Advancements in signal processing may allow for improved imaging and analysis of complex geologic targets found in seismic reflection data. A recent contribution to signal processing is the empirical mode decomposition ͑EMD͒ which combines with the Hilbert transform as the HilbertHuang transform ͑HHT͒. The EMD empirically reduces a time series to several subsignals, each of which is input to the same time-frequency environment via the Hilbert transform. The HHT allows for signals describing stochastic or astochastic processes to be analyzed using instantaneous attributes in the time-frequency domain. The HHT is applied herein to seismic reflection data to: ͑1͒ assess the ability of the EMD and HHT to quantify meaningful geologic information in the time and time-frequency domains, and ͑2͒ use instantaneous attributes to develop superior filters for improving the signal-to-noise ratio. The objective of this work is to determine whether the HHT allows for empirically-derived characteristics to be used in filter design and application, resulting in better filter performance and enhanced signal-to-noise ratio. Two data sets are used to show successful application of the EMD and HHT to seismic reflection data processing. Nonlinear cable strum is removed from one data set while the other is used to show how the HHT compares to and outperforms Fourier-based processing under certain conditions.

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Background Gas hydrates are solid structures composed of gas molecules encased in cages of water molecules. Most interesting to the energy community are hydrates that contain hydrocarbon gases. In the northern Gulf of Mexico, hydrates of this type form in water depths greater than 450m and often occur in mounds located where faults intersect the sea floor. Typically, these hydrates consist of sea water from which the salt has been excluded and gases that have migrated up faults from buried hydrocarbon reservoirs. In addition to the hydrates, the mounds contain large amounts of calcium carbonate and various other minerals precipitated by microbes that extract energy from the hydrocarbon fluids. It has been observed that microbial activity is an order of magnitude greater in the vicinity of mounds containing hydrate outcrops than elsewhere on the sea floor. The proliferation of microbes around gas hydrate sites is not coincidental; it is the result of a synergistic relationship between hydrocarbon gas hydrates and microbes, i.e. the carbonrich gases within hydrates provide sustenance for the microbes and biosurfactants produced by the microbes enhance the formation of hydrates. Recent laboratory experiments have shown that small concentrations of biosurfactants in the water wetting porous mineral surfaces have the effect of increasing the formation rates and decreasing the induction times of hydrates substantially. Moreover, biosurfactants exhibit surface specificities for particular mineral surfaces; that is, biosurfactant adsorption on a specific mineral surface results in hydrates nucleating and emanating from that surface. Surfaces of smectite clay, common in the northern Gulf, serve this purpose particularly well. Hydrates outcropping at the sea floor are in direct contact with large volumes of sea water. They are stable only marginally because salt, in sufficient concentration, impedes hydrate formation. For this reason hydrate outcrops can be ephemeral; significant changes may occur within rather short periods of time. In the absence of pressure fluctuations, whether the outcrops accumulate or dissociate is determined by variations in water temperature and rate of gas flow. Water temperature in the northern Gulf is influenced greatly by warm eddies shed from the Loop Current which enters the Gulf between the Yucatan Peninsula and Cuba (fig.1). These eddies drift slowly westward along the continental slope and can raise bottom-water temperatures by several degrees Celsius. Observations (Roberts et al., 1998) confirm that gas activity correlates with water temperature; gas flow increasing a few hours after the temperature increases (fig.2). Where hydrates occur within sediments, they are stable under the proper conditions of pressure, temperature, gas composition and salinity of pore fluid. If gas migrating up a fault encounters appropriate conditions as well as sediments of sufficient permeability, hydrates can form within the pore spaces and act to cement the sediment grains. This increases the shear modulus...

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