clear that salt is outcropping at the seafloor and these are not seep-related anomalies. These have been outlined with a unique class of polygon to distinguish them from authigenic seep carbonates (in bold pink on Figure 1). Two of these have been visited by the Alvin manned submersible and confirmed to be salt outcrops. Figure 9 shows three of these areas on the seafloor and the EW seismic profile E-E’ in Figure 10 shows one of these where the Alvin found salt on the surface.

Fig9_GOM anomalies Fig10_GOM anomalies

Fig11_GOM anomalies The seventh anomaly type is called “wb_anom_slump” (numbering 226). These are located at the base of over-steepened slopes caused by shallow vertical and horizontal salt movement; surface sediments become unstable and flow downslope to rest on low-slope seafloor in front of the steep slopes. Figure 11 is an example of interpreted slumps in front of the Sigsbee Escarpment where lateral movement of shallow salt appears to have over-steepened the seafloor, causing mass flow of sediment, Fig12_GOM anomaliesleaving a slump deposit in front and a scar on the Sigsbee Escarpment. The high positive amplitude of the slump is interpreted to be the result of winnowing of fines from the slump sediments, leaving a sand-rich layer on the basin floor and/or the presence of authigenic carbonate debris that accompanied the flow. Industry and government submersible and ROV investigations have found authigenic carbonate and chemosynthetic communities along the face of the Sigsbee that could be the source of the acoustically fast rubble. Figure 12 is a vertical seismic profile F-F’ showing the slump deposit. Many more slumps will be identified in the near future and added to this database

The eighth type of anomaly is called “wb_anom_fan” (numbering 25). Submarine channels and sand-rich turbidite fans have intermittently dominated portions of the deep water Gulf of Mexico for millions of years and are the source of subsurface sand reservoirs throughout the GOM. There are a few large, discrete recent channel/fan complexes on the seafloor (especially in Alaminos Canyon) that have high positive acoustic response on seismic data and are easily recognized (Figure 13) on amplitude maps. These recent examples are good analogues of reservoir geometries for subsurface exploration/development activities. Figure 14 is seismic profile G-G’showing the channel and the basin fill fan resulting from it.

As part of BOEM’s effort to confirm these seismically-recognizable features, we are in the process of researching which have been confirmed directly by visual observation (submersible, ROV, AUV, trawl, and core, by academia, government, and industry). Though this effort is far from complete, we are reporting observations of those compiled to date. “Anom_conf_orgs” are high positive amplitude anomaly locations reported to have chemosynthetic and coral communities on carbonate hardgrounds (totaling 218) and “anom_conf_coral” (totaling 4) that have reports of corals on hardgrounds being the primary benthic organisms. Significant gas hydrate occurrences (totaling 3) are called “anom_conf_hydrate.” “Anom_conf_oil” have had direct observations of oil seepage from high positive and low positive anomaly sites (totaling 15). 183 anomalies, both high flux/low positive and low-moderate flux, high positive, have been confirmed by multibeam sonar on the “RV Okeanos Explorer” during its 2011 and 2012 cruises to have gas plume anomalies in the water column, “anom_conf_gas.”

Fig13_GOM anomalies Fig14_GOM anomalies

Many of the seismic anomalies are located directly below surface oil slicks, or within a water depth’s distance. Though not directly observed to be seeping oil, we are calling these possible oil seeps - “anom_poss_oil_pos” for the high positive anomalies associated with oil slicks (662 of the total 22,087 high positive ) and “anom_poss_oil_neg” for the low positive associated with oil slicks (142 of the total 2670 low positive).

As noted earlier, this process of confirming these seismic amplitude anomalies is ongoing. An obvious limiting factor is our lack of knowledge of all the sites in the Gulf of Mexico that have been confirmed by direct observations. BOEM is asking all who download and use this database with knowledge of direct observations (of chemosynthetic/coral communities, hydrates, oil/gas seeps, etc.) of sites not indicated here to be confirmed to contact via email william.shedd@boem.gov or kody.kramer@boem.gov with lat/longs, maps, photos/videos, etc., so we can update our database for the benefit of all using this resource. This website will be referencing the source of these confirmations known to date and will give credit to all who contribute information in the future.

All of these files were also merged together into a single shapefile layer for inclusion in the MarineCadastre.gov map viewer.  This file is available for download below.