A practical autonomous path planner for turn-of-the-century planetary microrovers

With the success of Mars Pathfinder's Sojourner rover, a new era of planetary exploration has opened, with demand for highly capable mobile robots. These robots must be able to traverse long distances over rough, unknown terrain autonomously, under severe resource constraints. Based on the authors' firsthand experience with the Mars Pathfinder mission, this paper reviews issues which are critical for successful autonomous navigation of planetary rovers. No currently proposed methodology addresses all of these issues. We next report on the 'Wedgebug' algorithm, which is applicable to planetary rover navigation in SE(2). The Wedgebug algorithm is complete, correct, requires minimal memory for storage of its worked model, and uses only on-board sensors, which are guided by the algorithm to efficiently senses only the data needed for motion planning. The implementation of a version of Wedgebug on the Rocky7 Mars Rover prototype at the Jet Propulsion Laboratory is described, and experimental results from operation in simulated martian terrain are presented

First Descriptions of Endoparasite Fauna of Elasmobranch and Mesopelagic Teleost Bycatch Fishes from the Western North Atlantic Pelagic Longline Fishery

Natural mortality is a poorly known aspect of fisheries biology, despite its importance in stock assessments and population analysis. Of potential sources of mortality and morbidity in fishes, the effect of internal parasites is perhaps the least studied even though these organisms are known to inhibit nutrient uptake and stimulate an inflammatory response in fish. Parasite taxa of the pelagic elasmobranchs silky and night sharks and pelagic stingray (Carcharhinus falciformis, C. signatus and Pteroplatytrygon violacea), and the mesopelagic teleosts sailfin lancetfish, oilfish, snake mackerel, escolar and Atlantic pomfret (Alepisaurus ferox, Ruvettus pretiosus, Gempylus serpens, Lepidocybium flavobrunneum, and Brama brama) are described from the western North Atlantic and Gulf of Mexico. Parasite taxa included cestodes, trematodes, acanthocephalans, and nematodes. Suggested protocol revisions to current accepted laboratory methods will enhance future parasite taxa descriptions from pelagic marine fishes. This work serves as the first parasite taxa and load descriptions for pelagic stingray, lancetfish, oilfish, snake mackerel, escolar and pomfret

The nature of iron-oxygen vacancy defect centers in PbTiO3

The iron(III) center in ferroelectric PbTiO3 together with an oxygen vacancy forms a charged defect associate, oriented along the crystallographic c-axis. Its microscopic structure has been analyzed in detail comparing results from a semi-empirical Newman superposition model analysis based on finestructure data and from calculations using density functional theory. Both methods give evidence for a substitution of Fe3+ for Ti4+ as an acceptor center. The position of the iron ion in the ferroelectric phase is found to be similar to the B-site in the paraelectric phase. Partial charge compensation is locally provided by a directly coordinated oxygen vacancy. Using high-resolution synchrotron powder diffraction, it was verified that lead titanate remains tetragonal down to 12 K, exhibiting a c/a-ratio of 1.0721.Comment: 11 pages, 5 figures, accepted in Phys. Rev.

Investigation of the Davis Sandstone (Ft. Worth Basin, Texas) as a Suitable Formation for the GRI Hydraulic Fracture Test Site

The concept of the GRI Hydraulic Fracture Test Site (HFTS) was to provide a field laboratory to (1) validate three-dimensional hydraulic fracture models in tight gas sandstone and (2) develop technology in fracture diagnostics and stimulation. The Davis sandstone in the Fort Worth Basin, north-central Texas, was initially selected as a viable candidate formation for HFTS research based on the results of a cooperative well program initiated with Dallas Production. To gather comprehensive data on a specific site for HFTS research, the S.A. Holditch & Associates Data Well No. 1 was drilled in June 1991. The results of geological, petrophysical, and engineering analyses of the co-ops and data well are the basis of this report. These analyses indicate that in northern Parker and southern Wise Counties, Texas, the Davis sediments range from 250 to 350 feet thick. A broadly continuous, 100-foot thick interval in the upper part of the gross interval comprises the Davis Reservoir. The average permeability of the Davis Reservoir was found to be 0.08 millidarcies with an average closure stress of 0.45 pounds per square inch per foot. The shale barriers above and below the Davis had average closure stress of 0.63 to 0.73 pounds per square inch per foot and 0.88 to 0.98 pounds per square inch per foot, respectively. Hydraulic fracture azimuth was found to range from N10°E to N20°E. Drainage area from production analyses was calculated to be 48.7 acres in northwest Parker County. Natural fractures were encountered in the Davis, causing severe drilling problems in Data Well No. 1. Davis was therefore suspended.Bureau of Economic Geolog

A practical autonomous path planner for turn-of-the-century planetary microrovers

With the success of Mars Pathfinder's Sojourner rover, a new era of planetary exploration has opened, with demand for highly capable mobile robots. These robots must be able to traverse long distances over rough, unknown terrain autonomously, under severe resource constraints. Based on the authors' firsthand experience with the Mars Pathfinder mission, this paper reviews issues which are critical for successful autonomous navigation of planetary rovers. No currently proposed methodology addresses all of these issues. We next report on the 'Wedgebug' algorithm, which is applicable to planetary rover navigation in SE(2). The Wedgebug algorithm is complete, correct, requires minimal memory for storage of its worked model, and uses only on-board sensors, which are guided by the algorithm to efficiently senses only the data needed for motion planning. The implementation of a version of Wedgebug on the Rocky7 Mars Rover prototype at the Jet Propulsion Laboratory is described, and experimental results from operation in simulated martian terrain are presented

Analysis of Natural Fractures and Borehole Ellipticity Travis Peak Formation East Texas

This report summarizes petrographic studies of natural and coring-induced fractures in 7 cores from the Travis Peak Formation, a low-permeability gas sandstone in East Texas, and also presents an analysis of fracturing and wellbore elongation based on Borehole Televiewer, Formation Microscanner, and Ellipticity logs from 12 Travis Peak wells. Natural, vertical extension fractures in sandstone are open or only partly mineral-filled in the cored depth range (approximately -5,000 to -10,000 ft), and they are therefore potential gas reservoirs as well as a potentially important influence on commercial hydraulic fracture treatment. Crack-seal structure in fracture-filling quartz shows that fracturing and quartz cementation were contemporary; this result, together with evidence of timing of fracturing and the large water volumes that are inferred to have passed through the Travis Peak, suggests that natural hydraulic fracturing influenced fracture development. Healed transgranular microfractures that occur in sandstone can be used to ascertain natural fracture trends in core that lacks macrofractures, and coring-induced petal-centerline fractures can be used to infer stress orientations. Fractures trend ENE to E. In the upper Travis Peak, borehole ellipticity trends ENE, parallel to fracture trends, and in the lower Travis Peak ellipticity trends NNW, parallel to the direction of least horizontal stress.Bureau of Economic Geolog

Site Selection for Gri Cooperative Tight Gas Field Resesarch Volume 2: Geologic Characteristics of Selected Low-Permeability Gas Sandstones

Geological, engineering, and economic data on selected formations were compiled to provide a basis for siting the fourth Staged Field Experiment (SFE) for the Tight Gas Sands research program. The geologic units chosen are the Abo, Cleveland, and Frontier Formations, and the Mesaverde Group. Extrapolation potential is good for all formations except the Cleveland, whose thin deltaic package has no good analogy in other low-permeability sandstones. The Abo has the best potential for extrapolation to other low-permeability formations. The average thickness of reservoirs is about 250 ft in the Mesaverde and Abo, 160 ft in the Frontier, and 120 ft in the Cleveland. The deepest production depth varies from 4,750 ft (Abo) to 12,198 ft (Second Frontier sandstone). The estimated resource base ranges from 3 TCF (Abo) to 86 TCF (Mesaverde). Pre-stimulation production ranges from too small to measure (Cleveland, Frontier, Mesaverde) to 314 MCFD (Frontier). Post-stimulation production ranges from 3 MCFD (Mesaverde) to 12,250 MCFD (Cleveland). Permeability ranges from less than 0.0001 md (Frontier) to 1.3 md (Frontier). Natural fractures have been shown to be significant locally in the Mesaverde, but their contribution to reservoir permeability in the other formations is not well-documented.Bureau of Economic Geolog

Geology of the Lower Cretaceous Travis Peak Formation, East Texas: Characterization of a Tight Gas SandStone

Since 1982, the Gas Research Institute (GRI) Tight Gas Sands Project has supported geological investigations designed to develop knowledge necessary to efficiently produce low-permeability, gas-bearing sandstones. As part of that program, the Bureau of Economic Geology has conducted research on low-permeability sandstone in the Lower Cretaceous Travis Peak (Hosston) Formation in East Texas. The first phase of the study, which lasted from 1983 until 1986, involved extensive collection of core and production data in seven cooperative wells. Information gained from the cooperative wells, combined with geologic characterization of the Travis Peak throughout the study area, led to the drilling by GRI of three Staged Field Experiment (SFE) wells between 1986 and 1988. The SFE wells were drilled and completed by GRI specifically for the purpose of research on low-permeability gas reservoirs. This report summarizes the results of the geologic studies of the Travis Peak Formation, and it focuses on the contribution of geology to evaluation and completion of tight gas sandstone wells.Bureau of Economic Geolog

Geologic Evaluation of Critical Production Parameters for Coalbed Methane Resources: Part 1, San Juan Basin

The Fruitland Formation in the San Juan Basin is the major producer of coalbed methane in the Western U.S. Forty-three to forty-nine Tcf of methane occur in 245 billion short tons of Fruitland coal at depths between 400 and 4,200 ft. Thickest Fruitland coal seams trend northwest and occur in the northern part of the basin, northeast of a syndepositional, structural hinge line; they occur in coastal plain facies southwest of Pictured Cliffs barrier/strandplain sandstones. South of the hinge line, northeast-trending coal seams occur in floodplain facies between northeast-trending Fruitland fluvial systems. Face cleat trends in Fruitland coal seams are predominantly northeast in the southern two-thirds of the basin and northwest but variable in the northern third. Suggested targets for enhanced coalbed permeability are tectonic fractures and fractures associated with subtle folds. Fruitland Formation waters are evolved meteoric waters; water composition reflects hydrologic setting. Waters in the north-central San Juan Basin have high alkalinity and low chlorinity; waters in the southern part are Na-Cl type. Distribution of low-chloride ground water in the Fruitland Formation in the north-central basin coincides with the overpressured area and with flow patterns inferred from the head map. The Fruitland Formation acts regionally as a single hydrologic unit or homogeneous aquifer, but large pressure gradients locally indicate that Fruitland strata may be hydraulically disconnected and behave at the field scale as compartmentalized aquifers. Hydrologic studies defined reservoir characteristics and permeability boundaries in the Fruitland Formation. Geologic and hydrologic parameters were used to divide the San Juan Basin into areas in which coal beds have similar reservoir characteristics. Coalbed wells have negative declines early in their production history followed by exponential decline rates at less than 5 percent/year. Sandstone wells that exhibit coal-decline behavior probably are producing coalbed methane indirectly from coal seams.Bureau of Economic Geolog

Integrated Outcrop and Subsurface Studies of the Interwell Environment of Carbonate Reservoirs: Clear Fork (Leonaradian Age) Reservoirs, West Texas and New Mexico, Semi-Annual

Outcrop studies include stratigraphic and petrophysical analysis. Analysis of the detailed sequence- and cycle-scale architecture of the Clear Fork reservoir-equivalent outcrops in Apache Canyon is nearly complete. This work reveals two high-frequency transgressive-regressive sequences (HFS) in the lower Clear Fork composite depositional sequence and three HFS in the basal middle Clear Fork composite depositional sequence. A 1,800-ft transect of 1-inch-diameter samples was collected from one cycle at the Apache Canyon outcrop. The transect was sampled with 5-ft spacing, but there were some gaps due to cover and cliff, resulting in 181 samples. Permeability, porosity, and grain density were measured, and the spatial statistics are being analyzed geostatistically