The objective of this project is to develop and test a bottom-mounted seismic source for mapping gas hydrates in marine environments. The Naval Research Laboratory (NRL) will modify their existing Helmholtz resonator seismic source, which generates both compressional and shear waves, and develop a system for mounting it onto the seafloor. The resulting bottom-mounted configuration will be tested in the Mississippi Canyon 118 area of the Gulf of Mexico.

Geology and Geophysics Section, Naval Research Laboratory, Stennis Space Center, MS, 39529

There is general agreement in the hydrates research community that the next step for using seismology to help determine the location and concentration of gas hydrates in the marine environment is to use both compressional (P) waves and shear (S) waves. However, in deep water (where hydrates are likely to be found) it has been essentially impossible to directly generate shear waves, as this would require a seismic source mounted on or within the seafloor. At the water depths of interest, it is extremely difficult to generate seismic signals in a frequency band applicable for studying hydrates, which are generally located within the upper 700 meters of sediments. We have previously proposed to overcome the technical problems by adapting a seismic source developed by the Navy to a bottom-mounted configuration. Thus, we will use this seismic source, which has been proven to produce repeatable seismic source signals at frequencies appropriate for hydrates studies, to directly generate both P and S waves.

The value of shear waves is two-fold. First, the combination of P- and S- waves provides significant constraints on estimates of the physical properties of the sediments and sediment-hydrates that are sampled. Because the magnitude of change in shear modulus is greater than the potential change in compressive modulus for hydrated sediments, changes in S- wave velocity/amplitude will provide important constraints in interpretation. Second, shear waves provide an important test on interpretations of gas content within sediments. This occurs because shear waves are relatively insensitive to gas. Therefore, a reflection horizon interpreted to be the Bottom Simulating Reflector (BSR) using P-wave data could be tested using S-wave data. Similarly, seismic “blanking” zones, which are often attributed to gas pockets, can be confirmed using S-wave observations.

On March 25, 2010, NRL scientists embarked from Gulfport, MS aboard the R/V Cape Hatteras. The primary purpose of the cruise was to use DTAGS to shoot vertical hydrophone arrays to study the effects of geologic faulting on the efficiency of acoustic wave propagation. In stiff, well-consolidated sediments, sound waves traveling across faults tend to propagate slower and with lower amplitude than waves traveling along the faults. In soft, deep-water sediments, the magnitude of this effect is not known. To measure the amount of this effect (sediment anisotropy) the researchers introduced a sound and then listened with vertical arrays of hydrophones. The experiments were conducted in an area of the Gulf of Mexico where the faults have been extensively mapped and the water averages 800—900 m deep. This area, known as Mississippi Canyon lease block 118, is well-known for the occurrence of methane hydrate and is the location of the University of Mississippi’s gas hydrate observatory. A better understanding of the effects of anisotrophy on acoustic signals could lead to improved methods of locating and accessing methane hydrates in deep water sediments. In addition to the bottom-mounted DTAGS deployment in March 2010, other accomplishments include:

• Dockside, or shore tests, of the system took place in early February 2010 and ensured that the basic system was operational before subjecting the unit to greater depths.
• Design and fabrication to refit NRL’s existing resonator to ensure that it effectively couples with the sea floor was completed in January 2010.
• Several numerical simulations were executed and showed that shear waves generated by the Deep Towed Acoustics Geophysics System (DTAGS) in shallow water can be detected with geophones positioned on land. Land-based geophones, with no need for autonomous or even water-proof recording systems are far more readily available than the ocean bottom seismometer, or ocean bottom cables required for work at MC118. This greatly facilitates low-cost testing of the system before the expense of a ship is incurred.
• The prototype (small) Helmholz resonator was modified and tested in shallow water in the Gulf of Mexico.

(November 2011) 
The Naval Research Lab’s DTAGS deployment task is now complete and a summary report on the DTAGS deployment, entitled "Design and Deployment of a Deep-water Seafloor Sound Source", is available below under “Additional Information”. Due to FY2011 funding constraints and uncertainty regarding FY2012 budgets no additional tasks with NRL have been funded.

$150,000

$46,000

NETL - Robert Vagnetti (Robert.Vagnetti@netl.doe.gov or 304-285-1334)
Naval Research Lab – Warren Wood (warren.wood@nrlssc.navy.mil or 228-688-5311)

In addition to the information provided here, a full listing of project related publications and presentations as well as a listing of funded students can be found in the Methane Hydrate Program Bibliography [PDF].

Design and Deployment of a Deep-water Seafloor Sound Source [PDF-967KB] - May, 2011

2008 Hydrate Peer Review [PDF-2.51MB]