Sonar acoustic facies and sediment distribution on an area of the deep ocean floor

Long-range sidescan sonar can be used to map sediment distributions over wide expanses of deep ocean floor. Seven acoustic facies that arise from differing sediment or rock types have been mapped over the low-relief Saharan continental rise and Madeira abyssal plain. These have been calibrated with sampling, profiling and camera studies and the facies can be traced confidently on a regional scale using the sidescan data. The mapping of the sediment distribution shows that a complex interplay of turbidity current and debris flow processes can occur at a continental rise/abysaal plain transition over 1000 km from the nearest continental slope.     

The second order characteristic condition for plastic flow

In this paper the second order characteristic (discontinuous bifurcation) condition is derived for the granular flow (fully plastic) equations. This second order bifurcation equation is shown to be formally identical to the first order localization requirement during steady elastoplastic deformation provided the elastic compliance tensor is substituted for the product of the plastic multiplier with the flow Hessian. For isotropic yield and flow functions the invariant form of the characteristic condition is given in detail, as well as an alternative expression in adapted co‐ordinates. The characteristic condition can be regarded as defining a hardening function which is maximized to identify the critical angles. When the method is applied to 3D Coulomb flow, Mohr's 3D fracture plane conditions are obtained uniquely. Copyright © 2003 John Wiley & Sons, Ltd.     

Experiences with airborne and ground-based polarimetry

Polarimetry in the far-infrared and submillimeter has been valuable in tracing magnetic fields in bright Galactic clouds. We discuss the techniques we have developed and trends we have found in polarization vs column density and wavelength. The polarization spectrum has proven to be more interesting than had been anticipated. It is potentially valuable in explaining grain alignment and in characterizing dust species. One can expect the infrared cirrus to be a much simpler environment than the molecular clouds that have heretofore been explored at multiple wavelengths. Although cirrus observations must deal with low signals and wide extent, it is becoming possible to measure accurately the spectral energy distribution from mid-infrared to microwave frequencies and it should soon become feasible to determine the polarization spectrum over the same range.     

Preliminary attempt to characterize the rotation of seafloor in the Pito Deep area of the Easter Microplate using a submersible magnetometer

Several mechanisms have been proposed to account for the rotation of the nearly north-south abyssal hill fabric formed on the East Pacific Rise north of the Easter Microplate to the nearly east-west trends in the northern microplate interior. Proposed mechanisms include rigid microplate rotation, transform fault – parallel shear, and bookshelf faulting during the transfer of lithosphere from the Nazca Plate to the microplate. We used a submersible magnetometer on a NAUTILE dive program to measure the magnetic vector rotation of a pillow basalt and dike spur near Pito Deep, the present location of the tip of the propagating rift system that created the microplate. Our results, although too limited to draw strong conclusions from, suggest clockwise rotations of the seafloor magnetic vectors inconsistent with the transform-parallel shear model, and larger than can be explained solely by rigid microplate rotation.     

Nutrient attenuation in a shallow,gravel-bed river. II. Spatial and temporal changes in nitrogen dynamics and community metabolism

ABSTRACT

Instream processes alter the concentration and bioavailability of nutrients as they are transported downstream. By relating primary production and periphyton composition to changes in nutrient concentration in a gravel-bed river this study made inferences about recycling and attenuation. Where dissolved inorganic nitrogen (DIN) was abundant, concentrations decreased linearly with distance but by less than required to meet the nitrogen demand of primary production. Where DIN was barely measurable photosynthesis was reduced but only by 50%. We infer that recycling sustained primary production even when DIN concentrations were negligibly small. One implication is that DIN removal underestimates attenuation. Further experimental research on recycling and improved modelling is required to better quantify the length of streams adversely affected by nutrients.     

Planetary reflectance measurements in the region of planetary thermal emission

As planetary reflectance measurements are extended into the infrared, the emitted thermal radiation becomes larger than the reflected solar component. This paper describes a method for removal of the thermal component from planetary reflectance measurements and discusses the limitations involved. Examples are shown for the case of lunar observations where the temperature and emissivity are known and for Mercury where it is assumed the temperature and emissivity are unknown.     

Deep CCD photometry of stars in the globular cluster M3

Preliminary results on new CCD photometry inV andB bands of approximately 5000 stars in four overlapping fields in the globular cluster M3 are presented. The colour-magnitude diagram shows a very well-defined sequence with a small scatter, going fainter than previously published work on this cluster. The turn-off point found atV=19.1±0.2 mag and (B-V)=0.445±0.01, is slightly redder than those obtained in previous studies.Paper presented at the 11th European Regional Astronomical Meetings of the IAU on New Windows to the Universe, held 3–8 July, 1989, Tenerife, Canary Islands, Spain.