Seismic Water Bottom Anomalies Map Gallery

Mapping Methodology

The mapping of the water bottom horizon was conducted using specialized geophysical interpretation software. The process began with the manual interpretation of the horizon along selected seismic lines, which served as seed points for a 3D auto-tracking function that facilitated the mapping of the water bottom horizon across the entire survey area. Once the horizon was adequately mapped over the area of interest (AOI), the acoustic amplitude of the water bottom was extracted and displayed in plan view. 

Boundary polygons were delineated around regions exhibiting anomalously high-positive, low-positive, and negative amplitudes, the latter indicating a complete phase reversal of the water bottom horizon. To ensure the accuracy of the amplitude maps, the maps were verified against seismic profiles. In many cases, vertical 3-D seismic profiles beneath the amplitude anomalies revealed features such as blanking and visible fluid migration pathways, including vertical gas "chimneys" and faults, extending up to the water bottom. Some Water Bottom Anomaly example polygons are shown over a 2026 bathymetry-derived acoustic amplitude dataset for illustration, and should be distinguished from earlier images of polygons with seismic amplitude extractions; all anomaly polygons are based on 3-D seismic interpretation.

Collaborative research, archeological, and shallow hazard surveys involving BOEM, NOAA, industry contractors, and other stakeholders have confirmed hundreds of these anomalies as hydrocarbon seeps and carbonate hard grounds. This validation was achieved through various methods, including submersible and remotely operated vehicle (ROV) surveys, autonomous underwater vehicle (AUV) surveys, camera sled surveys, piston core sampling, trawls, and multibeam sonar, which identified gas plumes in the water column.

The following card images on the Seep Related Anomalies and Non-Seep Related Anomalies tabs were generated using a gulf-wide amplitude extraction from BOEM’s deepwater bathymetry map. The data was normalized.

Seep Related Anomalies

The following are images of amplitude anomaly extractions with the feature polygon type displayed. Most images are derived from a 2026 acoustic amplitude dataset; other images are from a legacy amplitude dataset. All anomaly polygons are based on 3-D seismic interpretation.

There are four classes of water bottom anomalies interpreted to be caused by hydrocarbon seepage: 1) High-Positive, 2) Low-Positive/Negative, 3) Pockmarks and 4) Water-Column Gas Plumes. Below is a visual explanation of the amplitude classifications (see Figure 2).

1. High-Positive Class

High-positive amplitude anomalies are characterized by significantly elevated acoustic responses compared to the surrounding seafloor. These anomalies frequently reveal hydrocarbon migration pathways when examined on vertical seismic profiles. Investigations conducted using submersibles or remotely operated vehicles (ROVs) have shown that most of these positive anomalies exhibit a slow to moderate rate of hydrocarbon seepage. The positive amplitude response observed in the seismic data is primarily attributed to the presence of authigenic carbonate hard grounds, which are formed by bacteria that metabolize hydrocarbons in the sediments at these seep sites, resulting in the production of calcium carbonate. These hardgrounds serve as ideal substrates for diverse and thriving chemosynthetic communities, as well as for various species of hard and soft corals. High-Positive anomaly data are subdivided into 9 types:

• Seep anomaly positives

  

  The most abundant, are high-positive amplitude anomalies (colored red in Figure 1), have not yet been confirmed as seep-site hardgrounds – they are strictly interpretations. Due to soft sediment cover and to the coarse vertical resolution of 3-D seismic (~30 feet), some of the high-positive anomalies visited by ROVs or submersibles did not visibly identify exposed authigenic carbonates at the seafloor.

• Seep anomaly confirmed organisms

  

  Marked in dark red in Figure 1 represent high-positive amplitude anomalies that have been verified as hardgrounds predominantly supporting chemosynthetic communities, although many also host corals to a lesser degree. These confirmations were achieved through various methods, including manned submersibles, ROVs,  AUVs, camera sleds, trawls, and piston cores, during research expeditions sponsored by academia, government, or industry.

• Seep anomaly confirmed corals

  

  Magenta in Figure 1 are high-positive amplitude anomalies that have been verified as authigenic carbonates, primarily inhabited by hard and/or soft corals. While these areas are predominantly colonized by corals, they also support chemosynthetic communities to a lesser extent. These polygons also cover the rough boundaries of the shelf marine sanctuaries “Biobanks”.

• Seep anomaly confirmed hydrates

  

  High-positive amplitude anomalies that have been verified to exhibit substantial natural gas hydrate exposures on the seafloor. These sites consistently host active chemosynthetic communities in close proximity to the hydrates. While numerous other locations in the GOA have been identified with hydrate exposures, these particular sites are regarded as significant. In this image, the hydrate exposures are surrounded by flows.

• Seep anomaly positives confirmed oil

  

  High-positive anomalies which have had direct observations of oil seepage from the seafloor by submersible, ROV, and/ or analysis of piston cores. All have had chemosynthetic and coral communities living on the carbonate hardgrounds.

• Seep anomaly positives possible oil

  

  High-positive anomalies located directly below sea surface oil slicks, or within one water depth’s distance. Though not directly observed to be seeping oil, these are classified as possible oil seeps due to the proximity to the slicks. These areas are likely to support chemosynthetic and coral communities residing on the carbonate hardgrounds.

• Seep anomaly flows

  

  Blue with dotted infill in Figure 1, are high-positive amplitude anomalies interpreted as sediment flows originating from high flux vent sites on steep slopes. These flows may have one of the following characteristics: 1) they contained hydrocarbons and were subsequently partially lithified, 2) they attracted chemosynthetic clams that consumed the available hydrocarbons and since they were not located at the active seep site, eventually died, or 3) they consist of acoustically faster sediment, e.g. sand. Flows that have been examined by submersibles often exhibit a combination of two or more of these features.

  The figure above illustrates flows (outlined in dark blue) that were partially lithified and contained dead clam shells, both of which resulted in a higher positive seismic response compared to the soft hemipelagic mud that predominates the deepwater Gulf of America.

• Seep anomaly confirmed carbonate

  

  Polygons that were previously classified as seep anomaly positives. These anomalies have been surveyed using side-scan sonar and/or high-resolution multibeam sonar, revealing significant acoustic backscatter at the seafloor. This backscatter is interpreted as clean carbonate hardgrounds, which may serve as suitable substrates for benthic communities. Direct visual observations of active communities in these areas have not yet been conducted.

• Seep anomaly confirmed buried carbonates

  

  Former seep anomaly positives polygons where 3-D seismic vertical profiles reveal clear subsurface migration pathways leading to the seafloor. However, subsequent visual investigations conducted by manned submersibles, ROVs, or AUVs indicate that there are no exposed carbonates at these sites. Additionally, remote sensing using multibeam or side-scan sonar shows no elevated acoustic backscatter suggesting the presence of buried authigenic carbonates.   
  
  Though clear seafloor indications of seepage such as bacterial mats, e.g., Beggiatoa and or brine seeps may be present, no suitable substrates for megafauna, such as corals or chemosynthetic organisms, e.g., tubeworms, mussels, etc., are present. If shown to be buried by mud, the anomalies in question will be removed from the list of potential hard substrates suitable for benthic organisms and the 2000’ buffer to protect any communities are removed.

2. Low-Positive/Negative Class

Similar to the High-Positive Class, the Low-Positive/Negative Class designation reflects the characteristics of the water bottom seismic horizon, represented by green polygons in Figure 1. Anomalies within this class exhibit an acoustically slower response compared to the typical hemipelagic mud found in the GOA. It is subdivided into 7 types:

• Seep anomaly negatives

  

  Represented by tiny green dots and circles in Figure 1, the seep anomaly negatives features display an anomalously low-positive amplitude response on seismic data compared to the typical hemipelagic muds. The most active and dynamic examples of this type exhibit a negative amplitude response, or acoustic trough, at the seafloor, which results from a complete phase reversal of the seafloor's usual positive acoustic impedance. These areas have been observed to experience rapid hydrocarbon flux, often accompanied by the expulsion of sediment and brine along with the hydrocarbons.

• Seep anomaly mud volcanoes

  

  Mud volcanoes are shown here within the boundaries of pockmarks. They are cones of sediment typically on low slopes that are built at high flux sites that do not exhibit high positive amplitude response. The rate of flux at the expulsion sites is too rapid for bacterial consumption of the hydrocarbons to convert them to authigenic carbonate hardgrounds, thus sessile chemosynthetic organisms and corals are usually not found on these features. These are currently unconfirmed by direct observation.

• Seep anomaly confirmed mud volcanoes

  

  Confirmed by direct observation. Just as with the high-positive anomalies, all negative anomalies were checked on the vertical 3-D seismic profiles to confirm they are caused by a seep with active migration.

• Seep anomaly confirmed mud volcanoes (gas) 

  

  These are mud volcanoes which have been observed seeping gas bubbles. The image shows a mud volcano (lime green) inside a pockmark (purple).

• Seep anomaly confirmed mud volcanoes (oil)

  

  Mud volcanoes which have been observed seeping oil.

• Seep anomaly negatives possible oil

  

  Those low-positive/negative anomalies that are either directly below sea-surface oil slicks or are within one water depth’s distance, and therefore could be the source of the slick, but not yet confirmed to be.

• Seep anomaly negatives confirmed oil

  

  These are low-positive or low-negative anomalies that have been visually confirmed to be seeping oil either through submersible, ROV, or piston core analysis.

3. Pockmarks Class

Due to the absence of hard substrates and the lack of ongoing seepage at most pockmarks, it is unlikely that chemosynthetic organisms or corals are associated with these features. The image to the right presents an amplitude map of an area containing numerous pockmarks, while the A-A’ vertical seismic profile below illustrates the pockmarks expression on the seafloor. Pockmarks found on the Florida Shelf are interpreted to be sinkholes formed by underlying karstic formations (see Sinkholes, Non-seep Related Anomalies).

• Seep anomaly pockmarks

  

  Outlined in purple in Figure 1, are circular to oval depressions interpreted to result from the removal of sediment due to rapid, and potentially explosive, gas expulsion. While a few pockmarks exhibit visible active migration pathways on vertical seismic profiles, the majority appear to be dormant, lacking discernible active migration. The rapid expulsion observed in these features is interpreted to be exclusively gas-related and is considered purely destructive, as it leads to the removal of sediment without any observations of sediment, brine, or oil expulsion.

4. Water-Column Gas Plumes Class

• Plumes EK60 & Plumes EM302 400ft diam

  This image shows the EK 60 and the EM302 datasets. EK60 is point data with low uncertainty of seep location.  The EM 302 plumes are represented as 400 ft diameter circles to reflect the uncertainty regarding the exact location of each seep on the seafloor, which can vary with water depth. Many of these circles are clustered closely together; clustering is a result of overlapping swath data acquisition, meaning that some of these clusters may represent a single seep originating from the same location on the seafloor. Within these clusters, some circles are positioned near the center of the swath, indicating a higher level of accuracy, while others are located further away from the center, suggesting a lower level of accuracy. This variability is the reason for the 400 ft diameter uncertainty associated with each seep location. 

  

• Seep anomaly high positives confirmed gas

  

  High positive anomalies that have either EK60 and/ or EM302 gas plumes identified to originate from within the anomaly polygon.

• Seep anomaly negatives confirmed gas

  

  Low positive/negative anomalies that have either EK60 and/ or EM302 gas plumes identified to originate from within the anomaly polygon. The negative gas anomaly is shown here inside a positive confirmed gas anomaly.

Non-Seep related Anomalies

The anomalies described below are non-seep related, and except for the two Cretaceous types, all are not likely to support benthic organisms.

• Anomaly Cretaceous

  

  The anomaly Cretaceous consists of long, linear high-positive amplitude anomalies that extend along the face of the northern Florida Escarpment. Through several dives conducted with manned submersibles and ROVs, these anomalies have been identified as outcrops of well-indurated carbonate strata dating back to the Cretaceous period. Each dive has confirmed that these locations serve as prime substrates for coral communities.

  Seismic data, along with paleontological information obtained from well control on the Florida Platform, establishes the Cretaceous age of these carbonate rocks. The polygons represented in the anomaly Cretaceous data are derived from 3-D seismic data.

• Anomaly Cretaceous Talus

  

  Anomaly Cretaceous talus refers to areas along the lower Florida Escarpment characterized by high-positive amplitude responses concentrated in valleys between discrete ridges. These features are interpreted as slump deposits of carbonates, likely dating back to the Cretaceous period. Given that the Cretaceous carbonate outcrops along the steep slopes of the Escarpment have been identified as suitable substrates for coral communities, it is probable that the talus deposits would also support similar coral communities. While chemosynthetic organisms have been observed at the base of the Escarpment during ROV and submersible dives, to our knowledge, none of the talus deposits have been directly investigated. Additionally, the Okeanos Explorer's multibeam sonar did not detect any active water column gas plumes in the mapped area.

• Anomaly Salt

  

  Anomaly salt, is represented by a distinct class of large, light pink polygons in Figure 1, setting them apart from authigenic seep carbonates. These areas in the Gulf of America are characterized by salt outcrops on the seafloor, exhibiting a high-positive amplitude response similar to that of hardground seep anomalies. However, vertical seismic profiles clearly indicate that these features are salt outcrops and not related to seepage. Two of these salt outcrop locations have been visited by the Alvin manned submersible, which confirmed their identity as salt formations. This figure illustrates the amplitude expression of these areas on the seafloor.

• Anomaly Slumps

  

  Anomaly slumps are found at the base of over-steepened slopes, which are the result of shallow vertical and horizontal salt movement. This movement destabilizes surface sediments, causing them to flow downslope and accumulate on the low-slope seafloor in front of the steep slopes. The figure above provides an example of interpreted slumps located in front of the Sigsbee Escarpment, where movement of shallow salt has led to the over-steepening of the seafloor. This process has triggered mass sediment flow, resulting in a slump deposit at the base and a scar on the Sigsbee Escarpment.

  The high-positive amplitude associated with the slump is interpreted to arise from the winnowing of finer sediments from the slump deposits, leaving behind a sand-rich layer on the basin floor. Additionally, the presence of authigenic carbonate debris that accompanied the flow may contribute to this amplitude response.

• Anomaly Fan

  

  Anomaly fan, yellow polygons with dotted fill in Figure 1, are interpreted to be sand-rich turbidite fans which have intermittently dominated sedimentation in portions of the deepwater Gulf of America for millions of years. Fans and channel systems expressed on the modern-day sea floor are important as they can be used to better understand the many subsurface fan & channel reservoirs throughout the basin.

  Several large, discrete recent channel/fan complexes can be observed on the seafloor, particularly in Alaminos Canyon, where they exhibit a high-positive acoustic response on seismic data and are easily identifiable on amplitude maps (above figure). These recent examples provide valuable analogues for understanding reservoir geometries relevant to subsurface exploration and development activities.

• Anomaly Channel

  

  Anomaly channel polygons represent erosional, deep-sea channels associated with turbidite fans, basin floor fans and basin fill fans, as well as major and minor shelf-slope-break bypass downcutting from recent lowstands. They are easily distinguished by their long, narrow, often sinuous character on amplitude and bathymetry displays. The channel feature polylines were closed into polygons for data management purposes and are not observed geologic features.

• Anomaly Relic Patch Reefs & Confirmed Relic Patch Reefs

  

  Anomaly relic patch reefs and confirmed relic patch reefs, also referred to as Potentially Sensitive Biological Features, or PSBFs, are small circular positive anomalies in 100 to 350 feet of water that are common in the region of the lower shelf near or next to the coral bio-banks of the northern Gulf of America, East and West Flower Garden Banks, Rankin and Bright Banks, Stetson Bank, etc., and to the east of the Birdfoot Delta, also referred to as the “Pinnacle Trend”. They appear similar to mud volcanoes in size and shape, but are generally acoustically fast, i.e., hardground and do not show any indication of subsurface gas/fluid migration up to the features.

  Recent multibeam mapping by the USGS and collaborative efforts with NOAA show high positive acoustic backscatter, suggesting hard surfaces. The confirmed patch reefs have had direct observation of present day live bottom communities by ROV deployments. We are hypothesizing that their origins were as patch reefs during past low sea-level stands because of their water depths, size and shape, and hard surfaces. 

• Anomaly Furrows

  

  Anomaly furrows are extensive fields of bottom current-scoured trenches on the floor of the ultra-deep Gulf of America that occur at and in front of the Sigsbee Escarpment. These features are first apparent just to the northeast of Green Knoll in the Green Canyon Protraction Area, trending NE-SW and extending intermittently over to the eastern side of Alaminos Canyon Protraction Area, trending E-W. They have been observed remotely by deep towed high resolution seismic, side-scan and multibeam sonar, and 3-D seismic data, and directly observed by manned submersibles by Texas A&M University (Bean, 2005). They were found to be as deep as 10 meters and as wide as 30 meters, with currents from 20 – 60 m/sec scouring the seafloor. BOEM mapped the furrow fields using 3-D seismic and created polygons around discrete packages of the linear depressions which are included in this Water Bottom Anomaly Map Gallery. Due to the limited vertical and horizontal resolution of the 3-D seismic used, BOEM was able to detect, but fully not resolve these features.

• Anomaly Sinkholes

  

  Anomaly sinkholes appear on the Florida Platform as circular depressions on the seafloor, similar to pockmarks, but have deep subsurface roots (600 to 800 ms), or possible vertical collapse or migration pathways leading up to them. Only three of them were recognized, whereas 47 pockmarks were mapped on the Florida platform. The figure above is an amplitude expression of a sinkhole on the Florida Platform.

Data Accessibility Statement

The Bureau of Ocean Energy Management (BOEM) provides access to seismic water bottom anomaly data to support scientific research, environmental analysis, and resource management. These data are derived from geophysical surveys conducted on the Outer Continental Shelf (OCS) and include interpreted anomalies that may indicate features such as gas pockets, hardgrounds, or other subsurface conditions.

Use and Licensing:

All data are provided under BOEM’s open data policy and may be used for research, planning, and decision-making purposes. Users are encouraged to cite BOEM as the source and include dataset version and retrieval date in any publications or derivative products.