Fledermaus Training August 17th - 19th, 2015
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About the Department of Earth and Atmospheric Sciences at the University of Houston
The Department of Earth and Atmospheric Sciences at the University of Houston (UH) is the largest geoscience program in the U.S. with ~260 MSc and PhD students and ~350 undergraduates. UH is the only public Carnegie Tier 1 research university within the greater Houston region, and the only university with a geoscience program that has two ranked NRC PhD programs (geology and geophysics) in Texas. UH is ranked #1 in the nation among Carnegie Tier 1 research universities for student diversity and there is a strong initiative at UH for inclusion of under-represented graduate and undergraduate students in scientific research projects and work-study laboratory programs.
UH scientists have led, and participated in, research cruises to numerous diverse locations ranging from the Parece Vela Basin in the Philippine Sea to Shatsky Rise in the western Pacific and Antarctica. This diverse range of marine activities was expanded in 2014 by the addition and success of UH's marine leg of the existing Geophysics Field Camp organized by Profs. Robert Stewart, Will Sager, and Shuhab Khan. The UH field camp is the only geophysics camp in the U.S. that trains undergraduate students in the acquisition and processing of both onshore and offshore data.
The Department has a wide range of research programs central to the earth sciences. These include sedimentology, carbonate petrology, sequence stratigraphy, micropaleontology, structural geology, tectonics, geodynamics, marine geology, petroleum systems and geochemistry, inorganic geochemistry, isotope geochemistry, igneous petrology, thermochronology, GIS, remote sensing, seismology, applied geophysics, applied rock physics, whole earth geophysics, potential fields, hydrology, atmospheric sciences, climate change, and air pollution sciences. Researchers at UH have been QPS Fledermaus clients since 2008 and have used Fledermaus on numerous projects, including the one detailed here. According to Alex Barnard, recent UH PhD recipient and lead on this project:
The QPS product suite is a giant leap forward for the marine community and has resulted in grand images of the subsea regions of Earth. 4D geo-spatial scenes created in the QPS suite have generated a buzz in multiple arenas ranging from scientific meeting sessions, to scientific literature and industry deliverables and even news reports. The product suite has benefited all of us with the unprecedented insights they provide, in both the time and space dimensions, into the submarine environment and have opened up many new and exciting possibilities. Together with the ongoing resolution revolution, the QPS suite user base continue to produce ever more intriguing images that not only capture our attention but will also propel generations to come.
Subsea Gas Emissions from the Barbados Accretionary Complex
Alex Barnard (TerraSond), Will W. Sager (UH), Jonathan E. Snow (UH) and Michael D. Max MD (Hydrate Energy International) conducted this multidisciplinary project that integrates bathymetry, backscatter, and water column acoustic data using the QPS product suite to provide water column features with a geological context. The study area was located ~100 km southeast of Barbados along the Barbados Ridge and is part of the Barbados Accretionary Complex (BAC).
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.
Multibeam Echo Soundings Acquisition and Processing Methods - Atlantis AT21-02
Multibeam echo sounder data were collected as part of the underway geophysical data on the cruise Atlantis AT21-02 (Fig.1). A Kongsberg-Simrad EM122 multibeam sonar system was used for mapping. Raw multibeam data from the cruise AT21-02 are available from NOAA (http://www.ngdc.noaa.gov/mgg/bathymetry/multibeam.html). A NW-SE trending cruise line is used in this work that includes data not being studied by other groups.
Figure 2. The research vessel Atlantis. (Woods Hole Oceanographic institution)
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.
Figure 3. QPS multibeam data processing workflow including the modules used. The DMagic module processes multibeam data into multi-resolution bathymetric surfaces, FM Geocoder Toolbox (FMGT) creates qualitative and quantitative acoustic backscatter mosaics, FMMidwater allows the visualization and analysis of water column data, and Fledermaus integrates data from all the modules and allows 4D visualization.
Figure 4. Base map: Sites 1 and 2 imaged by draping transparent multibeam bathymetry and hillshade layers over backscatter imagery in QPS Fledermaus. Insets: regions of relatively high backscatter that cover the seafloor at sites 1 and 2.
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 density8. 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., or an increase in reflectivity as the resonant frequency of single bubbles may be reached during their ascent
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).
The absence of the gas plumes above some craters can be explained by the site being inactive, at least during the survey, because emissions were not occurring during data acquisition. Alternatively, emission rates at the time of survey could have been below the acoustic detection limits and rapidly dissolved. Sites with no water column anomalies are likely to be at least temporarily inactive, although the recurrence interval of the emissions is not known. It is unlikely that gas emissions below the acoustic detection limits could produce the observed seafloor craters and mud volcanism.
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 6. Site 1 beam fan with time series amplitude versus range (m) for a nearly vertical beam (~20 off nadir). Note that the flare is characterized by positive amplitudes higher than background water column values. The location of Site 1 is given in Figures 1 and 4.
Figure 7. Integrated Fledermaus scene of Site 1 containing multibeam-bathymetry-shaded backscatter data with extracted water column data visualized as points. Both sites occur along north-south trending long-linear ridges indicating that the plumes are associated with a deep plumbing system. White arrows mark cratered regions with high backscatter. The shear on the plume shows that the current direction is towards 128 degrees (from the NW), and this view is orthogonal at 218 degrees resulting in a view of the maximum shear from the water column with a shear angle of ~21 degrees (white line). The flare is ~900 m tall, and is ~350 m offset from bottom to its top. The highest elevation in the foreground is -1470 m.
Figure 8. The 3D view clearly shows the current direction towards 128 degrees (SE) and the ~21 degrees offset on the plume from base to top. The top color bar corresponds to the bathymetric surface, while the bottom color bar indicates the sonar intensity return values for the plume (visualized as a point cloud). The white lines and numbers on the surface are contours and depth labels, respectively.
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.
The QPS Fledermaus suite of software provided processing, visualization, and analysis tools that were vital to this project. According to Jim Nash, Executive Vice President and Chief Operating Officer for TerraSond Ltd.:
Derisking uncertainty encountered when using geophysical tools to study the seafloor and subsurface is an important part of offshore oil and gas exploration. Multibeam system deliverables such as acoustic backscatter, bathymetry and water column data along with seafloor geochemical and heat flow measurements are used in frontier basin exploration to help focus gravity cores onto targets to provide direct evidence of the presence of effective and generating petroleum systems. The use of ground truth data is particularly important in frontier regions that include prospects with sparse 2D and little to no 3D seismic imaging data as critical decisions should not be based on interpretative methods. The added value of these methods and datasets described is relatively low cost, and it’s the holistic understanding of these integrated datasets that assists in reducing uncertainty, and therefore reduced risk in exploration.
Thanks to the crew and scientists of the R/V Atlantis during AT21-02, particularly Cindy Van Dover. Thanks to Bill Chadwick, Susan Merle, Daniel Price and Evan Robertson from NOAA, Erin Heffron, Lindsay Gee, and the staff at QPS, and Bramley Murton (NOCS). A special thank you to Dan McGinnis for providing us with a copy of the single bubble dissolution model SibuGUI. Thanks to Jingqiu Huang for discussion of acoustic imaging and Julia Wellner for organizing a full semester of gas-hydrate related talks at UH. We would also like to thank John Casey and an anonymous reviewer for their support and constructive comments that significantly improved this project.
This Client Spotlight is a short summary of research that was funded by the University of Houston, Department of Earth and Atmospheric Sciences Teaching Assistantship.
Seafloor regions of the study area were analyzed in detail using a 4D mash up of all the multibeam deliverables (bathymetry, backscatter, water column) in Fledermaus. The screen captures below document the relationships encountered between the seafloor morphology and the plumes identified in the water column. Also shown are the depositional lobes of a giant mud volcano that were highlighted using the Fledermaus slope map function.
Related References And Links
3 Deville, É., Mascle, A., Guerlais, S.-H., Decalf, C., Colletta, B., 2003. Lateral changes of frontal accretion and mud volcanism processes in the Barbados accretionary prism and some implications. AAPG Memoir 79, 656-674.
4 Doucet, M., Ware, C., Arsenault, R., Weber, T., Malik, M.A., Mayer, L., Gee, L., 2009. Advanced mid-water tools for 4D marine data fusion and analysis. In proceedings of OCEANS 2009, MTS/IEEE Biloxi-Marine Technology for Our Future: Global and Local Challenges, IEEE, pp. 1-9.
9 Max, M.D. (Ed), 2003. Natural gas hydrate in oceanic and permafrost environments (Second Edition). Kluwer Academic Pub.
10 Max, M.D., Johnson, A.H., Dillon, W.P., 2006. Economic geology of natural gas hydrate. Kluwer Academic Pub.
11 Murton, B. J., Biggs, J., 2003. Numerical modelling of mud volcanoes and their flows using constraints from the Gulf of Cadiz. Marine Geology 195(1), 223-236.
12 Pichot, T., Patriat, M., Westbrook, G., Nalpas, T., Gutscher, M.-A., Roest, W., Deville, É., Moulin, M., Aslanian, D., Rabineau, M., 2012. The Cenozoic tectonostratigraphic evolution of the Barracuda Ridge and Tiburon Rise, at the western end of the North America-South America plate boundary zone. Marine Geology 303, 154-171.
13 Ryan, W.B.F., S.M. Carbotte, J.O. Coplan, S. O'Hara, A. Melkonian, R. Arko, R.A. Weissel, V. Ferrini, A. Goodwillie, F. Nitsche, J. Bonczkowski, and R. Zemsky (2009), Global Multi-Resolution Topography synthesis, Geochem. Geophys. Geosyst., 10, Q03014.
16Schneider, J., Greinert, J., Chapman, N., Rabbel, W., Linke, P., 2010. Acoustic imaging of natural gas seepage in the North Sea: sensing bubbles under control of variable currents. Limnol. Oceanogr. Methods 8, 155e171.
17 Talukder, A.R., 2012. Review of submarine cold seep plumbing systems: leakage to seepage and venting. Terra Nova 24, 255-272.
Tinivella, U., Giustiniani, M., 2012. An overview of mud volcanoes associated to gas hydrate system. In: Nemeth, K. (Ed.), Updates in Volcanology - New Advances in Understanding Volcanic Systems. InTech, Rijeka, pp. 225-267.18
Westbrook, G., Smith, M., 1983. Long decollements and mud volcanoes: evidence from the Barbados Ridge Complex for the role of high pore-fluid pressure in the development of an accretionary complex. Geology 11, 279-283.
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