Seismic velocity modelling, fixed point optimization, and evaluation of positioning uncertainty in the central Labrador Sea region; methods, a software tool, and an application

Online Access: Get full text
doi: 10.4095/295859
Author(s): Li, Q.; Shimeld, J.; Dickie, K.; Dehler, S. A.; Mosher, D.; Desroches, K.
Source: Open-File Report - Geological Survey of Canada, No.7665, 116p. Publisher: Geological Survey of Canada, Calgary, AB, Canada
Note: In English
Summary: Conversion between seismic two-way time (TWT) and sediment thickness is required to implement Article 76 of the United Nations Convention on the Law of the Sea. The deep water sedimentary succession of the central Labrador Sea is used to illustrate our approach to this problem. Multiple available sources of sediment seismic velocity information are assembled and analyzed with their cons and pros for this purpose, including scientific boreholes, seismic wide-angle reflection/refraction data, and proxy observations based on the normal moveout of seismic reflections. The latter exhibit a high degree of scatter and are subject to many caveats. Therefore we preprocessed the borehole and wide-angle reflection/refraction measurements from widely distributed locations across the region of interest to create a regional model of sediment velocity versus burial depth. The velocity model is constructed by numerical fitting of the observations with a slowness (inverse velocity) function that has strong theoretical and empirical linkages with the first-order porosity reduction behaviour documented for deep water successions around the world. The mathematical form of the model is attractive because it yields physically plausible velocities at depths beyond the range of observation, and because the model parameters are readily interpretable in terms of geologically significant physical properties. The fitting procedure of sediment velocity model accommodates measurement error in both velocity and depth by employing the reduced major axis (RMA) method. With RMA modeling, the bootstrapping method is used to estimate confidence bounds. For the example from the Labrador Sea, the bootstrapping results indicate an overall certainty of ±6.0% at the 95% level of confidence. An analytical function is derived that allows the model to be used for precise depth-to-time conversion. For time-to-depth conversion, the Newton-Raphson method is employed that provides a predefined accuracy, such as within ±1.0 cm with computing efficiency. Comparison of the velocity model with global results from deep sea drilling and also deep water marine shales of the Gulf of Mexico demonstrates a remarkable level of correspondence. In addition to providing support for the velocity model and its underlying methodology, the comparison provide strong evidence that porosity reduction due to compaction is the predominant factor controlling seismic velocity within the deep water marine successions. The purpose of invoking Article 76 is to define outermost fixed points along the margin. There are several criteria. One is the maximum of 2500 m bathymetry isoline plus 60 nm criterion; another is the sediment thickness formula which requires the sediment thickness to be greater than 1% of its distance to the nearest foot of continental slope (FOS). Implementation of sediment thickness criteria is significantly optimized in this work by integrating the interpreted seismic horizons (seafloor and top of basement), FOS points, and the conversion between TWT and sediment thickness using the constructed velocity model. Positioning uncertainty is unavoidable for current techniques in the identification of outmost fixed points. The sources of uncertainty include FOS identification, positioning of survey equipment, seismic data processing, horizon identification, and conversion between TWT and sediment thickness. These uncertainty sources are integrated into the net positioning uncertainty according to the methodology suggested by United Nations agencies. A software tool kit is provided for the construction of the velocity model, conversion between TWT and sediment thickness, optimization the identification of fixed point, and uncertainty evaluation. They are characterized flexibility as well as efficiency, such as one page web application and look up table enabling to be embedded them in a document, batch processing of all seismic profiles in one region, interactive graphic application. A user manual is also provided with giving step by step demonstration in this report.
Year of Publication: 2015
Research Program: DSDP Deep Sea Drilling Project
IODP Integrated Ocean Drilling Program
ODP Ocean Drilling Program
Key Words: 07 Marine Geology and Oceanography; 20 Geophysics, Applied; Atlantic Ocean; Computer programs; DSDP Site 112; DSDP Site 113; Data processing; Deep Sea Drilling Project; Elastic waves; Expedition 303; Expeditions 303/306; Fixed point optimization; Geophysical methods; IODP Site U1305; Integrated Ocean Drilling Program; Labrador Sea; Leg 105; Leg 12; North Atlantic; Northwest Atlantic; ODP Site 646; ODP Site 647; Ocean Drilling Program; Optimization; Positioning uncertainty; Seismic methods; Seismic waves; Traveltime; Two-way time; Uncertainty
Coordinates: N520000 N640000 W0440000 W0640000
Record ID: 2016082120
Copyright Information: GeoRef, Copyright 2019 American Geosciences Institute. Produced under license from Her Majesty the Queen in Right of Canada, represented by the Minister of Natural Resources