Hartley Hoskins

Methane Hydrate and Free Gas on the Blake Ridge from Vertical Seismic Profiling

Seismic velocities measured in three drill holes through a gas hydrate deposit on the Blake Ridge, offshore South Carolina, indicate that substantial free gas exists to at least 250 meters beneath the bottom-simulating reflection (BSR). Both methane hydrate and free gas exist even where a clear BSR is absent. The low reflectance, or blanking, above the BSR is caused by lithologic homogeneity of the sediments rather than by hydrate cementation. The average methane hydrate saturation above the BSR is relatively low (5 to 7 percent of porosity), which suggests that earlier global estimates of methane in hydrates may be too high by as much as a factor of 3.

Ascension Fracture Zone, Ascension Island, and the Mid-Atlantic Ridge

Near 7° S. latitude, the Ascension fracture zone offsets the Mid-Atlantic Ridge right-laterally over 230 km. North of the fracture zone, which trends about N. 80° E., the ridge crest is perpendicular to its trend, but to the south, near 8° S., the initially perpendicular trend changes to nearly northerly. Ascension Island lies approximately 50 km south of the fracture on magnetic anomaly 4, with an inferred age of 7 m.y. It is not on any major tectonic trend and there is no evidence that it is part of a volcanic chain. Spreading rates in the region increase from north to south, proportional to the distance from the pole of rotation of the African and South American plates, and may be slightly different on the east and west sides of the ridge. Normal to subnormal heat-flow values prevail except for one high value east of the northern ridge axis. The Ascension fracture valley is wide and filled with thick sediments implying an anomalously high age. Earthquake epicenters are aligned along the ridge crest, but near the fracture zone they define an activity belt south of it and more nearly east-west trending. The data suggest a shift of the fracture zone to an east-west trend about 10 m.y. ago, followed by a reorientation of the southern ridge axis that proceeded from south to north and has not been completed. The hypothesis accounts for most observations except the heat-flow pattern, the absence of epicenters on the southernmost ridge crest, and some small structural features.

Geophysical Studies in the Angola Diapir Field

Marine geophysical data on the continental slope west of Angola, Africa, indicate a large (50,000 km2) diapir field. Heat-flow values on the crests of the diapiric structures are systematically 2 to 3 times larger than those measured between these structures. A steady-state model of thermal conductivity contrasts, based on a detailed survey over the diapirs, strongly suggests that the cores of the diapirs are salt.

Velocity structure in upper ocean crust at Hole 504B from vertical seismic profiles

Hole 504B provides the only opportunity to directly correlate seismic velocity structure to the lithology and physical properties of upper ocean crust, providing a baseline for comparison with seismic measurements elsewhere. We determine P and S velocities from vertical seismic profiles (VSPs) obtained on Ocean Drilling Program (ODP) Legs 111 and 148. Four issues are considered: the location of the seismic layer 2/3 boundary, P to S wave conversion by scattering, transverse isotropy, and Poissons ratio as an indicator of lithology, porosity, and structure. (1) In the P velocity profile, the change in slope marking the layer 2/3 boundary coincides with the top of the sheeted dike unit. Seismic layer 2 is composed of the extrusives and the lithologic transition zone, the layer in which flows and dikes interfinger. (2) Even in these normal incident VSPs, several second arrivals with velocities indicative of vertically polarized shear energy are observed. P to S wave conversion within the upper 110 m of basement occurs by scattering from surface roughness and volume heterogeneities and does not depend on angle of incidence as predicted by plane boundary transmission coefficient analysis. (3) Vertical velocities determined from the VSP differ by <10% from horizontal velocities obtained from the oblique seismic experiment (OSE) on Deep Sea Drilling Project (DSDP) Leg 92. The P wave velocity structure is determined by small and intermediate (<1 cm) pore structure with no measurable anisotropy. The large-scale, well-oriented vertical fractures, which are formed tectonically, do not have a detectable effect on compressional wave velocities. (4) High Poissons ratio in the upper 300 m of basement coincides with an extrusive layer composed of pillows and thin flows. Low Poissons ratio at 850–1150 m below seafloor (mbsf) coincides with the downhole decrease in bulk porosity caused by the transition from extrusives to dikes. Relatively large-aspect ratio cracks are required to produce such low values of Poissons ratio. The cracks were likely created by hydraulic fracturing when hot dikes encountered low-temperature seawater.

Geophysical Study of the Easternmost Walvis Ridge, South Atlantic: Morphology and Shallow Structure

The landward termination of Walvis Ridge consists of two east-trending basement ridges of probable basaltic composition enclosing a relatively important sedimentary basin. East of long. 10° E., the southern ridge disappears under the sediments of the continental margin. The trends of the basement ridges are in good agreement with the inferred direction of initial opening. Since its formation, the Walvis Ridge has probably dammed sediment coming from the south. The proposed identification of layer A, a very strong horizon over which the reflectors are nearly undisturbed, may indicate that no major tectonic phase has affected this area since the shift of the pole of opening for the south Atlantic in Late Cretaceous-early Tertiary time. [NOT CONTROLLED OCR]

Seismic attenuation in upper ocean crust at Hole 504B

Seismic attenuation and its relationship to borehole stratigraphy in the upper 1.8 km of ocean basement at Hole 504B are determined from analysis of vertical seismic profile (VSP) data. VSP data provide unambiguous measurements of seismic amplitude decay along a vertical propagation path through the crust, and ancillary borehole measurements enable detailed modeling of the relative contributions from geometrical spreading, scattering, and intrinsic loss mechanisms to this decay. About 60% of the total observed amplitude decay occurs in the pillow basalt section and is due mostly to geometrical spreading and scattering from impedance contrasts. The remaining amplitude decrease is concentrated in two layers, at 500–650 and 800–900 meters below seafloor (mbsf) (225–375 and 525–625 m below basement), across which amplitude rapidly decays and the frequency characteristics of the downgoing wave field are significantly and permanently modified. Attenuation in these layers is not due to scattering but rather to an intrinsic mechanism that can be characterized by Q of 10 and 8, respectively. It is likely that the Q structure of both of these intervals is formed with the crust near the ridge and thus related to fundamental ocean crust forming processes. The shallow interval coincides with a change in alteration mineralogy deposited by late-stage fluid flow and may separate lower lavas that were emplaced within the rift zone from upper lavas that were emplaced by off-axis flow through large lava tubes. Intrinsic attenuation in the deeper horizon is probably due to an increase in porosity and cracking associated with either intracrustal deformation or subhorizontal faulting. There is negligible attenuation of seismic frequencies in the dikes below 1000 mbsf (∼725 m subbasement).

Offset-vertical seismic profiling for marine gas hydrate exploration - is it a suitable technique? First results from ODP Leg 164

Abstract : Walkaway vertical seismic profiles were acquired during Ocean Drilling Project (ODP) Leg 164 at the Blake Ridge to investigate seismic properties of hydrate-bearing sediments and the zone of free gas beneath them. An evaluation of compressional (P-) wave arrivals Site 994 indicates P-wave anisotropy in the sediment column. We identified several shear (S-) wave arrivals in the horizontal components of the geophone array in the borehole and in data recorded with an ocean bottom seismometer deployed at the seafloor. S-waves were converted from P-waves at several depth levels in the sediment column. One of the most prominent conversion points appears to be the bottom simulating reflector (BSR). It is likely that other conversion points are located in the zone of low P-wave reflectivity above the BSR. Modeling suggests that a change of the shear modulus is sufficient to cause significant shear conversion without a significant normal-incidence P-wave reflection.

[Comment on ’”Debate over free exchange of data roils geohysical world” by Julie Wakefield”] Geophysics data: Neither free nor cheap in today's political, economic climate

A Feb. 14, 1995, Eos article, “Debate Over Free Exchange of Data Roils Geophysical World,” discussed a controversial proposal to develop a three-tiered system for meteorological data exchange. In response to the call for related Forum pieces, I offer the following comments. Environmental data does not come cheap. The real costs of environmental data are frequently “buried” in the budgets of the multipurpose infrastructures that gather the information. Costs include instrumentation and calibration, siting and access, technical support, and data processing. Many producers and consumers probably do not know the actual “cost per information unit.” Smart systems, with communications links, are reducing acquisition costs; but the trend is countered by increasingly sophisticated and costly sensor technology and acquisition schemes.

Ocean crustal drilling at the Hawaii-2 Observatory

In preparation for a permanent borehole seismometer at the Hawaii-2 Observatory (H2O) site, Ocean Drilling Project Leg 200 drilled Hole 1224D 1.48 km northeast of the H2O junction box. A reentry cone and cemented casing were installed through 28 m of soft, red clay and 30 m into basaltic basement. The cased basement interval consisted of massive basalt flows that had been cemented by calcite and it should provide good coupling for the seismometer to true earth motion. We also drilled a second single-bit hole, which was cored and logged, within 20 m of the first to a depth of 145 m into basement. The second hole was left with a free-fall funnel so that it also could be reentered using the wireline reentry technology to carry out other borehole experiments at the site. A suite of shipboard physical properties, well log, chemical and microbiological analyses that can be used to characterize the crust surrounding the observatory, was carried out. Measurements of ship noise during the drilling operations were also acquired on the shallow buried seismometer at the Hawaii-2 observatory.