Camelia C. Knapp

Application of the empirical mode decomposition and Hilbert-Huang transform to seismic reflection data

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 (EMD) which combines with the Hilbert transform as the Hilbert-Huang transform (HHT). 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: (1) assess the ability of the EMD and HHT to quantify meaningful geologic information in the time and time-frequency domains, and (2) 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.

Mantle earthquakes in the absence of subduction? Continental delamination in the Romanian Carpathians

The Vrancea seismogenic zone of Romania is a steeply NW-dipping volume (30 × 70 × 200 km) of intermediate-depth seismicity in the upper mantle beneath the bend zone of the Eastern Carpathians. It is widely held that the source of this seismicity is the remnant of a Miocene-age subduction zone. However, recent deep seismic-reflection data collected over the Eastern Carpathian bend zone image an orogen lacking (1) a crustal root and (2) dipping crustal-scale fabrics routinely imaged in modern and ancient subduction zones. Here, we use these data to evaluate the lithospheric structure of the Eastern Carpathians as it relates to the Vrancea seismogenic zone. Crustal architecture obtained from these data indicate the 140-km-wide orogen is only supported by ∼33-km-thick crust, while the adjacent Transylvanian and Focsani basins have ∼37- (possibly up to ∼46 km) and 42-km-thick crust, respectively. Because the Vrancea seismogenic zone is located beneath the east side of the thin orogenic crust, we infer that the lower orogenic crust was removed through continental delamination and is now represented by the mantle seismicity observed in the Vrancea seismogenic zone. These data and their interpretation suggest an alternate means of generating mantle seismicity in the absence of subduction processes.

Preserved extent of Jurassic flood basalt in the South Georgia Rift: A new interpretation of the J horizon

At the end of the Triassic, ∼200 m.y. ago, the Central Atlantic Magmatic Province (CAMP), one of the largest igneous provinces in the world, was emplaced within a very short period of time. The flows, sills, and dikes that mark the event are predominantly preserved in Triassic rift basins along the Atlantic margins. Conventional wisdom implies that the areally largest of the CAMP flows is preserved in the South Georgia Rift, a Triassic rift basin buried beneath the Atlantic Coastal Plain. The extent of this flow has been mapped on the basis of a prominent seismic reflection referred to as the J horizon. This seismic horizon has been used as a time marker for estimating the end of rifting in the southern United States and the beginning of seafloor spreading. Reanalysis of existing well and seismic data, however, shows that the extent of the flood basalt is limited to a few areas, and that the J horizon coincides with the base of the Coastal Plain. This reopens the question of how the CAMP relates to the rift-drift transition of eastern North America.

STRUCTURE OF A CARBONATE/HYDRATE MOUND IN THE NORTHERN GULF OF MEXICO

A one-kilometer-diameter carbonate/hydrate mound in Mississippi Canyon Block 118 has been chosen to be the site of a multi-sensor, multi-discipline sea-floor observatory. Several surveys have been carried out in preparation for installing the observatory. The resulting data set permits discussing the mound’s structure in some detail. Samples from the water column and intact hydrate outcrops show gas associated with the mound to be thermogenic. Lithologic and bio-geochemical studies have been done on sediment samples from gravity and box cores. Pore-fluid analyses carried out on these cores reveal that microbial sulfate reduction, anaerobic methane oxidation, and methanogenesis are important processes in the upper sediment. These microbial processes control the diffusive flux of methane into the overlying water column. The activity of microbes is also focused within patches near active vents. This is primarily dependent upon an active flux of hydrocarbon-rich fluids. The geochemical evidence suggests that the fluid flux waxes and wanes over time and that the microbial activity is sensitive to such change. Swath bathymetry by AUV combined with sea-floor video provides sub-meter resolution of features on the surface of the mound. Seismic reflection profiling with source-signature processing resolves layer thicknesses within the upper 200-300m of sediment to about a meter. Exploration-scale 3-D seismic imaging shows that a network of faults connects the mound to a salt diapir a few hundred meters below. Analyses of gases from fluid vents and hydrate outcrops imply that the faults act as migration conduits for hydrocarbons from a deep, hot reservoir. Source-signature-processed seismic traces provide normal-incidence reflection coefficients at 30,000 locations over the mound. Picking reflection horizons at each location allows a 3-D model of the mound’s interior to be constructed. This model provides a basis for understanding the movement of fluids within the mound.

A multiscale stratigraphic analysis of shallow unconsolidated sediments: A new approach for hydrogeophysical characterization in heterogeneous environments for contaminant remediation

Reconstruction of shallow stratigraphy of unconsolidated sediments in heterogeneous environments is a topic of primary interest in several environmental, hydrological, geotechnical, and engineering applications. The identification of porous layers and their assessment of their saturation, the characterization of sediments, the identification of bedrock and the analysis of shallow stratigraphy are some examples of topics of primary interest in near-surface applications, especially, in areas with near-surface contaminants. The purpose of this research is to integrate geophysical methods with geological and hydrological data at the P Reactor Area, Savannah River Site (SRS), South Carolina, in order to develop new approaches for hydrogeophysical characterization in heterogeneous environments.

Integrated Hydrogeophysical and Hydrogeologic Driven Parameter Upscaling for Dual-Domain Transport Modeling

Our research project is motivated by the observations that conventional characterization approaches capture only a fraction of heterogeneity affecting field-scale transport, and that conventional modeling approaches, which use this sparse data, typically do not successfully predict long term plume behavior with sufficient accuracy to guide remedial strategies. Our working hypotheses are that improved prediction of contaminant transport can be achieved using a dual-domain transport approach and field-scale characterization approaches.