Access Keys:
Skip to content (Access Key - 0)

Versions Compared

Key

  • This line was added.
  • This line was removed.
  • Formatting was changed.

...

Figure 1. Locations of Site 1 and 2, geologic features, and ocean currents of northern South America within the Lesser Antilles Arc and the Barbados Accretionary Complex (adapted from Deville et al. (3) and Pichot et al. (12)). Data were pushed into ESRI ArcGIS using the Fledermaus FMGIS transfer function to create an ArcGIS map of the bathymetry.

Mud volcanoes, craters and faults have been mapped across the accretionary prism (Fig. 1), and gas emissions from the seafloor are spatially associated with mud diapirs1. However, previous to this project, active gas emissions into the water column have neither been identified in this region nor associated with vent locations.

...

The multibeam data were imported into the QPS product suite for processing, analysis and visualization (Fig. 3). Using the QPS suite allows the operator to process the bathymetry, backscatter, and water column data, fuse it together, and analyze it in 4D4,5. The time dimension effectively provides the user with visualization of the data as it was collected using the acquisition time stamps, and also allows comparison of multiple passes or surveys. Based on the sounding density, a 50x50 meter cell size was chosen to create a map of seafloor bathymetry.

...

Extraction of the flares from the background water column data using FMMidwater identifies the curvilinear shape of the anomalies and the extent of their resolvable travel time through the water column. Two sites have been identified along the cruise transect with water column anomalies that occur along narrow (~2 km), 7~10 km long, north-south trending ridges and valleys with ~400 m of relief (Table 1). Flares within the water column occur at two sites at water depths between ~1500 and ~600 m depth (Figs. 5 and 6). At Site 1 two closely-spaced flares are both ~900 m tall whereas at Site 2 the flares are shorter, ~600 m tall. Each has the higher amplitude backscatter values occurring in the central and lower portions of the flare, perhaps because this is the region of greatest bubble density6, or an increase in reflectivity as the resonant frequency of single bubbles may be reached during their ascent8. Both sites occur on ridges adjacent to valleys. The steepest ridge slopes are ~20 degrees and the ridge slope morphology has measured length-displacement ratios between 4 and 7, commonly associated with elliptical escarpments interpreted to mark the presence of faults and relays2,19.

Table 1. Location of the identified seep sites (WGS 84).

...

Both sites display flares in the water column directly above hummocky seafloor with relatively high backscatter values. Large escarpments have been associated with mud volcanoes in the BAC region (northern Guiana Basin) by Langseth et. al.7, who suggested that faults focus flow into migration pathways for ascending gas and fluids. The presence of flares in proximity with long linear ridges implies that these seafloor features are part of a deep plumbing system related to subjacent high pore fluid pressure, where the ridges are the surface termination of the faulting1,17,20

Bathymetric expressions of gas and fluid emissions are also present at these sites where flares are absent in the water column. These regions are interpreted as the result of gas emissions that caused sediment redistribution including mud volcanism, which is typically associated with gas plumes emitted from the seafloor14,15. Multiple craters observed at Site 1 may be satellite craters that formed on the flanks of a feeder channel18. At Site 2, both craters and mud volcanoes occur beneath flares in the water column (Figs. 4 and 5).

...

As bubbles rise they will tend to behave as Lagrangian drifters, making them useful as ocean current indicators16.  The calculated horizontal shear of the gas plumes is consistent with a current direction and velocity similar to that of North Atlantic Deep Water in this region.  Bubble ascent ~900 m through the water column is made possible through the formation of Natural Gas Hydrate (NGH) shells on gas bubble walls9,10

The tallest gas plume (located in Site 1) reaches from the seafloor up to ~600 m water depth. The observed top of the gas plume does not correspond exactly to the three horizontal hydrographic water masses in the water column that have been identified by correlating amplitude values in the multibeam water column data with temperature data from conductivity, temperature, depth casts. These layers are: 1) the shallow surface (mixed) layer above 100 m; 2) a middle layer ~100-250 m; and 3) a lower layer 250-2000 m.

Disappearance of the gas plumes at ~600 m depth is likely related to the dissolution of the NGH gas shell armoring and shards at the top of the Gas Hydrate Stability Zone (GHSZ) within the water column. Although it is not possible to quantify methane volume flux based on the available data the widespread occurrence of mud volcanoes across the BAC suggest that it is substantial11.

Figure 5. Along track water column data (view is perpendicular to the cruise track). Two regions where acoustic flares occur in the water column, Sites 1 and 2. The flares are interpreted as gas plumes that ascend up to ~900 m through the water column. In this transect a maximum (dilation) filter is used to return the highest values for a neighborhood surrounding a pixel to identify targets in the water column (4). In this image all of the data in the beam fan is visualized – all of the along track data is stacked to create the image. Therefore this image includes some distortion because it is not corrected for the beam angle in order to incorporate all of the information for all the beams. The amplitude values are the raw time series sample value present in the source sonar file, anomaly amplitudes are higher than the background water column values (Fig. 6).

...

Figure 9. For comparison and indication of project scale, the AT21-02 EM122 survey has been overlaid on a map of a section of the Gulf of Mexico (white box). It would cover most of Galveston Bay / Houston Ship Channel. Regional bathymetry grid exported from GeoMapApp.(13)


Figure 10. The survey area overlaid on regional map downloaded from GeoMapApp (13). The northeastern tip of Venezuela and the islands of Trinidad and Tobago are in the left corner, closest to the viewer. The survey area is centered on the screen, as indicated by the grey widgets. 

...

Quality Positioning Services B.V. | privacy statement | disclaimer | Terms and Conditions of Sale and Service | Powered by Atlassian Confluence / Adaptavist Builder