Ross Chapman

Geophysical and geochemical signatures associated with gas hydrate–related venting in the northern Cascadia margin

This paper presents a comprehensive, multidisciplinary study of cold vents associated with near-seafloor gas hydrate. Several cold vents characterized by seismic blank zones have been identified on the northern Cascadia margin near Ocean Drilling Program (ODP) Site 889/890. The most prominent vent site (Bullseye vent) has been the subject of intense geophysical and geochemical studies, including two- and three-dimensional (2D/3D) seismic imaging, heat flow measurements, piston coring with measurements of sediment physical properties and pore-fluid geochemistry, seafloor video observation, and sampling with the unmanned submersible ROPOS. The main seismically derived constraining observations are: (1) blanking increases with seismic frequency, (2) at low frequencies, layers can be traced through the zones, (3) blank zones widen with depth, (4) blank zones are underlain by a bottom simulating reflector (BSR), and (5) no velocity anomalies were detected across the vents. Constraints from piston core and thermal probe analyses are: (1) massive hydrate was recovered just below the seafloor at Bullseye vent, and (2) chemical alteration of sediments was observed by reduced magnetic susceptibility, increased thermal conductivity, and an elevated sulfate/methane interface. Additional constraints are: (1) no thermal anomaly was observed, (2) widespread carbonates and active chemosynthetic communities were found, and (3) elevated levels of methane were detected in the water column above Bullseye vent. We present a model for the seismic blanking at Bullseye vent that honors the constraints from all observations. The cold vents represent channels or networks of filamentous fractures containing hydrate and/or free gas. Free gas can be present within the hydrate stability field only in fractures, which may be coated with hydrate that prevents the inflow of water. The overall concentration of hydrate or gas within the vent must be small, because there was no observable velocity anomaly.

Thermogenic gas hydrates in the northern Cascadia margin

Gas hydrates are ice-like solids that form in rigid cage structures under specific conditions of pressure, temperature, and gas and water concentration. Marine gas hydrates are stable in pore spaces of sediments in water depths greater than ∼300 m beneath the slopes of active and passive continental margins [Kvenvolden, 1988]. The lower limit of hydrate occurrence in marine sediments is determined by the geothermal gradient, so that the zone of hydrate stability is generally contained within the first few hundred meters of sediment. Continental hydrates occur in polar permafrost regions in the Arctic and Siberia. Most of the hydrates that have been discovered contain methane derived from microbial processes. Other hydrocarbons can also form hydrates, but in different structures of the surrounding water cages. Structure I, the most prevalent form, contains mostly (>99%) microbial methane, a small amount of ethane, and traces of C2+ hydrocarbons [Sassen et al, 2001]. Structure II and structure H hydrates contain significant quantities of thermogenic methane and larger, more complex hydrocarbons formed at high temperatures from fossil organic matter (i.e.,kerogen) or oil [Sassen and MacDonald, 1994].The gas origin is inferred from measurements of the carbon-13 isotopic ratio (δ13); microbial methane is depleted in 13C (δ13 <−60‰) relative to thermogenic methane (δ13 from −20‰ to −50‰).

New seismic study of deep sea gas hydrates results in greatly improved resolution

A multichannel seismic survey has resulted in greatly improved resolution of structural details of deep sea gas hydrates off the west coast of Canada, revealing numerous geological features not before evident. The survey using the Naval Research Laboratory deep-towed acoustic/geophysics system (DTAGS), provided high-resolution images and layer velocities more than 10 times better than those obtained in the past using conventional systems. Vertical resolution within 2 m and horizontal resolution within 20 m were achieved. The work demonstrates that high-resolution seismic surveys with deep towed multichannel systems can provide important new information about pathways of fluid and gas migration that control the formation of gas hydrates. Conventional surface-towed seismic systems are unable to do this.

Estimates of geoacoustic model parameters from inversions of horizontal and vertical line array data

This paper describes results from geoacoustic inversion of low-frequency acoustic data recorded at a receiving array divided into two sections, a sparse bottom laid horizontal array (HLA) and a vertical array (VLA) deployed in shallow water. The data are from an experiment conducted by the Norwegian Defence Research Establishment (FFI) in the Barents Sea, using broadband explosives (shot) sources. A two-layer range-independent geoacoustic model, consistent with seismic profiles from the area, described the environment. Inversion for geoacoustic model parameters was carried out using a fast implementation of the hybrid adaptive simplex simulated annealing (ASSA) inversion algorithm, with replica fields computed by the ORCA normal mode code. Low-frequency (40-128 Hz) data from six shot sources at ranges 3-9 km from the array were considered. Estimates of sediment and substrate p-wave velocities and sediment thickness were found to be consistent between independent inversions of data from the two sections of the array

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.

Gas Hydrate Offshore Vancouver Island, Northern Cascadia Margin

This chapter reviews the extensive geophysical studies and Ocean Drilling Program (ODP) results that provide constraints on the occurrence, distribution, and concentration of deep-sea gas hydrate beneath the northern Cascadia margin offshore Vancouver Island. Most of this information comes from a wide range of seismic surveys and includes the mapping of the bottom-simulating reflector (BSR), as well as estimating gas-hydrate and free-gas concentrations. Recent additional constraints on the distribution and concentration of gas hydrate come from sea-floor-towed, controlled-source electromagnetic surveying and sea-floor compliance studies. These surveys and studies have been primarily deployed around a cold vent field, where seismic data show several broad blank zones, interpreted as fault-related conduits for focused fluid-gas migration, and where gas hydrate has been recovered in piston cores at the sea floor. Results from the ODP Leg 146 and the recently completed Integrated Ocean Drilling Program (IODP) Expedition 311 further constrain concentration estimates for gas hydrate and free gas in the sediments along the margin and also give insight into the complex formation mechanisms and controlling factors for gas hydrate occurrence in an accretionary complex. This summary was first presented in September 2004 at the AAPG Hedberg Research Conference on gas hydrates. Subsequently, 1 yr later, the drilling of IODP Expedition 311 resulted in a significant amount of new information and insight into the occurrence and formation processes of gas hydrate at the northern Cascadia margin. This chapter provides only a short summary of the results from that IODP Expedition. Reviews of the results from that drill coring and the downhole measurements are in progress.

Short range geoacoustic inversion with a vertical line array

During the SW06 (Shallow Water 06) experiments, short range geoacoustic inversion experiments were carried out at a source‐receiver range of 230 m to determine the sea bed properties at fine spatial scales. Acoustic signals were collected at 16 hydrophones on a vertical line array from source depths of 15‐65 m in 10 m intervals. For this experimental geometry, the angular coverage for reflectivity versus grazing angle is 7‐25°. This paper combines the information extracted from low‐frequency (100‐900 Hz) and mid‐frequency (1500‐4500 Hz) LFM signals transmitted from the same source position to estimate multi‐layered geoacoustic models. The model parameterization was based on the number of resolved sub‐bottom reflections in the data. An inversion approach using adaptive simplex differential evolution was applied to the data to invert for the sub‐bottom sound speeds and layer thicknesses. The temporal variation of the water column sound speed profile was included in the inversion. The estimated sound speeds ...

Benchmark workshop for geoacoustic inversion techniques in range dependent waveguides

This paper summarizes the results from the ONR/SPAWAR Geoacoustic Inversion Techniques (IT) Workshop held in May 2001. The format of the workshop was a blind test to estimate unknown geoacoustic profiles by inversion of synthetic acoustic field data for vertical and horizontal array geometries in range dependent shallow water waveguides. The fields were calculated using COUPLE/RAM for three range‐dependent test cases: a monotonic slope; a shelf break; and an intrusion in the sediment. Geoacoustic profiles were generated to simulate sand, silt and mud sediment environments. Different approaches for inverting the field data were presented at the workshop: model‐based methods based on normal modes, parabolic equation or ray theory; perturbative methods; methods using transmission loss (TL) data; methods using vertical or horizontal array data. The geoacoustic profiles inverted by the different methods are compared using a metric based on calculation of transmission loss using the estimated profiles for geome...

Matched mode geoacoustic inversion of broadband signals in shallow water

A matched mode geoacoustic inversion is presented for broadband acoustic signals recorded in experiments at a shallow water region of the New England Bight. First the modal behavior of the waveguide in the presence of 3D effects due to the water column and bottom bathymetry is examined. Spatial measurements of the temperature and salinity profiles and bathymetry provide data for calculating 3D propagation of the broadband acoustic wave field. Frequency shift of the broadband signal along one of the source-receiver paths in the experiment is attributed to internal waves propagating in the water column. Geoacoustic model parameters of the seabed along a source-receiver track with constant bathymetry are estimated by matched mode processing through matching the phase of the mode signals recorded at a Vertical Line Array. A warping transform was applied to identify and isolate modes, and the filtered modes were inverse transformed back to the time domain to reconstruct the mode signals. The inversion results are compared with the results from other inversion methods to assess the performance of the inversion scheme for estimating reliable values of the geoacoustic parameters in the experiment region.

Assessment of geoacoustic inversion methods

Sound transmission in shallow water is strongly affected by the physical and acoustic properties of the ocean bottom. Over the past decade, sophisticated methods have been developed for estimating parameters of geoacoustic models that account for the interaction of sound with the bottom. The performance of the methods has been compared in benchmarking exercises for range-independent and range-dependent shallow water environments using simulated data. This paper extends the comparison of geoacoustic inversion methods to assess performance using data from experiments at sites where the ocean bottom environment was well known from independent ground truth information. There are several aspects to performance assessment. The comparison presented here shows the accuracy of estimates from various inversion methods compared to the ground truth data about the ocean bottom sediments. The methods that are compared include matched field inversion; perturbation techniques based on modal waveumber estimation; bottom l...