Seismic Source Characteristics of Nuclear Explosions in Water-filled Cavities

—?During the period from 1975 to 1979, the former Soviet Union conducted a series a six nuclear explosions in a water-filled cavity in salt which was created in 1968 by a tamped 27?kt explosion at a depth of 597?m at the Azgir test site at the north end of the Caspian Sea. Broadband, near-regional seismic data recorded from these tests have been processed and analyzed in an attempt to characterize the seismic source characteristics of these explosions and assess their relevance to the cavity decoupling evasion scenario. The results of these analyses indicate that the explosions in the water-filled cavity were not decoupled, but rather show evidence of enhanced seismic coupling with respect to that which would be expected from tamped explosions of the same yields in salt. Theoretical finite difference simulations of these tests have been conducted in which the complex, nonlinear interactions between the shock effects in both the water and surrounding salt medium have been explicitly modeled. The results of these simulations indicate that the most prominent yield dependent features of the observed seismic source functions can be largely explained by the dynamic interactions between the expanding and contracting steam bubbles generated by the explosions in water and the shock-wave reflections from the cavity wall. More specifically, it has been found that the shock-wave reflection from the cavity wall retards the expansion of the steam bubble in a yield dependent fashion relative to that expected in the open ocean, resulting in a smaller maximum bubble radius and a shorter bubble oscillation period.     

Concentrations and transport of trace metals in the St. Lawrence River

Samples of raw water were collected in the St. Lawrence River during six sampling trips from August 1990 to April 1992. Water samples were analyzed for both dissolved and particulate phases for five trace metals. Partition coefficients (Kd) and metal fluxes were calculated in order to determine metal transport. A mass balance equation was used for the determination of the major metal sources to the St. Lawrence River and an estimation of metal loadings to the estuary was made. Average dissolved metal concentrations were found to be Cd 10 ng/L, Co 74 ng/L, Cu 64 ng/L, Fe 69 µg/L and Mn 700 ng/L. Particulate concentrations were (in µg/g) 1.68 for Cd, 31 for Co, 73 for Cu, 25 mg/g for Fe and 1.69 mg/g for Mn. Co, Fe and Mn were transported essentially in the particulate phase while Cd and Cu were predominantly found in the dissolved phase at 56% and 48% of the total metal concentration respectively. Log Kd values varied from 5.1 (for Cu) to 6.8 (for Mn). In the dissolved phase the major sources were found to be the Great Lakes and the Ottawa River whereas in the particulate phase Québec tributaries appear to be the most important. Industrial inputs are quite important in both the dissolved and the particulate phases for Cd, whereas other sources are very variable, especially for the dissolved phase.     

Practical techniques for 3D resistivity surveys and data inversion1

Techniques to reduce the time needed to carry out 3D resistivity surveys with a moderate number (25 to 100) of electrodes and the computing time required to interpret the data have been developed. The electrodes in a 3D survey are normally arranged in a square grid and the pole-pole array is used to make the potential measurements. The number of measurements required can be reduced to about one-third of the maximum possible number without seriously degrading the resolution of the resulting inversion model by making measurements along the horizontal, vertical and 45° diagonal rows of electrodes passing through the current electrode. The smoothness-constrained least-squares inversion method is used for the data interpretation. The computing time required by this technique can be greatly reduced by using a homogeneous half-space as the starting model so that the Jacobian matrix of partial derivatives can be calculated analytically. A quasi-Newton updating method is then used to estimate the partial derivatives for subsequent iterations. This inversion technique has been tested on synthetic and field data where a satisfactory model is obtained using a modest amount of computer time. On an 80486DX2/66 microcomputer, it takes about 20 minutes to invert the data from a 7 by 7 electrode survey grid. using the techniques described below, 3D resistivity surveys and data inversion can be carried out using commercially available field equipment and an inexpensive microcomputer.     

Near bottom magnetic profile across the Red Sea

Results are presented from a high precision geophysical profile made at an altitude of about 100 m above the sea floor with the Deep Two instrument package, crossing the Red Sea at 17°30N. The emphasis is on the analysis and interpretation of the magnetic field, including an inversion which removes the distortions due to bathymetry and the orientation with respect to the earth's main field vector. The spreading rates are determined precisely and found to be highly asymmetric: only 5 mm yr^-1 to the east and up to 10 mm yr^-1 to the west. We conclude that the axis of spreading is located on a volcanic ridge, rather than on the axial graben, based on the presence of a zone of high magnetization. The magnetization high (40 Am^-1) is about twice as great as found on the Mid-Atlantic Ridge with the same instrument and analysis. The quality of the recording of the magnetic anomalies in the oceanic crust is much greater than expected for such a low spreading rate.     

Deep-Tow magnetics near 20°S on the East Pacific Rise: A study of short wavelength anomalies at a very fast spreading center

Six Deep-Tow magnetic profiles across the axis of the East Pacific Rise [EPR] in two small areas between 19°25 and 20°10S were collected during the 1983 Protea 1 cruise of the R/V Melville. These near-bottom profiles are of extremely high resolution allowing the interpretation of very short wavelength features. We have inverted the magnetic field data to determine the rock magnetization distribution near the axis of this ultrafast speading center (162 mm yr^-1). The solutions reveal large amplitude (up to 35 A m^-1) short wavelength (1–3 km) variations in magnetization. Specifically all crossings show a narrow (0.5 to 1.5 km) low in magnetization superimposed on a broader (2.5 to 4 km) high directly over the ridge axis. Four profiles in the northern area (19°25 to 19°33S) also show symmetrical near-axis (within 4 km) lows which are remarkably continuous along strike. Explanations for the short-wavelength variations are discussed which fall into the following categories: (1) variations in the thickness of the magnetized layer, (2) variations in rock chemistry (e.g. alteration due to hydrothermal activity), and (3) paleofield intensity variations. None of the mechanisms discussed alone adequately explain the observed phenomena in the study area or on a world-wide scale. Further sampling and high resolution surveying will be required in order to accurately determine the relative importance of the mechanisms discussed.     

Tectonics of the Nereus Deep,Red Sea: A deep-tow investigation of a site of initial rifting

The Nereus Deep (23°N) lies in the central portion of the Red Sea, in a region which marks a transition between the nearly continuous axial rift valley of the southern Red Sea and the northern Red Sea, where a well defined axial rift is absent. The deep-tow survey and associated heat flow measurements reported here show that the Nereus Deep is a short segment of axial rift, and it is the northernmost deep where petrology, heat flow, magnetics, and morphology all indicate classic seafloor spreading. Heat flow measured in the Nereus Deep is characterized by non-linear gradients and closely-spaced variability indicative of active hydrothermal circulation associated with seafloor spreading. The two axial highs which we have mapped in Nereus differ markedly in that the southernmost appears younger or at least has had a more recent phase of volcanism. The two axial highs are offset left laterally approximately 2 km. This small offset or bend in the axial course has been labelled the Nereus shear zone, and, despite its small extent, it mimics many of the major features of small offset, slow-slipping transform faults. This shear zone may result from shear stresses associated with misalignments in succeeding volcanic episodes. The Nereus Deep appears to represent one of the earliest phases of seafloor spreading. The Red Sea seems to be opening towards the north, and the Nereus Deep is near the tip of propagation, but it is clear from this study that rift propagation in a site of initial rifting differs greatly from that observed along a well developed, fast spreading center like the East Pacific Rise.     

Phytoplankton distribution in a frontal region of Tokyo Bay,Japan in November 1985

Phytoplankton distribution in a frontal region of Tokyo Bay was investigated in relation to hydrography in November 1985. The frontal region was observed from the central to the mouth area of the bay and consisted of a series of fine scale discontinuities of salinity and temperature. Among them the Kenzaki offshore front (KOF) and the Yokosuka inshore front (YIF) were most prominent in terms of the duration and the magnitudes of the hydrographic gaps. Three major phytoplankton assemblages were observed: (A) neritic and offshore diatoms in the mouth area of Tokyo Bay, (B) a diatomLeptocylindrus danicus and dinoflagellates in the central area, and (C) bloom forming cryptophyceans, dinoflagellates andL. danicus from the inner Tokyo Bay. The KOF was an approximate boundary of the outside assemblage (A) and the intermediate population (B), and the YIF was that of (B) and the inner bay population (C). Species changes across the fronts were rather gradual in the KOF making a strong contrast to distinct jumps in temperature, salinity,in vivo chlorophyll fluorescence and nitrate plus nitrite. An outward surface flow of the inside population along the western coast off Yokosuka was detected.     

Distribution of riverine sediment chemistry on the shelf,slope and rise of the Gulf of Papua

The tectonically active islands of the Indo-Pacific Archipelago deliver much sediment to the ocean margins. In the Gulf of Papua on the south coast of Papua New Guinea (PNG), the chemical composition of surface sediment from grab samples indicates that Fly River muds are dispersed to the north and east, where they are joined by sediment plumes from the other large rivers along the south coast of PNG. This is the likely source of terrestrial sediment on the Papuan Plateau and the northern Coral Sea Abyssal Plain. The sediment is transported through submarine troughs and canyons offshore, far to the east of the riverine inputs. Immediately south and 30–50 km offshore from the Fly and Purari deltas is a platform of algal and reef carbonate materials, containing little or no terrestrial surface sediment.     

A review of present techniques and methodological advances in analyzing Th in aquatic systems

The short-lived thorium isotope ²³⁴Th (half-life 24.1 days) has been used as a tracer for a variety of transport processes in aquatic systems. Its use as a tracer of oceanic export via sinking particles has stimulated a rapidly increasing number of studies that require analyses of ²³⁴Th in both marine and freshwater systems. The original ²³⁴Th method is labor intensive. Thus, there has been a quest for simpler techniques that require smaller sample volumes. Here, we review current methodologies in the collection and analysis of ²³⁴Th from the water column, discuss their individual strengths and weaknesses, and provide an outlook on possible further improvements and future challenges. Also included in this review are recommendations on calibration procedures and the production of standard reference materials as well as a flow chart designed to help researchers find the most appropriate ²³⁴Th analytical technique for a specific aquatic regime and known sampling constraints.