Ralph A. Stephen

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.

A long in situ section of the lower ocean crust: results of ODP Leg 176 drilling at the Southwest Indian Ridge

Ocean Drilling Program Leg 176 deepened Hole 735B in gabbroic lower ocean crust by 1 km to 1.5 km. The section has the physical properties of seismic layer 3, and a total magnetization sufficient by itself to account for the overlying lineated sea-surface magnetic anomaly. The rocks from Hole 735B are principally olivine gabbro, with evidence for two principal and many secondary intrusive events. There are innumerable late small ferrogabbro intrusions, often associated with shear zones that cross-cut the olivine gabbros. The ferrogabbros dramatically increase upward in the section. Whereas there are many small patches of ferrogabbro representing late iron- and titanium-rich melt trapped intragranularly in olivine gabbro, most late melt was redistributed prior to complete solidification by compaction and deformation. This, rather than in situ upward differentiation of a large magma body, produced the principal igneous stratigraphy. The computed bulk composition of the hole is too evolved to mass balance mid-ocean ridge basalt back to a primary magma, and there must be a significant mass of missing primitive cumulates. These could lie either below the hole or out of the section. Possibly the gabbros were emplaced by along-axis intrusion of moderately differentiated melts into the near-transform environment. Alteration occurred in three stages. High-temperature granulite- to amphibolite-facies alteration is most important, coinciding with brittle^ductile deformation beneath the ridge. Minor greenschist-facies alteration occurred under largely static conditions, likely during block uplift at the ridge transform intersection. Late post-uplift low-temperature alteration produced locally abundant smectite, often in previously unaltered areas. The most important features of the high- and low-temperature alteration are their respective

Finite‐difference synthetic acoustic logs

The finite‐difference method is a powerful technique for studying the propagation of elastic waves in boreholes. Even for the simple case of an open borehole with vertical homogeneity, the snapshot format of the method displays clearly the interaction between the borehole and the rock, and the origin and evolution of phases. We present an outline of the finite‐difference method applied to the acoustic logging problem, including a boundary condition formulation for liquid‐solid cylindrical interfaces which is correct to second order in the space increments. Absorbing boundaries based on the formulations of Reynolds (1978) and Clayton and Engquist (1977) were used to reduce reflections from the grid boundaries. Results for a vertically homogeneous sharp interface model are compared with the discrete‐wavenumber method and excellent agreement is obtained. The technique is also demonstrated by considering sharp and continuous transitions (damaged zones) at the borehole wall and by considering the effects of wa...

Broadband seismology in the oceans: Lessons from the Ocean Seismic Network Pilot Experiment

The fundamental objective of the Ocean Seismic Network Pilot Experiment (OSNPE)—which was carried out over a period of about 4 months at a site 225 km southwest of Oahu, Hawaii—was to learn how to make high-quality, broadband seismic measurements in the deep oceans. The OSNPE results demonstrate that broadband data of quality similar to that of quiet land stations can be acquired with seafloor seismographs, but that the location of the seismometer—whether it be on the seafloor, surficially buried within the seabed, or in a deep borehole—has a profound effect on data quality. At long-periods ( 0.1 Hz), data quality was best for a seismometer deployed 242 m below the seafloor in a borehole.

Viscoelastic Waves in Layered Media

Preface 1. One-dimensional viscoelasticity 2. Three-dimensional viscoelasticity 3. Viscoelastic P, SI and SII waves 4. Framework for single-boundary reflection-refraction and surface-wave problems 5. General P, SI, and SII waves incident on a viscoelastic boundary 6. Numerical models for general waves reflected and refracted at viscoelastic boundaries 7. General SI, P, and SII waves incident on a viscoelastic free surface 8. Rayleigh-type surface wave on a viscoelastic half space 9. General SII waves incident on multiple layers of viscoelastic media 10. Love-type surface waves in multilayered viscoelastic media 11. Appendices 12. References Index.

Seismic energy partitioning and scattering in laterally heterogeneous ocean crust

We present finite difference forward models of elastic wave propagation through laterally heterogeneous upper oceanic crust. The finite difference formulation is a 2-D solution to the elastic wave equation for heterogeneous media and implicitly calculatesP andSV propagation, compressional to shear conversion, interference effects and interface phenomena. Random velocity perturbations with Gaussian and self-similar autocorrelation functions and different correlation lengths (a) are presented which show different characteristics of secondary scattering. Heterogeneities scatter primary energy into secondary body waves and secondary Stoneley waves along the water-solid interface. The presence of a water-solid interface in the model allows for the existence of secondary Stoneley waves which account for much of the seafloor ‘noise’ seen in the synthetic seismograms for the laterally heterogeneous models.‘Random’ incoherent secondary scattering generally increases aska (wavenumber,k, and correlation length,a) approaches one. Deterministic secondary scattering from larger heterogeneities is the dominant effect in the models aska increases above one. Secondary scattering also shows up as incoherence in the primary traces of the seisograms when compared to the laterally homogeneous case. Cross-correlation analysis of the initialP-diving wave arrival shows that, in general, the correlation between traces decreases aska approaches one. Also, because many different wave types exist for these marine models, the correlation between traces is range dependent, even for the laterally homogeneous case.

An implicit finite-difference formulation of the elastic wave equation

The problem with existing finite‐difference formulations of the elastic wave equation is that they have a limited stability range, and the necessity of taking small time steps can result in excessively high computation costs. It is possible to formulate a finite‐difference scheme which is stable for arbitrarily large time steps. However, the solutions obtained by the unconditionally stable scheme are unacceptably inaccurate for time steps outside the stability range of finite‐difference schemes currently in use.

Hydroacoustic events located at the intersection of the Atlantis (30°N) and Kane (23°40′N) Transform Faults with the Mid‐Atlantic Ridge

We investigate the characteristics of T-phase events located at the ends of two segments of the Mid-Atlantic Ridge. Our motivations for the study were to understand whether T-phase locations represent earthquake epicenters (and thus whether accurate geological inferences can be made from their spatial patterns) and to understand further the relationship between T-phase event characteristics and earthquake properties. We examine the characteristics of 158 T-phase events with respect to both event location water depth and source to receiver distance. The propagation paths of the T-phases are also modeled to study the effects of encountering seafloor topography. We find that existing models for T-phase excitation and propagation cannot explain adequately all of our observations. The amplitudes (Received Levels) of T-phases at the hydrophones show no dependence on event water depths, in contrast to current excitation models which predict a decrease in event magnitude with increasing water depth. The Received Levels are observed to decrease with increasing source to receiver distance, and events from the two study areas exhibit different trends in relative Received Levels between hydrophones, once attenuation is taken into account. Our acoustic ray trace model is able to reproduce similar trends in relative amplitudes at the hydrophones based on 1-D topography between the event and each hydrophone, but the variances in both the observed data and model are high. We observe a pattern of short T-phase onset times for shallow water events and long onset times for deep water events, where onset time is defined as the time interval between the appearance of the T-phase envelope above the ambient noise and its first peak. This suggests that the onset time may be a function of several variables, including efficiency of energy conversion based on local topography, efficiency of propagation based on event water depth, and hypocentral depth in the crust. The results of this study underscore the complexity of T-phase excitation and propagation and argue that current models of T-phase excitation and propagation need to be improved to explain the observed characteristics of T-phase data.

The North Pacific Acoustic Laboratory deep-water acoustic propagation experiments in the Philippine Sea

A series of experiments conducted in the Philippine Sea during 2009-2011 investigated deep-water acoustic propagation and ambient noise in this oceanographically and geologically complex region: (i) the 2009 North Pacific Acoustic Laboratory (NPAL) Pilot Study/Engineering Test, (ii) the 2010-2011 NPAL Philippine Sea Experiment, and (iii) the Ocean Bottom Seismometer Augmentation of the 2010-2011 NPAL Philippine Sea Experiment. The experimental goals included (a) understanding the impacts of fronts, eddies, and internal tides on acoustic propagation, (b) determining whether acoustic methods, together with other measurements and ocean modeling, can yield estimates of the time-evolving ocean state useful for making improved acoustic predictions, (c) improving our understanding of the physics of scattering by internal waves and spice, (d) characterizing the depth dependence and temporal variability of ambient noise, and (e) understanding the relationship between the acoustic field in the water column and the seismic field in the seafloor. In these experiments, moored and ship-suspended low-frequency acoustic sources transmitted to a newly developed distributed vertical line array receiver capable of spanning the water column in the deep ocean. The acoustic transmissions and ambient noise were also recorded by a towed hydrophone array, by acoustic Seagliders, and by ocean bottom seismometers.

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.