Seismic Water Bottom Anomalies Map Gallery

Introduction:

Figure1 GOM anomaliesSince 1998, geoscientists at the Bureau of Ocean Energy Management (BOEM), formerly Minerals Management Service (MMS), have identified and mapped over 31,000 water bottom (seafloor) acoustic 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 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 acoustic amplitude response relative to typical background, with most areas having overlapping seismic coverage by two or more surveys. These results cover over 230,000 square kilometers of seismic data interpretation.

Mapping Methodology

Figure 2 GOM anomaliesThe water bottom horizon was mapped over these surveys using hand-interpreted seed-lines and Schlumberger’s geophysical interpretation software GeoFrame’s automatic picking function to fill the gaps between. Next, the water bottom’s acoustic amplitude was extracted and displayed in plan view. Boundary polygons were drawn around areas with anomalously high-positive and low-positive amplitudes, as well as negative amplitudes (complete phase-reversal of the water bottom horizon). The amplitude maps were cross-checked with vertical seismic profiles to verify correctness in the autopicked interpretation. In vertical 3-D seismic profiles beneath most of the amplitude anomalies, blanking and/or visible fluid-migration pathways (e.g., vertical gas “chimneys” and faults) are often visible in the subsurface up to the water bottom. As through March 2014, BOEM, NOAA, and industry contractors have confirmed over 500 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 nineteen types of anomaly, however, not all anomalies nor anomaly types are visible in Figure 1 due to its scale and resolution. You will need to download the anomaly files to properly view all mapped features.

Anomaly Types:

Seep-Related Anomalies

There are three classes of waterbottom anomalies interpreted to be caused by hydrocarbon seepage: 1) High-Positive, 2) Low-Positive/Negative, 3) Pockmarks, and 4) Water-Column Gas Plumes.

  1. High-Positive Class

    Exhibits a high-positive amplitude response relative to the background water bottom response, often shows hydrocarbon migration pathways on vertical seismic profiles. 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 authigenic carbonate hard grounds created by bacteria living off the hydrocarbon in the sediments at these seeps, producing calcium carbonate in the process. These hardgrounds provide suitable substrates for thriving chemosynthetic communities and both hard and soft coral types. High-Positive anomaly shapefiles are subdivided into 7 types:
    • seep_anomaly_positives, the most abundant and colored red in Figure 1, are high-positive amplitude anomalies which have not yet been confirmed as seep-site hardgrounds – they are purely seismic interpretations. Due to the coarse vertical resolution of 3-D seismic (~30 feet), some of the high-positive anomalies visited by ROVs or submersibles have not had exposed authigenic carbonates at the seafloor due to soft sediment cover. There may be 28 feet of authigenic carbonate buried by 2 feet of hemipelagic mud, but the 3-D seismic amplitude will still show a strong high-positive anomaly (see Figure 2, seafloor amplitude map and Figure 3, seismic profile A–A’).
    • seep_anomaly_confirmed_organisms, dark red in Figure 1 are high-positive amplitude anomalies that have been confirmed to be hardgrounds with predominantly chemosynthetic communities, though most have corals to a minor extent. Confirmed via manned submersibles, ROVs, AUVs, camera sleds, trawls, and piston cores either by academia, government, or industry sponsored cruises.
    • seep_anomaly_confirmed_corals, magenta in Figure 1 are high-positive amplitude anomalies that have been confirmed to be authigenic carbonates predominantly colonized by hard and/or soft corals, but have had chemosynthetic communities to a minor extent.
    • seep_anomaly_confirmed_hydrate are high-positive amplitude anomalies that have been confirmed to have significant seafloor natural gas hydrate exposures, but have always had active chemosynthetic communities near or adjacent to the hydrates.
    • seep_anomaly_positives_confirmed_oil are high-positive anomalies which have had direct observations of oil seepage from the seafloor, and all have had chemosynthetic and coral communities living on the carbonate hardgrounds.
    • seep_anomaly_positives_possible_oil are 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, we are calling these possible oil seeps due to their proximity to the slicks. They are likely to have chemosynthetic and coral communities living on the carbonate hardgrounds.
    • seep_anomaly_flows, blue with dotted infill in Figure 1, are high-positive amplitude anomalies interpreted to be flows of sediment out of high flux vent sites on steep slopes. They either 1) contained hydrocarbons and were subsequently partially lithified, 2) attracted chemosynthetic clams which consumed what hydrocarbon was available (and since they were not located at the active seep site, subsequently died), or 3) are made up of acoustically faster sediment (i.e., sand). The flows that have been visited by submersibles are often a combination of two 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 hemipelagic 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; note the lack of migration pathways in the subsurface below the flow.
  2. Low-Positive/Negative Class

    In the same style as Class 1 High-Positive, the Low-Positive/Negative class name also reflects the nature of the water bottom seismic horizon (green polygons in Figure 1). This class of anomalies exhibit acoustically slower response than that of typical hemipelagic mud of the GOM. It is subdivided into five types:
    • seep_anomaly_negatives, tiny green dots and circles in Figure 1, show an anomalously low-positive amplitude response on seismic compared to the typical hemipelagic muds. The most active and dynamic of this type exhibit a negative amplitude response, or acoustic trough, at the seafloor, resulting from a total phase reversal of the seafloor’s typical positive acoustic impedance. These areas have been observed to have rapid hydrocarbon flux, often with sediment and brine being expelled with the hydrocarbons (see Figure 2, seafloor amplitude map and Figure 3, NW-SE seismic profile A-A’).
    • seep_anomaly_mud_volcanoes 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 unconfirmed by direct observation.
    • seep_anomaly_confirmed_mud_volcanoes have been 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_negatives_possible_oil are those low-positive/negative anomalies that are either directly below sea-surface oil slicks or are within 1 water depth’s distance, and therefore could be the source of the slick, but not yet confirmed to be.
    • seep_anomaly_negatives_confirmed_oil are those low-positive/negative anomalies that have been visually confirmed, through submersible or ROV, to be seeping oil.
  3. Pockmarks Class
    • seep_anomaly_pockmarks, appearing as purple dots in Figure 1, are circular to oval depressions interpreted to be created by the removal of sediment through rapid, and possibly, explosive gas expulsion. Few pockmarks have visible active migration pathways on vertical seismic profiles, but most appear to be dormant and without discernible active migration. Rapid expulsion is interpreted to be exclusively gas and appear to be purely destructive due to the removal of sediment. No sediment, brine, or oil expulsion has been observed during direct observations. Due to a lack of hard substrate and absence of continuing seepage at most pockmarks, 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.
      Fig5_GOM anomalies Fig6_GOM anomalies
  4. Water-Column Gas Plumes Class

    Fig7_GOM anomaliesDetected by NOAA’s RV Okeanos Explorer during cruises in 2011, 2012, and 2014, this class represents plumes of gas found in the water column, originating from seafloor seeps, as detected by EK60 single beam sonar and the EM302 multibeam sonar. Due to the relatively similar acoustic properties of oil and water, these sonars are unable to discern if oil seepage is associated with any of these gas plume sites. We are including the files of both the EK60 and EM302, as they have different swath widths of detection (EK 60 is around 20-60 feet depending on water depth, and the EM 302 is 3.7 times the water depth); see Figure 7. Many of these plumes helped confirm BOEM seep-related anomaly polygons included in the shapefiles below. Updates of the Okeanos Explorer 2014 cruise, as well as other sonar surveys by other vessels will be made in the future. The two shapefiles are called:
    • plumes_EK60_points – Points with a triangle symbol. Low uncertainty of seep location.
    • plumes_EM302_400ft_diam – These are downloaded as 400 foot diameter circles to indicate the uncertainty of the exact seep location on the seafloor, which varies with water depth. You will note many of the circles are clumped nearby each other. Due to the nature of overlapping swath data acquisition, some of those clumps may actually represent a single seep from the same spot on the seafloor; some are near the center of the swath (most accurate) and some well away from the center of the swath (less accurate). This is why there is a 400 foot diameter of uncertainty.

Non-Seep-Related Anomalies

The anomalies described below are non-seep related, and all but the first, “anomaly_Cretaceous”, are not likely to support benthic organisms.

  • anomaly_Cretaceous, outlined in blue in Figure 1 and directly following the northern Florida Escarpment are long, linear high-positive amplitude anomalies extend along the face of the escarpment and, from several dives with manned submersibles and ROVs, have been found to be outcrops of Cretaceous-aged, well-indurated carbonate stratum. Each dive has proven these locations act as prime substrate for coral communities. Seismic data and paleontological data from well control on the Florida Platform establish the Cretaceous age of these rocks. The polygons in the anomaly_Cretaceous shapefiles 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 8 is a bathymetry/amplitude map across the Florida Escarpment with the Cretaceous carbonate outcrops outlined in blue. Figure 9 is a seismic traverse (D-D’) across the escarpment.
    Fig8_GOM anomalies Fig9_GOM anomalies
  • anomaly_salt, have been outlined with a unique class of large, light pink polygons in Figure 1 to distinguish them from authigenic seep carbonates. They are areas in the Gulf of Mexico where salt outcrops on the seafloor and has a very similar high-positive amplitude response as hardground seep anomalies, but from vertical seismic profiles it’s clear that salt is outcropping and these are not seep related anomalies. Two of these have been visited by the Alvin manned submersible and confirmed to be salt outcrops. Figure 10 shows three of these areas on the seafloor and the E-W seismic profile (E-E’) in Figure 11 shows one of these where the Alvin found salt on the surface.
      Fig10_GOM anomalies
  • anomaly_slump 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 12 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, leaving 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. Figure 13 is a vertical seismic profile (F-F’) showing the slump deposit.
  • 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 deep water Gulf of Mexico for millions of years and are the source of many of the subsurface sand reservoirs throughout the basin. 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 (see Figure 14) on amplitude maps. These recent examples are good analogues of reservoir geometries for subsurface exploration/development activities. Figure 15 is seismic profile G-G’ illustrating the channel and resulting basin-fill fan

File Details:

These files were generated using ESRI® ArcGIS ArcMap™ 10.0 in a Microsoft® Windows® 7 operating system, and have been tested to be backwards compatible through version 9.3.1.

Datum Information: All of the anomaly polygons were drawn in NAD 27.

File formats: For your convenience, the anomaly files have been provided in Layer Package Format (.lpk), which is similar to a zipped folder. The single file contains all 19 anomaly types, metadata, projection information, and visual symbology. Alternatively, if you would rather download each type of anomaly individually, you may do so with the individual zipped folders below. The individual zipped folders contain the standard shapefile suite of files (.shp, .dbf, .prj, etc.) and also a layer file (.lyr) which contains the visual attributes and descriptions of each shapefile. We have also made .DWG files available for CAD users. The DWG files are in R2010 format.

NOTE: The single file downloads will be available via the web shortly.

Instructions to View the Anomaly Polygons:

If using ESRI® ArcGIS ArcMap™ 9.3.1 or later

Method 1: Layer Package file with ArcMap™ already running

  1. Download the Layer Package file (.lpk) BOEM Seafloor Anomalies March 2014 Layer Package.
  2. Open the ArcMap™ project you will be using.
  3. Navigate to the downloaded .lpk file and open it.
  4. The files will be unpacked and loaded to your project automatically. Note - Unpacking may take minutes.

Method 2: Layer Package file with ArcMap™ not already running

  1. Download the Layer Package file (.lpk) BOEM Seafloor Anomalies March 2014 Layer Package.
  2. Navigate to the downloaded .lpk file and open it.
  3. The file will automatically launch ArcMap™ and unpack the files to a blank project. Note - Unpacking may take minutes.

Method 3: (not recommended) Loading individual shapefiles instead of the Layer Package file

  1. Download the desired individual anomaly zip files from below.
  2. Unzip/extract the files, ensuring you keep the shapefile suite files together for any given anomaly type. The shapefiles will not display correctly if separated from each other or from their layer .lyr files!
  3. Open the ArcMap™ project you will be using.
  4. For each anomaly type you would like to load into your project, click and drag its layer .lyr file into your map. DO NOT drag the shapefile .shp into your map unless you want to strip the file of all formatting, descriptions, and metadata!
  5. Alternatively, you can import each anomaly .lyr file using ArcCatalog™.

IMPORTANT LAYER PACK FILE LOADING TIME NOTICE:

When initially loading the Layer Package file to your project, please use patience. There are over 31,000 polygons in the files, so they may take up to a few minutes to load in to your project. Your project may freeze temporarily while the file is being unpacked.

If using any other GIS software

  1. Download the desired individual anomaly zip files from below.
  2. Unzip/extract the files, ensuring you keep the shapefile suite of files together for each given anomaly type. The shapefiles will not display correctly if separated from each other!
  3. Open your GIS project and load the shapefiles.

If using CAD software

  1. Download the zipped folder BOEM Seafloor Anomalies March 2014 DWG Format.
  2. Unzip/extract the .DWG files.
  3. Open your CAD project and import the .DWG files.

You can assist in making this database more complete!

A significant limiting element in this dataset is our lack of knowledge of all the sites in the Gulf of Mexico that have been confirmed by direct observations. Should you discover through your use of these files, that you may have evidence to confirm the presence or absence of chemosynthetic and/or coral communities, hydrates, oil & gas seeps, etc., BOEM asks that you 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.