Making the connection between Geosat and TOPEX/Poseidon

We can presently construct two independent time series of sea level, each at a precision of a few centimeters, from Geosat (1985–1988) and TOPEX/Poseidon (1992–1995) collinear altimetry. Both are based on precise satellite orbits computed using a common geopotential model, JGM-2 (Nerem et al. 1994). We have attempted to connect these series using Geosat-T/P crossover differences in order to assess long-term ocean changes between these missions. Unfortunately, the observed result are large-scale sea level differences which appear to be due to a combination of geodetic and geopotential error sources. The most significant geodetic component seems to be a coordinate system bias for Geosat sea level (relative to T/P) of −7 to −9 cm in the y-axis (towards the Eastern Pacific). The Geosat-T/P sea height differences at crossovers (with JGM-2 orbits) probably also contain stationary geopotential-orbit error of about the same magnitude which also distort any oceanographic interpretation of the observed changes. We also found JGM-3 Geosat orbits have not resolved the datum errors evident from the JGM-2 Geosat -T/P results. We conclude that the direct altimetric approach to accurate determination of sea level change using Geosat and T/P data still depends on further improvement in the Geosat orbits, including definition of the geocenter. Received: 11 March 1996; Accepted: 19 September 1996     

Structure and age of the Cerro de Pasco Cu-Zn-Pb-Ag deposit,Peru

The world-famous Cu-Zn-Pb-Ag deposit at Cerro de Pasco, Peru, consists of texturally massive pyrite, texturally massive sphalerite-galena-pyrite, and veins containing pyrite and enargite. Historically the deposit has been considered to be the hydrothermal product of the adjacent Miocene volcanic and intrusive complex (locally known as the Vent). However, both the texturally massive sulfides of the deposit and the pre-Miocene strata are cut by the Longitudinal fault, one of the largest faults in the district, but the Vent is not. Imbrication by the Longitudinal fault zone (duplex structures) has thickened the deposit so that it is amenable to open-pit mining. Dikes and pyrite-enargite veins pass from the Vent into the massive sulfides; fragments of massive pyrite occur in the Vent. Thus, no matter what their origin, the texturally massive sulfides are older and, therefore, genetically unrelated to the Vent.     

A new method to describe seagrass habitat sampled during fisheries-independent monitoring

We explain a new method of quantifying seagrass cover and describing seagrass species composition during fisheries-independent monitoring. This new method is similar to a point-intercept method developed to estimate arboreal crown cover, but it uses an aquascope designed for shallow water. The method does not require a diver. Seagrass cover (cover ratio) distinguished different percentage cover categories in 0.25-m² seagrass plots. Estimates of species composition determined by using the new method were most similar to those obtained by using estimates of aboveground biomass. Within each 141-m² area sampled with a 21.3-m fish seine, we accurately estimated seagrass cover ratio and species composition with six observations that typically required less than 6 total minutes. Within such areas, 42 trials were conducted to evaluate the precision with which different observers estimated seagrass cover ratio and species composition. In 98% of the trials, observers attained statistically similar estimates of cover ratio, and in 100% of the trials in areas with multiple seagrass species, observers attained statistically similar estimates of species composition. We conclude that the new method provided efficient and reasonably accurate means to quantify seagrass cover and species composition.     

Applications of satellite altimetry to oceanography and geophysics

Satellite-borne altimeters have had a profound impact on geodesy, geophysics, and physical oceanography. To first order approximation, profiles of sea surface height are equivalent to the geoid and are highly correlated with seafloor topography for wavelengths less than 1000 km. Using all available Geos-3 and Seasat altimeter data, mean sea surfaces and geoid gradient maps have been computed for the Bering Sea and the South Pacific. When enhanced using hill-shading techniques, these images reveal in graphic detail the surface expression of seamounts, ridges, trenches, and fracture zones. Such maps are invaluable in oceanic regions where bathymetric data are sparse. Superimposed on the static geoid topography is dynamic topography due to ocean circulation. Temporal variability of dynamic height due to oceanic eddies can be determined from time series of repeated altimeter profiles. Maps of sea height variability and eddy kinetic energy derived from Geos-3 and Seasat altimetry in some cases represent improvements over those derived from standard oceanographic observations. Measurement of absolute dynamic height imposes stringent requirements on geoid and orbit accuracies, although existing models and data have been used to derive surprisingly realistic global circulation solutions. Further improvement will only be made when advances are made in geoid modeling and precision orbit determination. In contrast, it appears that use of altimeter data to correct satellite orbits will enable observation of basin-scale sea level variations of the type associated with climatic phenomena.