3-D seismic investigations of northern Cascadia marine gas hydrates
Notice bibliographique
Résumé
This dissertation presents results from 3-D (parallel 2-D) high resolution seismic surveys and associated studies over an area with deep sea gas hydrate occurrence. The study area is located on the accretionary prism of the northern Cascadia subduction zone offshore Vancouver Island, Canada. The major objectives of this study were the imaging of a gas/fluid vent field found in the study area and detailed mapping of the tectonic setting and geological controls on fluid/gas venting. Secondary objectives were the characterization of the gas hydrate occurrence and constraints on the seismic nature of the bottom-simulating reflector (BSR) and its spatial distribution. The main grid was 40 lines at 100 m spacing with eight perpendicular crossing lines of multichannel and single channel seismic reflection, and 3.5 kHz subbottom profiler data. In addition to the main 3-D seismic grid, two smaller single channel grids (25 m spacing) were collected over the vent field. The multichannel seismic data acquired with the Canadian Ocean Acoustic Measurement System (COAMS) streamer required correction for irregular towing depth and shot point spacing. A new array element localization (AEL) technique was developed to calculate receiver depth and offset. The individual receiver depths along the COAMS streamer varied between 10-40 m, which resulted in the occurrence of a prominent receiver ghost that could not be completely removed from the seismic data. The ghost resulted in limited vertical resolution and a coarse velocity depth function. The vent field is characterized by several blank zones that are related to near-surface deformation and faulting. These zones are 80-400 m wide and can be traced downward through the upper 100-200 m thick slope sediment section until they are lost in the accreted sediments that lack coherent layered reflectivity. The blank zones are also characterized by high amplitude rims that are concluded to result from the interference effect of diffractions. These diffractions result due to relatively sharp discontinuities in the sediment physical properties at the blank zone boundary. 2-D vertical incidence seismic modeling suggests an increase in P-wave velocity inside of the blank zone with only minor changes in density. Blanking is believed to be mainly the effect of increased hydrate formation within the fault planes. The faults are conduits for upward migrating fluids and methane gas that is converted into hydrate once it reaches the hydrate stability field. Carbonate formations at the seafloor can also contribute to blanking especially at higher frequencies. Free gas may be present in case of full hydrate saturation or strong fluid flow. Geochemical analyses of pore water and water-column samples carried out in cooperation with Scripps Institute of Oceanography indicate relatively low fluid fluxes of less than 1 mm/yr and there is no heat flow anomaly present over the vent field. Methane concentrations of 20 n-moles/L (about 8 times the ocean background concentration) were detected in water-column samples of the first 100-200 m above the main blank zone of the vent field. Venting is also believed to be strongly episodic with a recently more quiet time. However, the observed carbonate crusts indicate a long-term activity of the vents.
Récupéré en direct depuis OpenAlex et désinversé. Les résumés ne sont pas conservés dans cette base de données : les index inversés représentent 8,6 Go des 9,3 Go de texte de la base, et le serveur dispose de 13 Go libres.
Comment cette classification a été obtenuedéplier
Prédiction machine sur la base complète
Imitation des enseignantsNi prévalence calibrée, ni vérité terrain. Validation humaine à venir. Le volet Gemma est une étiquette directe du modèle pour chaque travail de la base, lue sur la notice réduite au titre. Le volet Codex est un classifieur appris des 10 348 étiquettes directes de Codex et calibré sur les taux pondérés de l'échantillon; les champs sans appui suffisant ne portent aucun appel Codex. Le mode candidate est l'union des deux volets; le consensus est leur intersection. Ces sorties portent le statut machine_predicted_unvalidated et ne sont pas des étiquettes humaines.
Scores du classifieur distillé par catégorie (deux têtes)
| Catégorie | Codex | Gemma |
|---|---|---|
| Métarecherche | 0,000 | 0,000 |
| Méta-épidémiologie (sens strict) | 0,000 | 0,000 |
| Méta-épidémiologie (sens large) | 0,000 | 0,000 |
| Bibliométrie | 0,001 | 0,001 |
| Études des sciences et des technologies | 0,000 | 0,000 |
| Communication savante | 0,000 | 0,000 |
| Science ouverte | 0,000 | 0,000 |
| Intégrité de la recherche | 0,000 | 0,000 |
| Charge utile insuffisante (le modèle a refusé de juger) | 0,001 | 0,000 |
Scores machine (provisoires)
Les deux têtes enseignantes du modèle étudiant, lues sur ce travail. Un score ordonne la base pour la relecture; il n'affirme jamais une catégorie, et le statut de validation accompagne chaque rangée tel quel.
Scores de référence d'un modèle non mature (critères de maturité non atteints, 7 itérations). Un score ordonne; il n'affirme jamais une catégorie.
score_only:v0-immature-baseline · tel quel depuis la passe de notation : score_only signifie que le nombre peut ordonner les travaux, et qu'aucune étiquette de catégorie n'en découleClassification
machine, non validéePrédiction automatique; un appel candidat d’une seule source (Gemma direct ou Codex distillé), pas un consensus.
Le détail, modèle par modèle et score par score, se trouve en fin de page sous « Comment cette classification a été obtenue ».