Thomas M. McGee

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.

Hydrocarbon gas hydrates in sediments of the Mississippi Canyon area, Northern Gulf of Mexico

Abstract The Gulf of Mexico Hydrates Research Consortium has begun installing a seafloor observatory to monitor gas hydrate outcrops and the hydrate stability zone in Mississippi Canyon Area Lease Block 118. Relevant background information concerning the Mississippi Canyon Area and gas hydrate occurrences in the northern Gulf of Mexico is presented. Microbial influences and possible scenarios of hydrate accumulation are considered. The design of the observatory was based on field data recorded in the Mississippi Canyon Area, principally lease block 118 (MC118) and the vicinity of lease block 798 (MC798). Swath bathymetry by autonomous underwater vehicle played a large part, as did seismic imaging within the hydrate stability zone and core sampling. These data and the results of their analyses are discussed in detail. Discussion and interim conclusions are presented.

A seafloor observatory to monitor gas hydrates in the Gulf of Mexico

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 450 m and often occur in mounds located where faults intersect the seafloor. Typically, these hydrates consist of seawater from which the salt has been excluded and gases that have migrated up faults from buried hydrocarbon reservoirs. In addition to hydrates, the mounds contain large amounts of calcium carbonate and various other minerals precipitated by microbes that extract energy from hydrocarbon fluids.

Pushing the Limits of High-Resolution in Marine Seismic Profiling

It has become commonplace to record high-resolution marine seismic profiles in digital format and to base the choice of sampling rate on the traditional assumption that seismic signals are of limited bandwidth. The truth is that seismic signals are causal and the Heisenberg uncertainty principle prohibits causal signals being bandlimited. Since only signals with finite bandwidth can be digitized without loss of information, the implication is that at least some resolution is always lost when seismic signals are digitized. A research program to improve seismic resolution, begun in 1987 at the University of Utrecht and now continuing at the University of Mississippi, has investigated ways to minimize that loss and to recover from seismograms as much geologic information as possible. This paper addresses theoretical considerations encountered by the program and presents experimental results which demonstrate the seismic resolution it is currently possible to achieve in shallow and deep water using commercial...

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.

Can Fractures in Soft Sediments Host Significant Quantities of Gas Hydrates

The Gulf of Mexico Hydrate Research Consortium has collected several types of data in and around Mississippi Canyon Lease Block 798 (MC798), an area of the northern Gulf of Mexico where fine-grained sediment occurs at the sea floor and where hydrates have been sampled. Swath bathymetry, heat-flow measurements, core samples, and subbottom profiles were collected. Hydrate was grown in the laboratory in sediments subsampled from the cores to demonstrate that the surficial sediments in MC798 are conducive to hydrate formation. Herein, data are presented and results discussed. It is postulated that significant quantities of hydrate could form in fine-grained sediments by filling fracture porosity produced by polygonal faulting. Analyses of cores combined with laboratory experiments indicate that conditions in MC798 are conducive to the formation of polygonal faults. Heat-flow measurements indicate that the hydrate stability zone is about 400 m (1312 ft) thick. Its upper 100 ms or so appears on two-dimensional (2-D) subbottom profiles to be fine grained. Small, near-vertical fractures indicated by features called brooms are common there. Thus, it is possible that a polygonal fault system exists in the upper 100 ms (75 m [246 ft] at 1500 m/s [4921 ft/s]). It is acknowledged that 2-D profiles cannot demonstrate this conclusively. Conclusive proof would require a three-dimensional (3-D) data set with sufficient resolution to demonstrate interconnectivity among the small faults. If polygonal faulting exists, gas and water could circulate through the fractures and be exposed to smectite-rich clays, a situation favorable to hydrate formation. X-ray images of pressure cores have documented hydrate accumulation within small, nearly vertical fractures in fine-grained sediments. Thus, it is possible that polygonal fault systems could host significant accumulations of hydrate in the Gulf of Mexico.

A Remote Station to Monitor Gas Hydrate Outcrops in the Gulf of Mexico

Abstract: Outcrops of gas hydrates will be monitored via a remote station installed on the continental slope of the Northern Gulf of Mexico. The project, driven by the need to initiate collection of a data base for assessing stability of the sea floor, will also address other factors associated with the formation and dissociation of gas hydrates. A group of experts has been organized to supervise the assembly and operation of the station, which will monitor physical and chemical parameters of sea water and sea floor sediments on a more or less continuous basis.

Geologic characteristics in the vicinity of the Deepwater Horizon oil spill

On April 20, 2010, the Deepwater Horizon offshore drilling rig experienced a catastrophic explosion and fire. At the time, it was drilling about 70km southeast of the mouth of the Mississippi River. A couple of days later it lay on the sea floor in more than 1500m of water. The drilling riser that extended from the sea floor to the water surface was destroyed and the blow-out preventer malfunctioned, causing a massive leak of crude oil and natural gas. Early attempts by British Petroleum (BP) and its drilling sub-contractors failed to cap the well. It continued to flow, at times intermittently, for more than 100 days. In total, it is estimated that at least five million barrels of oil were released into the Gulf of Mexico, undoubtedly the largest oil spill in U.S. history. Due to the proprietary nature of petroleum exploration and production, few details have been made public concerning the well from which the Deepwater Horizon spill originated. There is information about the geologic characteristics in the general vicinity of that well site, however. One source of information is the Gulf of Mexico Hydrate Research Consortium (GoMHRC) which has studied a hydrate/carbonate mound in Mississippi Canyon Block 118 (MC118) about 23 kilometers northwest of the spill site in preparation for installing a sea-floor observatory. It is managed by the University of Mississippi. Another source is a remarkable earthquake that occurred about 18 kilometers northeast of the spill site. Fig.1 shows the location of the spill site relative to that of the sea-floor observatory and the earthquake epicenter. Presented below is a discussion of what can be learned from these two sources.

A remote, multi-sensor station to monitor conditions near the sea floor within the hydrate stability zone

The possibility of designing and developing a remote, multi sensor, monitoring station for long term investigation and research of the near-sea-floor hydrocarbon system within the hydrate stability zone of the northern Gulf of Mexico has been discussed for some years. A program was initiated in 1999 to design and assemble a station which will monitor physical and chemical parameters of the sea water and sea-floor sediments on a more-or-less continuous basis over an extended period of time. It is planned that an operational station will be installed in at least a thousand meters of water by 2004. The heart of the station will be a net of vertical arrays of sensors. Each array will occupy the lower portion of the water column and a hole bored into the sea floor. Sensors will include hydrophones to record compression waves, thermistors to measure temperature within the water and the sediment, and optic fibers connected to an optical spectrometer to identify and quantify hydrocarbon gases. The lowermost sensor of each array will be a three-component accelerometer pushed into the sediments at the bottom of the hole to record compression and shear waves. Peripheral sensors will include a sea-floor positioning system and an acoustic doppler current profiler. Also, if sufficient electrical power is available, several video systems with pan and tilt capability will be installed. The video images would be posted on a web site in near-to-real time and made available for educational and public outreach purposes.

A single-channel seismic reflection method for quantifying lateral variations in BSR reflectivity

Abstract Results of seismic inversion techniques and logs of deep-sea bore holes indicate that bottom simulating reflectors (BSRs) which exhibit high reflection amplitudes are underlain by a thin layer of free gas. Often, however, BSRs exhibit relatively low amplitudes and display significant lateral variability. In these cases the structure is not well understood and remains a topic of research. Waveform inversion has been used to investigate the distribution of propagation speeds in the vicinity of BSRs, but the technique is not practical in some situations because it requires multi-channel data sets that include large offset distances between sources and receivers. Such data are not available in many instances, so it has become attractive to consider other methods of achieving the same end. A method that is applicable to single-channel, short-offset data is discussed here. It was originally developed to help characterize shallow submarine sediments for engineering and environmental purposes. Of course, no single-channel method can provide information concerning speeds of propagation such as is available from multi-channel methods. In this case the single-channel method has an advantage, however, in that it is self-calibrating. That allows it to provide, after correction for wave-front divergence, true reflection amplitudes without considering source characteristics or referencing to a known, or inferred, propagation parameter such as speed or density. These true amplitudes then yield reflection coefficients by correcting for transmission losses. Use of the method is illustrated with the help of synthetic data. It is demonstrated that the accuracy of results is improved by using a rapid digitizing rate during data acquisition. The method is then applied to a set of real data that previously had been analyzed by full-waveform inversion. The results are noisy, largely due to the data having been digitized at a rather slow rate and the length of recording being too short; however, average values of reflection coefficients at the sea floor and the BSR compare well with average values obtained by the inversion procedure. It is concluded that the single-channel method provides reasonable values for reflection coefficients. This suggests that, with judicious constraints on density variations, single-channel data could provide information on the structure of propagation speed in the vicinity of BSRs. Moreover, it would involve substantially less effort than is required for full-waveform inversion.