Seismic Attenuation and Velocity Dispersion to Discriminate Gas Hydrates and Free Gas Zone, Makran Offshore, Pakistan

We have estimated the seismic attenuation in gas hydrate and free-gas-bearing sediments from high-resolution P-cable 3D seismic data from the Vestnesa Ridge on the Arctic continental margin of Svalbard. P-cable data have a broad bandwidth (20–300 Hz), which is extremely advantageous in estimating seismic attenuation in a medium. The seismic quality factor (Q), the inverse of seismic attenuation, is estimated from the seismic data set using the centroid frequency shift and spectral ratio (SR) methods. The centroid frequency shift method establishes a relationship between the change in the centroid frequency of an amplitude spectrum and the Q value of a medium. The SR method estimates the Q value of a medium by studying the differential decay of different frequencies. The broad bandwidth and short offset characteristics of the P-cable data set are useful to continuously map the Q for different layers throughout the 3D seismic volume. The centroid frequency shift method is found to be relatively more stable than the SR method. Q values estimated using these two methods are in concordance with each other. The Q data document attenuation anomalies in the layers in the gas hydrate stability zone above the bottom-simulating reflection (BSR) and in the free gas zone below. Changes in the attenuation anomalies correlate with small-scale fault systems in the Vestnesa Ridge suggesting a strong structural control on the distribution of free gas and gas hydrates in the region. We argued that high and spatially limited Q anomalies in the layer above the BSR indicate the presence of gas hydrates in marine sediments in this setting. Hence, our workflow to analyze Q using high-resolution P-cable 3D seismic data with a large bandwidth could be a potential technique to detect and directly map the distribution of gas hydrates in marine sediments.

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Time–frequency analysis for delineating gas hydrates and free gas in the Mahanadi offshore, India

Exploration Geophysics ◽

10.1080/08123985.2021.1889365 ◽

2021 ◽

pp. 1-14

Author(s):

Jitender Kumar ◽

Kalachand Sain ◽

K. P. Arun

Keyword(s):

Frequency Analysis ◽

Gas Hydrates ◽

Time Frequency Analysis ◽

Free Gas ◽

Time Frequency

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Geophysical methods to quantify gas hydrates and free gas in the shallow subsurface: Review and Outlook

DGG/EAGE Workshop - Geophysics for Unconventionals ◽

10.3997/2214-4609.20143844 ◽

2012 ◽

Author(s):

Christian Berndt

Keyword(s):

Gas Hydrates ◽

Geophysical Methods ◽

Free Gas ◽

Shallow Subsurface

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Velocity-attenuation model from check-shot drift trends in North Sea well data

Geophysics ◽

10.1190/geo2019-0419.1 ◽

2020 ◽

Vol 85 (2) ◽

pp. D65-D74 ◽

Cited By ~ 1

Author(s):

Andrew J. Carter ◽

Veronica A. Torres Caceres ◽

Kenneth Duffaut ◽

Alexey Stovas

Keyword(s):

North Sea ◽

Velocity Dispersion ◽

Seismic Attenuation ◽

P Wave ◽

Linear Wave ◽

Velocity Models ◽

Velocity Attenuation ◽

Attenuation Models ◽

Well Data ◽

Attenuation Values

Seismic attenuation distorts phase and narrows bandwidth in seismic surveys. It is also an exploration attribute, as, for example, gas or overpressure, may create attenuation anomalies. Compensating attenuation in imaging requires accurate models. Detailed attenuation models may be obtained using full-waveform inversion (FWI) or attenuation tomography, but their accuracy benefits from reliable starting models and/or constraints. Seismic attenuation and velocity dispersion are necessarily linked for causal linear wave propagation such that higher frequencies travel faster than lower frequencies in an attenuative medium. In publicly released well data from the Norwegian North Sea, we have observed systematic positive linear trends in check-shot drift when comparing (lower frequency) time-depth curves with (higher frequency) integrated sonic transit times. We observe velocity dispersion consistent with layers having constant seismic attenuation. Adapting a previously published method, and assuming an attenuation-dispersion relationship, we use drift gradients, measured over thick stratigraphic units, to estimate interval P-wave attenuation and tentatively interpret its variation in terms of porosity and fluid mobility. Reflectivity modeling predicts a very low attenuation contribution from peg-leg multiples. We use the attenuation values to develop a simple regional relationship between P-wave velocity and attenuation. Observed low drift gradients in some shallower units lead to an arch-shaped model that predicts low attenuation at both low and high velocities. The attenuation estimates were broadly comparable with published effective attenuation values obtained independently nearby. This general methodology for quickly deriving a regional velocity-attenuation relationship could be used anywhere that coincident velocity models are available at seismic and sonic frequencies. Such relationships can be used for fast derivation (from velocities) of starting attenuation models for FWI or tomography, constraining or linking velocity and attenuation in inversion, deriving models for attenuation compensation in time processing, or deriving background trends in screening for attenuation anomalies in exploration.

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Seismic and seafloor evidence for free gas, gas hydrates, and fluid seeps on the transform margin offshore Cape Mendocino

Journal of Geophysical Research Atmospheres ◽

10.1029/2001jb001679 ◽

2003 ◽

Vol 108 (B5) ◽

Cited By ~ 17

Author(s):

Anne M. Tréhu ◽

Debra S. Stakes ◽

Cindy D. Bartlett ◽

Johanna Chevallier ◽

Robert A. Duncan ◽

...

Keyword(s):

Gas Hydrates ◽

Free Gas ◽

Transform Margin

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Seismic attenuation and velocity dispersion in heterogeneous partially saturated porous rocks

Geophysical Journal International ◽

10.1111/j.1365-246x.2011.05291.x ◽

2011 ◽

Vol 188 (3) ◽

pp. 1088-1102 ◽

Cited By ~ 50

Author(s):

J. Germán Rubino ◽

Klaus Holliger

Keyword(s):

Velocity Dispersion ◽

Seismic Attenuation ◽

Porous Rocks ◽

Partially Saturated

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Characterising Gas Hydrate Deposits on New Zealand's Southern Hikurangi Margin using Seismic Reflection Data

10.26686/wgtn.17057975.v1 ◽

2021 ◽

Author(s):

◽

Hanyan Wang

Keyword(s):

Gas Hydrates ◽

Gas Hydrate ◽

Research Area ◽

Low Velocity ◽

Seismic Reflection Data ◽

Free Gas ◽

Reflection Data ◽

Geological Conditions ◽

Gas Hydrate Deposits ◽

Seismic Lines

<p>Reprocessed Bruin 2D seismic data (recorded in 2006) from New Zealand Hikurangi Margin are presented and analyzed to show the presence of gas hydrates. We choose six seismic lines that each showed bottom-simulating reflections (BSRs) that are important indicators for the presence of gas hydrate. The aim is to obtain a higher resolution image of the shallow subsurface structures and determine the nature of the gas hydrate system in this area. To further investigate the presence of Gas Hydrates was undertaken. There is a strong correlation between anomalous velocities and the depths of BSRs, which supports the presence of gas hydrates in the research area and is useful for detecting areas of both free gas and gas hydrate along the seismic lines. The combination of high-resolution seismic imaging and velocity analysis is the key method for showing the distribution of gas hydrates and gas pockets in our research area. The results indicate that the distribution of both free gas and gas hydrate is strongly localized. The Discussion Chapter gives several concentrated gas hydrate deposits in the research area. Idealized scenarios for the formation of the gas hydrates are proposed. In terms of identifying concentrated gas hydrate deposits we propose the identification of the following key seismic attributes: 1) existence of BSRs, 2) strong reflections above BSRs in the gas hydrate stability zone, 3) enhanced reflections related to free gas below BSRs, 4) appropriate velocity anomalies (i.e. low velocity zones beneath BSRs and localized high-velocity zones above BSRs). This study contributes to the understanding of the geological conditions and processes that drives the deposition of concentrated gas hydrate deposits on this part of the Hikurangi Margin.</p>

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Numerical Modeling of Fluid Effects on Seismic Properties of Fractured Magmatic Geothermal Reservoirs

10.5194/se-2016-144 ◽

2016 ◽

Author(s):

Melchior Grab ◽

Beatriz Quintal ◽

Eva Caspari ◽

Hansruedi Maurer ◽

Stewart Greenhalgh

Keyword(s):

Numerical Modeling ◽

Velocity Dispersion ◽

Seismic Velocity ◽

Seismic Attenuation ◽

Geothermal Reservoir ◽

Modeling Technique ◽

Hydraulic Permeability ◽

Seismic Properties ◽

Fluid Content ◽

Geothermal Reservoirs

Abstract. Seismic investigations of geothermal reservoirs over the last 20 years have sought to interpret the resulting tomograms and reflection images in terms of the degree of reservoir fracturing and fluid content. Since the former provides the pathways and the latter acts as the medium for transporting geothermal energy, such information is needed to evaluate the quality of the reservoir. In conventional rock physics-based interpretations, this hydro-mechanical information is approximated from seismic velocities computed at the low frequency (field-based) and high frequency (lab-based) limits. In this paper, we demonstrate how seismic properties of fluid-filled, fractured reservoirs can be modeled over the full frequency spectrum using a numerical simulation technique which has become popular in recent years. It is based on Biot's theory of poroelasticity and enables the modeling of the seismic velocity dispersion and the frequency dependent seismic attenuation due to wave-induced fluid flow. These properties are sensitive to key parameters such as the hydraulic permeability of fractures as well as the compressibility and viscosity of the pore fluids. Applying the poroelastic modeling technique to the specific case of a magmatic geothermal system under stress due to the weight of the overlying rocks requires careful parameterization of the model. This includes consideration of the diversity of rock types occurring in the magmatic system and examination of the confining pressure-dependency of each input parameter. After the evaluation of all input parameters, we use our modeling technique to determine the seismic attenuation factors and phase velocities of a rock containing a complex interconnected fracture network, whose geometry is based on a fractured geothermal reservoir in Iceland. Our results indicate that in a magmatic geothermal reservoir the overall seismic velocity structure mainly reflects the lithological heterogeneity of the system, whereas indicators for reservoir permeability and fluid content are deducible from the magnitude of seismic attenuation and the critical frequency at which the peak of attenuation and maximum velocity dispersion occur. The study demonstrates how numerical modeling provides a valuable tool to overcome interpretation ambiguity and to gain a better understanding of the hydrology of geothermal systems, which are embedded in a highly heterogeneous host medium.

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