Two‐dimensional numerical models of steady state convection show that convection cells of aspect ratio as large as 8.6 are possible for variable viscosity convection in the upper mantle. Our models include the effects of variable viscosity, viscous dissipation, internal heating, heat flow through the bottom, and the adiabatic gradient. The large aspect ratio of the convection cells is primarily due to the large viscosity contrast between the lithosphere and the asthenosphere. It appears possible for multiple convection cells to occur in a low‐viscosity zone while the surface velocities give the appearance of a single cell. The details of the viscosity law relevant to mantle materials and conditions are presently uncertain but are of crucial importance; temperature, viscosity, and flow patterns are inextricably entwined. Convection decreases the overall temperature gradient; consequently, generally accepted temperatures for most of the mantle are too high. The controversies over plate‐mantle decoupling and passive versus active plates are probably due to oversimplifications that disregard hydrodynamic concepts.
For many underwater observation and construction activities the accurate measurement of pressure is a key requirement.The pressure measurement may be used as primary observation data such as in Tsunami detection, wave and tide gauges, and platform leveling applications, o r the pressure measurements may be used as associated observation data such as in depth sensors for ROV's, profiling instruments, and towed arrays.In all these applications pressure transducers employing quartz resonator technology have been successfully used in underwater systems that required the highest resolution, accuracy, and stability. This paper describes the construction, operation, and performance of the quartz resonator technology with specific examples of underwater applications. In addition, advances in materials and electronics promise to extend the usefulness of these devices within virtual instrumentation arrays aimed a t synoptic observations.
Deep sounding seismic reflection data show undeformed reflectors at depths down to 11 kilometers beneath the continental rise and abyssal plain and 7 kilometers in basins of the lower slope. Weak reflectors are visible beneath the salt of the Sigsbee Scarp and within salt ridges separating the lower slope basins.
A reliable downhole sensor network will dramatically improve reservoir management practices and enable the construction of "intelligent" downhole well completion and control systems. Fiber optic technology will play a seminal role in the architecture of downhole imaging and control systems because of advantages of power, performance and reliability over conventional electronics.Results from a field test of a fiber optic seismic borehole receiver prototype demonstrate that a multi-level, fiber optic hydrophone system can improve the economics of Vertical Seismic Profiling (VSP) & cross-well surveys, and quicken the birth of permanent monitoring networks.
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