The Kerguelen hotspot (Indian Ocean) has produced basalt for …130 Myr, among the longest known volcanic records from a single source. New and published 40Ar/39Ar age determinations from the Kerguelen Plateau, Broken Ridge, Rajmahal Traps, and Bunbury basalts, and of Indian and Antarctic dikes help to document the hotspot’s history. Using radiometric dates and crustal structure determined from geophysical data and drilling results, we calculate the magmatic output of the Kerguelen hotspot through time. Output rates have varied in ways not predicted by current geodynamic models; maximum eruption volumes postdate the initial surface manifestation of the hotspot as well as break-up between Antarctica and India by ∶15 Myr, and magma output rates were high, as well as geographically diverse, over an interval of 25 Myr, from …120 to …95 Ma. We propose two alternatives to the standard mantle plume paradigm, one involving multiple plume sources, and another consisting of a single, but dismembered plume source.

/pdf/kerguelen-hotspot-magma-output-since-130-ma-1br9jassfl.pdf

Kerguelen Hotspot Magma Output since 130 Ma

[1] We present a three-dimensional, S velocity model of the SE Asian-western Pacific upper mantle with 400-km lateral resolution. Using the novel Automated Multimode Inversion technique, we processed 4038 vertical-component seismograms and extracted 22,708 linear equations with uncorrelated uncertainties that constrain upper mantle structure. We used time-frequency windows to select signal with negligible proportion of scattered energy. The windows included the fundamental Rayleigh mode and S and multiple S waves. The observed range of S velocity variations is the widest (17–18%) in the upper 150 km of the mantle. High-velocity continental roots can reach beyond the present extent of the overlying Archean-Proterozoic crust by 500 km. Beneath some Precambrian units the roots are absent, which can be attributed to deformation and gradual destruction of the ancient lithosphere. At 120–150 km, S velocity beneath some cratons reaches 4.8 km/s; this can be accounted for by thermal and compositional effects. Beneath the Hainan Island area a low-velocity anomaly is observed from near the surface to the bottom of our model; the hot spot-type volcanism here may be caused by the deep-mantle Hainan plume. A low-velocity mantle domain underlies the south central Sea of Japan, surrounded on the surface by intraplate volcanoes. A deep-seismicity gap is present near 40°N in the Pacific slab subducting below and may result from a plume-slab interaction. A high-velocity anomaly is present in the transition zone beneath the northern boundary of the Yangtze Craton. We propose that the anomaly corresponds to subducted continental lithosphere, stagnant atop the 660-km discontinuity.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2000JB000073

Upper mantle beneath Southeast Asia from S velocity tomography

Gas in an unconsolidated sand reservoir encased in shale often results in a dramatic increase in amplitude of the seismic reflection from the shale/gas-sand interface. Unfortunately, reflection amplitude appears not to vary linearly with water (brine) saturation, and thus cannot be used to estimate gas quantity. Previously presented theoretical velocity computations for a Tertiary sedimentary section, which demonstrate that compressional-wave velocity in an unconsolidated gas sand varies nonlinearly with brine saturation, qualitatively agree with laboratory velocity measurements on a sand specimen composed of pure quartz grains. However, significant departure of measured and theoretical velocities at high brine saturation indicates that the technique for partially saturating the sand specimen by flowing a gas-brine mixture through the specimen does not provide a sufficiently uniform distribution. The gas preferentially seeks larger pores. In a subsequent experiment on a specimen composed of spherical glass beads of nearly uniform size, the previous, as well as a modified, fluid injection technique was used. For the latter, brine only was injected into the pore space previously filled with a mixture of gas and brine in nearly equal proportions. This resulted in a more uniform distribution of the gas-brine mixture. For approximately equal brine saturations, this modified technique more » resulted in a measured compressional-wave velocity approximately one-half of the velocity measured for the previously used fluid injection technique. This result implies that if the gas-brine mixture is uniformly distributed in a reservoir, the fluid compressibility is the weighted-by-volume average of the constituent compressibilities. « less

Effect of brine-gas mixture on velocity in an unconsolidated sand reservoir. [Laboratory velocity measurements in unconsolidated sand at various brine saturations]

http://basin.earth.ncu.edu.tw/Course/SeminarII/abstract2014_1/2015.01.08_Hsueh-Fen%20Wang/Gas%20hydrate%20occurrence%20on%20the%20continental%20slope%20of%20the%20northern%20South%20China%20Sea.pdf

Gas hydrate occurrence on the continental slope of the northern South China Sea

Attenuation at low seismic frequencies in partially saturated rocks: Measurements and description of a new apparatus

Tectonics of the Bay of Bengal: new insights from satellite-gravity and ship-borne geophysical data

Introduction: Importance of different forms of energy, renewable, non-renewable, efficiencies of all energies.- Oil & Gas scenario.- Alternatives. Heavy Oil. Coal Bed Methane. Hydrogen. Hydrates. Others.- Gas hydrates. What are gas hydrates. Structure. Shape and forms.- Stability conditions. Availability of methane. Pressure and Temperature conditions. Water chemistry. Higher hydrocarbons.- Geological indicators. Geophysical surveys. Geophysical processing. Geophysical indicators. Bottom Simulating Reflectors. Top of the hydrates (Gradational boundary). Attenuation. Shear waves (P-S conversion). Velocity.- Quantification of gas hydrates. Seismic Attributes. Reflection coefficient. Blanking. Velocity. Amplitude versus Offset. Modelling. Inversion. Rock Physics.- The Road Ahead.- Organization of the book

Exploration of Gas Hydrates: Geophysical Techniques

Any propagating wave undergoes attenuation, which is primarily governed by the physical properties of the medium, determined in terms of quality factor (Q). Research into the characteristics of both P- and S-wave Q with reference to gas-hydrates exploration remains in its infancy. Presence of gas-hydrates increases the Q, and this again depends on the nature of distribution and amount of hydrates within the sediments. Thus, estimation of Q provides useful input for both the detection and quantitative assessment of gas-hydrates. Here we propose a simple technique of deriving Q from prestack surface seismic reflection data based on the logarithm of spectral ratio (LSR), and apply the method to marine multi-channel seismic (MCS) data collected on the western margin of India where a bottom simulating reflector (BSR), which is a prime marker for gas-hydrates, has already been identified. The Q (256 ± 11) estimated over the region with a strong BSR is found to be more than double the Q (101 ± 9) derived for the region without any BSR or a weak BSR. The anomalously high Q with respect to the background can be used to detect gas-hydrates in areas where the BSR is not very clearly observed on seismic sections.

Seismic quality factor observations for gas-hydrate-bearing sediments on the western margin of India

Exploration of Gas Hydrates

Multi-channel seismic reflection data from the western continental margin of India (WCMI) have been analyzed to construct a plausible model for gas hydrate formation. A reflector at 2950 ms two way travel time (TWT) on one of the sections is interpreted to represent the base of the layer of the methane hydrate, identified by a bottom simulating reflector (BSR) that lies almost 500 ms beneath the sea floor. BSRs of similar origin are common world wide, where they are usually interpreted to mark the base of gas hydrate bearing clastic sediment, with or without underlying free gas. In this study we present a model with the contrasting physical properties that produce synthetic wavelets that match with the observed BSR amplitude and waveforms for varying source-receiver offsets of multi-channel seismic reflection data. The preliminary results presented here put important constraints on models that predict the distribution and formation of hydrate. Offset-dependent amplitude recovery also gives an appropriate response for hydrate characterization.

Naresh Kumar Thakur

Papers

Tectonics of the Bay of Bengal: new insights from satellite-gravity and ship-borne geophysical data

Exploration of Gas Hydrates: Geophysical Techniques

Seismic quality factor observations for gas-hydrate-bearing sediments on the western margin of India

Exploration of Gas Hydrates

Probable gas hydrate/free gas model over western continental margin of India