Seismic mapping and modeling of near‐surface sediments in polar areas

Geophysics ◽  
2003 ◽  
Vol 68 (2) ◽  
pp. 566-573 ◽  
Author(s):  
Tor Arne Johansen ◽  
Per Digranes ◽  
Mark van Schaack ◽  
Ida Lønne
Keyword(s):  
Water Saturation ◽  
Geological Mapping ◽  
P Wave ◽  
Unconsolidated Sediments ◽  
Polar Regions ◽  
Seismic Velocities ◽  
S Wave ◽  
Delta Front ◽  
Near Surface ◽  
Water Saturated

A knowledge of permafrost conditions is important for planning the foundation of buildings and engineering activities at high latitudes and for geological mapping of sediment thicknesses and architecture. The freezing of sediments is known to greatly affect their seismic velocities. In polar regions the actual velocities of the upper sediments may therefore potentially reveal water saturation and extent of freezing. We apply various strategies for modeling seismic velocities and reflectivity properties of unconsolidated granular materials as a function of water saturation and freezing conditions. The modeling results are used to interpret a set of high‐resolution seismic data collected from a glaciomarine delta at Spitsbergen, the Norwegian Arctic, where the upper subsurface sediments are assumed to be in transition from unfrozen to frozen along a transect landward from the delta front. To our knowledge, this is the first attempt to study pore‐fluid freezing from such data. Our study indicates that the P‐ and S‐wave velocities may increase as much as 80–90% when fully, or almost fully, water‐saturated unconsolidated sediments freeze. Since a small amount of frozen water in the voids of a porous rock can lead to large velocity increases, the freezing of sediments reduces seismic resolution; thus, the optimum resolution is obtained at locations where the sediments appear unfrozen. The reflectivity from boundaries separating sediments of slightly different porosity may depend more strongly on the actual saturation rather than changes in granular characteristics. For fully water‐saturated sediments, the P‐wave reflectivity decreases sharply with freezing, while the reflectivity becomes less affected as the water saturation is lowered. Thus, a combination of velocity and reflectivity information may reveal saturation and freezing conditions.

Seismic velocities and electrical resistivity of recent volcanics and their dependence on porosity, temperature, and water saturation

Geophysics ◽  
1981 ◽  
Vol 46 (10) ◽  
pp. 1415-1422 ◽  
Author(s):  
A. W. Ibrahim ◽  
George V. Keller
Keyword(s):  
Water Saturation ◽  
Strong Dependence ◽  
Electric Resistivity ◽  
P Wave ◽  
Seismic Velocities ◽  
Fine Grained ◽  
Wave Velocities ◽  
Electrical Resistivities ◽  
P Wave Velocities ◽  
Water Saturated

Variation of P‐wave velocities and electrical resistivities of several suites of water‐saturated recent volcanics was investigated. Both P‐velocities and resistivities exhibited strong dependence on porosity. Resistivity was also dependent upon degree of water saturation and temperature. P‐wave velocities, while showing a strong dependence on porosity, appear to be independent of water saturation and temperature. Volcanics, in general, exhibit higher resistivities compared to other igneous rocks and sediments. Electric resistivity of fine‐grained basalts is anomalously low, probably due to higher content of disseminated iron. Pyroclastics and volcanic breccia, on the other hand, exhibit higher resistivities in relation to fine‐grained basalts.

Detection of near-surface hydrocarbon seeps using P- and S-wave reflections

2016 ◽  
Vol 4 (3) ◽  
pp. SH21-SH37 ◽  
Author(s):  
Mathieu J. Duchesne ◽  
André J.-M. Pugin ◽  
Gabriel Fabien-Ouellet ◽  
Mathieu Sauvageau
Keyword(s):  
P Wave ◽  
Unconsolidated Sediments ◽  
Fluid Migration ◽  
Wave Reflections ◽  
S Wave ◽  
Abrupt Decrease ◽  
Seismic Reflection Data ◽  
Near Surface ◽  
Wave Data ◽  
Hydrocarbon Seeps

The combined use of P- and S-wave seismic reflection data is appealing for providing insights into active petroleum systems because P-waves are sensitive to fluids and S-waves are not. The method presented herein relies on the simultaneous acquisition of P- and S-wave data using a vibratory source operated in the inline horizontal mode. The combined analysis of P- and S-wave reflections is tested on two potential hydrocarbon seeps located in a prospective area of the St. Lawrence Lowlands in Eastern Canada. For both sites, P-wave data indicate local changes in the reflection amplitude and slow velocities, whereas S-wave data present an anomalous amplitude at one site. Differences between P- and S-wave reflection morphology and amplitude and the abrupt decrease in P-velocity are indirect lines of evidence for hydrocarbon migration toward the surface through unconsolidated sediments. Surface-gas analysis made on samples taken at one potential seeping site reveals the occurrence of thermogenic gas that presumably vents from the underlying fractured Utica Shale forming the top of the bedrock. The 3C shear data suggest that fluid migration locally disturbs the elastic properties of the matrix. The comparative analysis of P- and S-wave data along with 3C recordings makes this method not only attractive for the remote detection of shallow hydrocarbons but also for the exploration of how fluid migration impacts unconsolidated geologic media.

Seismological studies in a rock body at Chalk River, Ontario and their relation to fractures

1982 ◽  
Vol 19 (8) ◽  
pp. 1535-1547 ◽  
Author(s):  
C. Wright
Keyword(s):  
Wave Velocity ◽  
Test Site ◽  
P Wave ◽  
Seismic Velocities ◽  
S Wave ◽  
P Waves ◽  
Rock Body ◽  
Near Surface ◽  
Wave Velocities ◽  
P Wave Velocity

Seismological experiments have been undertaken at a test site near Chalk River, Ontario that consists of crystalline rocks covered by glacial sediments. Near-surface P and S wave velocity and amplitude variations have been measured along profiles less than 2 km in length. The P and S wave velocities were generally in the range 4.5–5.6 and 2.9–3.2 km/s, respectively. These results are consistent with propagation through fractured gneiss and monzonite, which form the bulk of the rock body. The P wave velocity falls below 5.0 km/s in a region where there is a major fault and in an area of high electrical conductivity; such velocity minima are therefore associated with fracture systems. For some paths, the P and 5 wave velocities were in the ranges 6.2–6.6 and 3.7–4.1 km/s, respectively, showing the presence of thin sheets of gabbro. Temporal changes in P travel times of up to 1.4% over a 12 h period were observed where the sediment cover was thickest. The cause may be changes in the water table. The absence of polarized SH arrivals from specially designed shear wave sources indicates the inhomogeneity of the test site. A Q value of 243 ± 53 for P waves was derived over one relatively homogeneous profile of about 600 m length. P wave velocity minima measured between depths of 25 and 250 m in a borehole correlate well with the distribution of fractures inferred from optical examination of borehole cores, laboratory measurements of seismic velocities, and tube wave studies.

Rock physics characterization of chalk by combining acoustic and electromagnetic properties

Geophysics ◽  
2021 ◽  
pp. 1-65
Author(s):  
Hemin Yuan ◽  
Majken C. Looms ◽  
Lars Nielsen
Keyword(s):  
Reservoir Characterization ◽  
Water Saturation ◽  
Rock Physics ◽  
Cost Effective ◽  
Geological Mapping ◽  
P Wave ◽  
Electromagnetic Properties ◽  
Model Parameters ◽  
Near Surface

The characterization of shallow subsurface formations is essential for geological mapping and interpretation, reservoir characterization, and prospecting related to mining/quarrying. To analyze elastic and electromagnetic properties, we characterize near-surface chalk formations deposited on a shallow seabed during the Late Cretaceous–Early Paleogene (Maastrichtian-Danian). Electromagnetic and elastic properties, both of which are related to mineralogy, porosity, and water saturation, are combined to characterize the physical properties of chalk formations. We also perform rock physics modeling of elastic velocities and permittivity and analyze their relationships. We then use measured ground penetrating radar and P-wave velocity field data to determine the key model parameters, which are essential for the validity of the models and can be used to evaluate the consolidation degree of the rocks. Based on the models, a scheme is developed to estimate the porosity and water saturation by combining the two rock physics templates. The predictions are consistent with previous findings. Our templates facilitate fast mapping of near-surface porosity and saturation distributions and represent an efficient and cost-effective method for near-surface hydrological, environmental, and petrophysical studies. In the current formulation, the method is only applicable to rock type (chalk) comprising a single mineral (pure calcite). It is possible to tailor the formulation to include more than one mineral; however, this will increase the uncertainty of the results.

Seismic velocities of unconsolidated sands: Part 2 — Influence of sorting- and compaction-induced porosity variation

Geophysics ◽  
2007 ◽  
Vol 72 (1) ◽  
pp. E15-E25 ◽  
Author(s):  
Michael A. Zimmer ◽  
Manika Prasad ◽  
Gary Mavko ◽  
Amos Nur
Keyword(s):  
Compressional Wave ◽  
P Wave ◽  
Unconsolidated Sediments ◽  
Seismic Velocities ◽  
Porosity Variation ◽  
Unconsolidated Sands ◽  
Wave Velocities ◽  
Grain Size Distributions ◽  
Gassmann Fluid Substitution ◽  
Water Saturated

Unaccounted-for porosity variation in unconsolidated sediments can cloud the interpretation of the sediment’s seismic velocities for factors such as fluid content and pressure. However, an understanding of the effects of porosity variation on the velocities can permit the remote characterization of porosity with seismic methods. We present the results of a series of measurements designed to isolate the effects of sorting- and compaction-induced porosity variation on the seismic velocities and their pressure dependences in clean, unconsolidated sands. We prepared a set of texturally similar sand and glass-bead samples with controlled grain-size distributions to cover an initial porosity range from 0.26 to 0.44. We measured the compressional- and shear-wave velocities and porosity of dry samples over a series of hydrostatic pressure cycles from [Formula: see text]. Over this rangeof porosities, the velocities of the dry samples at a given pressure vary by [Formula: see text]. However, the water-saturated compressional-wave velocities, modeled with Gassmann fluid substitution, demonstrate a consistent increase with decreasing porosity. In both the dry and water-saturated cases, the porosity trend at a given pressure is approximately described by the isostress (harmonic) average between the moduli of the highest-porosity sample at that pressure and the moduli of quartz, the predominant mineral component of the samples. Empirical power-law fit coefficients describing the pressure dependences of the dry bulk, shear, and constrained (P-wave) moduli from each sample also demonstrate no significant, systematic relationship with the porosity. The porosity dependence of the water-saturated bulk and constrained moduli is primarily contained in the empirical coefficient representing the modulus at zero pressure.

Shallow velocity structure and poisson's ratio at the Tarzana, California, strong-motion accelerometer site

1996 ◽  
Vol 86 (6) ◽  
pp. 1704-1713 ◽  
Author(s):  
R. D. Catchings ◽  
W. H. K. Lee
Keyword(s):  
Urban Areas ◽  
Velocity Structure ◽  
Strong Motion ◽  
P Wave ◽  
S Wave ◽  
Near Surface ◽  
Velocity Models ◽  
Wave Velocities ◽  
Poisson's Ratios ◽  
Poisson’S Ratios

Abstract The 17 January 1994, Northridge, California, earthquake produced strong ground shaking at the Cedar Hills Nursery (referred to here as the Tarzana site) within the city of Tarzana, California, approximately 6 km from the epicenter of the mainshock. Although the Tarzana site is on a hill and is a rock site, accelerations of approximately 1.78 g horizontally and 1.2 g vertically at the Tarzana site are among the highest ever instrumentally recorded for an earthquake. To investigate possible site effects at the Tarzana site, we used explosive-source seismic refraction data to determine the shallow (<70 m) P-and S-wave velocity structure. Our seismic velocity models for the Tarzana site indicate that the local velocity structure may have contributed significantly to the observed shaking. P-wave velocities range from 0.9 to 1.65 km/sec, and S-wave velocities range from 0.20 and 0.6 km/sec for the upper 70 m. We also found evidence for a local S-wave low-velocity zone (LVZ) beneath the top of the hill. The LVZ underlies a CDMG strong-motion recording site at depths between 25 and 60 m below ground surface (BGS). Our velocity model is consistent with the near-surface (<30 m) P- and S-wave velocities and Poisson's ratios measured in a nearby (<30 m) borehole. High Poisson's ratios (0.477 to 0.494) and S-wave attenuation within the LVZ suggest that the LVZ may be composed of highly saturated shales of the Modelo Formation. Because the lateral dimensions of the LVZ approximately correspond to the areas of strongest shaking, we suggest that the highly saturated zone may have contributed to localized strong shaking. Rock sites are generally considered to be ideal locations for site response in urban areas; however, localized, highly saturated rock sites may be a hazard in urban areas that requires further investigation.

Shallow near-surface effects

Geophysics ◽  
2016 ◽  
Vol 81 (5) ◽  
pp. T221-T231 ◽  
Author(s):  
Christine E. Krohn ◽  
Thomas J. Murray
Keyword(s):  
Time Delay ◽  
Velocity Gradient ◽  
P Wave ◽  
Unconsolidated Sediments ◽  
P Waves ◽  
Large Computer ◽  
Near Surface ◽  
S Waves ◽  
High Attenuation ◽  
Pulse Broadening

The top 6 m of the near surface has a surprisingly large effect on the behavior of P- and S-waves. For unconsolidated sediments, the P-wave velocity gradient and attenuation can be quite large. Computer modeling should include these properties to accurately reproduce seismic effects of the near surface. We have used reverse VSP data and computer simulations to demonstrate the following effects for upgoing P-waves. Near the surface, we have observed a large time delay, indicating low velocity ([Formula: see text]), and considerable pulse broadening, indicating high attenuation ([Formula: see text]). Consequently, shallowly buried geophones have greater high-frequency bandwidth compared with surface geophones. In addition, there is a large velocity gradient in the shallow near surface (factor of 10 in 5 m), resulting in the rotation of P-waves to the vertical with progressively smaller amplitudes recorded on horizontal phones. Finally, we have found little indication of a reflection or ghost from the surface, although downgoing reflections have been observed from interfaces within the near surface. In comparison, the following have been observed for upgoing S-waves: There is a small increase in the time delay or pulse broadening near the surface, indicating a smaller velocity gradient and less change in attenuation. In addition, the surface reflection coefficient is nearly one with a prominent surface ghost.

Measurement of acoustic properties of near‐surface soils using an ultrasonic waveguide

Geophysics ◽  
2004 ◽  
Vol 69 (2) ◽  
pp. 460-465 ◽  
Author(s):  
Rob Long ◽  
Thomas Vogt ◽  
Mike Lowe ◽  
Peter Cawley
Keyword(s):  
Urban Areas ◽  
Surrounding Medium ◽  
Longitudinal Mode ◽  
Acoustic Properties ◽  
P Wave ◽  
S Wave ◽  
Surface Soils ◽  
Near Surface ◽  
Experimental Laboratory ◽  
Ultrasonic Waveguide

A technique is presented that uses a circular ultrasonic waveguide to measure the bulk shear (S‐wave) and longitudinal (P‐wave) velocities of unconsolidated media, with particular application to near‐surface soils. The technique requires measuring the attenuation characteristics of the fundamental longitudinal mode that propagates along an embedded bar, from which the acoustic properties of the surrounding medium are inferred. The principles behind the technique are discussed, and the results of an experimental laboratory validation are presented, followed by details of in‐situ soil property measurements obtained at various sites in urban areas of the United Kingdom.

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