DE-AI26-06NT42878 - Bottom Source Task

Goal
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 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.

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

Background
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 a source that has been proven to produce repeatable seismic source signals at frequencies appropriate for hydrates studies to a configuration that will directly generate both P and S waves.

Potential Impact
The value of shear waves is multi-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. As the magnitude of change in shear modulus is greater than 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 with sediments. This arises because shear waves are relatively insensitive to gas. Therefore, a reflection horizon interpreted to be the 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.

Accomplishments

1. The prototype (small) Helmholz resonator was modified and tested in shallow water in the Gulf of Mexico.
2. Efforts have been made to collaborate with Bob Hardage and Paul Murray at the University of Texas who also require three component geophones in the MC118 area to complete their work. It may be possible to “piggy-back” the DTAGS on their cruise, which may occur in late fall of 2008, or early spring of 2009. This collaboration would allow a direct comparison between the surface-towed source proposed by University of Texas and the bottom-mounted source proposed by NRL, using the same bottom-mounted geophones.
3. Several numerical simulations have been run showing that shear waves generated by DTAGS in shallow water can be detected with geophones positioned on land. Land-based geophones, with no need for autonomous or even water proofed 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.

Current Status
After consultation with the DOE program manager, the cruise planned for September of 2008 was canceled due to the delayed installation of three component geophones at MC 118 (originally scheduled for spring of 2008). The contingency plan, to use existing NRL geophones, was not possible due to unexpected delays in refurbishing the phones and manufacturing pressure cases for use at MC118. The funds for this cruise will be carried over at no cost to DOE, but significant costs are associated with postponing this cruise. We (NRL) are still negotiating these costs, but they could be as high as $112,000.

Project Start Date: June 1, 2006
Project End Date: December 31, 2008

DOE Contributions: $150,000
NRL Contribution: $46,000

Contact Information:
NETL – Traci Rodosta (traci.rodosta@netl.doe.gov or 304-285-1345)
Naval Research Lab – Warren Wood (warren.wood@nrlssc.navy.mil or 228-688-5311)

Addditional Information:
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].

2008 Hydrate Peer Review [PDF-2.51MB]