In this thesis seafloor cold vents are examined using autonomous underwater vehicle (AUV) and remotely operated vehicle (ROV) data on the Northern Cascadia margin. These data were collected in a 2009 joint cruise between the Monterey Bay Aquarium Research Institute (MBARI) and Natural Resources Canada (NRCan). High- resolution bathymetry data, acoustic reflectivity (backscatter) data, and 3.5 kHz sub bottom profiler data were examined for cold-vent-related features that include pockmarks, chemosynthetic biological communities (CBC), and authigenic carbonate. Additionally subsequent ROV observations, sediments from push cores and seafloor video/photos were used to ground truth AUV data. Numerous prolific venting sites were examined in detail and a model for the evolution of venting was generated. Vents are categorized as juvenile, intermediate, or mature depending on the presence and or absence of cold-vent-features. High near-surface reflection amplitudes are coincident with an anomalous area of seafloor backscatter. In June of 2012, NEPTUNE (North East Pacific Time-series Underwater Networked Experiment) collected a near-surface push core with their ROV ROPOS (Remotely Operated Platform for Ocean Sciences) in the high reflective area. The retrieved core showed stacked turbidites in the top 0.5 meters of the sediment column. Closely spaced high-velocity turbidite sands are highly reflective and inhibit acoustic penetration to depth. The presence of high-density, high-velocity sands in the near surface is linked to steady ocean bottom currents. These bottom currents progress northeast to southwest over the study area and differentially erode the surface sediments by removing muds and leaving heavy sands over the exposed area. / Graduate / 0373 / 0374 / jonfurlong@hotmail.com

This paper presents results of a seismic tomography experiment carried out at the Bullseye cold vent site offshore Vancouver Island. In the experiment, a seismic air gun survey was recorded on an array of five ocean bottom seismometers (OBS) deployed around the vent. The locations of the shots and the OBSs were determined to high accuracy by an inversion based on the shot travel times. A three-dimensional tomographic inversion was then carried out to determine the velocity structure around the vent, using the localized source and receiver positions. The inversion indicates a relatively uniform velocity field around and inside the vent. The velocities are close to the values expected for sediments containing no hydrate, which supports previous claims that the bulk concentrations of gas hydrates are low at the site. However, the largest resolved velocity anomalies of + 25 m/s are spatially within the limits of the acoustic blank zone seen in multichannel seismic data near the Bullseye vent. The velocity inversion is consistent with zones of high concentration (15-20 % of the pore space) in the top 50-100 m of sediment.

Using the NEPTUNE-Canada cable-linked ocean-floor observatory we plan continuous, real-time monitoring of the gas hydrate-associated, “Bullseye” cold vent offshore Vancouver Island. Our group inferred the presence of a massive gas hydrate deposit there, based on the significant resistivity anomaly in our controlled-source electromagnetic (CSEM) dataset, as well as anomalously heightened shear moduli, from seafloor compliance data. This interpretation was confirmed by drilling by IODP expedition 311 (site U1328), which indicated a 40 m thick gas hydrate layer near the surface. Sporadic venting and variations in blanking in yearly single-channel seismic surveys suggest the system is evolving in time. We are preparing two stationary semi-permanent imaging experiments: a CSEM and a seafloor compliance installation. These are designed not only to assess the extent of the gas hydrate deposit, but also for long-term monitoring of the gas hydrate/free gas system. The principle of the CSEM experiment is to input a particular electromagnetic signal at a transmitter (TX) dipole on the seafloor, and to record the phase and amplitude of the response at several seafloor receiver (RX) dipoles, at various TX-RX separations. The data are sensitive to the underlying resistivity, which is increased when conductive pore water is displaced by electrically-insulating gas hydrate. The experiment is controlled onshore, and can be expanded to include a downhole TX. Repeated soundings at this site, over several years, will allow measurement of minute changes in resistivity as a function of depth, and by inference, changes in gas hydrate or underlying free gas distribution. Similarly, the displacement of pore fluids by solid gas hydrate will affect elastic parameters. Thus, seafloor compliance data, the transfer function between pressure and seafloor displacement time series, most sensitive to shear modulus as a function of depth, will be gathered continuously to monitor the evolution of the gas hydrate distribution.

The study focuses mainly on two gas hydrate-related targets, located on the Northern Cascadia Margin, offshore Vancouver Island: (1) a recently identified 70-80-m high carbonate mound, Cucumber Ridge, located ~3.5-km west of Ocean Drilling Program (ODP) Site 889 and Integrated Ocean Drilling Program (IODP) Site U1327, and (2) a large cold vent, Bullseye vent, which is up to ~500 m in diameter and was drilled by IODP at Site U1328. The objective of this thesis is to analyze seismic data that provide indicators of locally focused fluid flow and characteristics of the gas hydrate occurrence associated with these two features. A grid of closely-spaced single channel seismic (SCS) data was collected at Cucumber Ridge in July/August 2001, and deep-towed multichannel seismic (MCS) lines were collected using Deep-towed Acoustics and Geophysics System (DTAGS) at the Bullseye vent area and at Cucumber Ridge in October 2002. The high-resolution SCS data, with a frequency bandpass of 40-150 Hz, recorded coherent reflectivity down to about 400 m beneath the seafloor, and provide excellent images of the subseafloor structure of Cucumber Ridge and of the gas hydrate bottom-simulating reflector (BSR) beneath it. Cucumber Ridge is interpreted to have developed as a structural topographic high in the hanging wall of a large reverse fault formed at the base of the current seaward slope. The fault zone provides pathways for fluids including gas to migrate to the seafloor where diagenetic carbonate forms and cements the near-surface sediments. Over the seismic grid, heat flow was derived from the depth of the BSR. A simple 2-D analytical correction for theoretical heat flow variations due to topography is applied to the data. Across the mound, most of the variability in heat flow is explained by topographic effects, including a local 6 mW/m2 negative anomaly over the central mound and a large 20 mW/m2 positive anomaly over the mound steep side slope. However, just south of the mound, there is a 6-7 mW/m2 positive anomaly in a 2-km-long band that has predominantly flat seafloor. Most of this anomaly is probably unrelated to topographic effects, but rather likely due to warm upward fluid flow along faults or fracture zones. Towed ~300 m above seafloor, the high frequency (220-1k Hz) DTAGS signal can provide high vertical resolution images with increased lateral resolution. The major problems of DTAGS are significant nonlinear variations of the source depths and receivers locations. New routines were developed for optimal DTAGS data processing, mainly including (1) cable geometry estimation by node depths, direct arrivals and seasurface reflections using a Genetic Algorithm inversion method, (2) acoustic image stitching based on accurate relative-source positioning by crosscorrelation of redundant data between two adjacent shots, and (3) velocity inversion of wide-angle traveltimes using a nonlinear global grid search method. The final processed DTAGS images resolve multiple seismic blanking zones and fine details of subseafloor features in the slope sediments. At Bullseye vent, where a 35-m-thick near-surface massive hydrate layer was drilled at U1328, the DTAGS data resolved the upper part of layer as a dipping diffraction zone, likely corresponding to a fracture zone. The inverted velocity structure in upper 100 m sediments successfully revealed a 17-m-thick layer of high velocity (~1650 m/s) just below seafloor, probably related to carbonate presence. A local high velocity zone, with a positive velocity anomaly of ~40-80 m/s in the upper 50 m beneath seafloor, was observed over the ~100-m wide region between U1328 and the deepest part of a seafloor depression; the high velocity zone is consistent with the dipping diffraction zone in the DTAGS image and with the massive hydrate drilled at U1328.