Crosswell seismic imaging in a contaminated basalt aquifer

Geophysics ◽  
2004 ◽  
Vol 69 (1) ◽  
pp. 16-24 ◽  
Author(s):  
Thomas M. Daley ◽  
Ernest L. Majer ◽  
John E. Peterson
Keyword(s):  
Seismic Source ◽  
P Wave ◽  
Low Velocity ◽  
S Waves ◽  
High Attenuation ◽  
Well Spacing ◽  
Simultaneous Acquisition ◽  
Environmental Laboratory ◽  
New Type ◽  
Borehole Seismic

Multiple seismic crosswell surveys have been acquired and analyzed in a fractured basalt aquifer at Idaho National Engineering and Environmental Laboratory. Most of these surveys used a high‐frequency (1000–10,000 Hz) piezoelectric seismic source to obtain P‐wave velocity tomograms. The P‐wave velocities range from less than 3200 m/s to more than 5000 m/s. Additionally, a new type of borehole seismic source was deployed as part of the subsurface characterization program at this contaminated groundwater site. This source, known as an orbital vibrator, allows simultaneous acquisition of P‐ and S‐waves at frequencies of 100 to 400 Hz, and acquisition over larger distances. The velocity tomograms show a relationship to contaminant transport in the groundwater; zones of high contaminant concentration are coincident with zones of low velocity and high attenuation and are interpreted to be fracture zones at the boundaries between basalt flows. The orbital vibrator data show high Vp/Vs values, from 1.8 to 2.8. In spite of the lower resolution of orbital vibrator data, these data were sufficient for constraining hydrologic models at this site while achieving imaging over large interwell distances. The combination of piezoelectric data for closer well spacing and orbital vibrator data for larger well spacings has provided optimal imaging capability and has been instrumental in our understanding of the site aquifer's hydrologic properties and its scale of heterogeneity.

Orbital vibrator seismic source for simultaneous P‐ and S‐wave crosswell acquisition

Geophysics ◽  
2001 ◽  
Vol 66 (5) ◽  
pp. 1471-1480 ◽  
Author(s):  
Thomas M. Daley ◽  
Dale Cox
Keyword(s):  
Seismic Energy ◽  
Seismic Source ◽  
P Wave ◽  
S Wave ◽  
Circularly Polarized ◽  
S Waves ◽  
Polarized Waves ◽  
Coordinate Rotation ◽  
Processing Algorithms ◽  
Borehole Seismic

A recently developed borehole seismic source, the orbital vibrator, was successfully deployed in a crosswell survey in a fractured basalt aquifer. This seismic source uses a rotating eccentric mass to generate seismic energy. Source sweeps with clockwise and counter‐clockwise rotations are recorded at each source location. Because this source generates circularly polarized waves, unique processing algorithms are used to decompose the recordings into two equivalent linearly oscillating, orthogonally oriented seismic sources. The orbital vibrator therefore generates P‐ and S‐waves simultaneously for all azimuths. A coordinate rotation based on P‐wave particle motion is used to align the source components from various depths. In a field experiment, both P‐ and S‐wave arrivals were recorded using fluid‐coupled hydrophone sensors. The processed field data show clear separation of P‐ and S‐wave arrivals for in‐line and crossline source components, respectively. A tensor convolutional description of the decomposition process allows for extension to multicomponent sensors.

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.

scholarly journals Relative moment tensors and deep Yakutat seismicity

2019 ◽  
Vol 219 (2) ◽  
pp. 1447-1462 ◽  
Author(s):  
Alexandre P Plourde ◽  
Michael G Bostock
Keyword(s):  
Principal Components ◽  
Moment Tensor ◽  
Synthetic Data ◽  
Principal Component ◽  
P Wave ◽  
Inversion Method ◽  
Low Velocity ◽  
Stress Regime ◽  
Moment Tensors ◽  
S Waves

SUMMARY We introduce a new relative moment tensor (MT) inversion method for clusters of nearby earthquakes. The method extends previous work by introducing constraints from S-waves that do not require modal decomposition and by employing principal component analysis to produce robust estimates of excitation. At each receiver, P and S waves from each event are independently aligned and decomposed into principal components. P-wave constraints on MTs are obtained from a ratio of coefficients corresponding to the first principal component, equivalent to a relative amplitude. For S waves we produce constraints on MTs involving three events, where one event is described as a linear combination of the other two, and coefficients are derived from the first two principal components. Nonlinear optimization is applied to efficiently find best-fitting tensile-earthquake and double-couple solutions for relative MT systems. Using synthetic data, we demonstrate the effectiveness of the P and S constraints both individually and in combination. We then apply the relative MT inversion to a set of 16 earthquakes from southern Alaska, at ∼125 km depth within the subducted Yakutat terrane. Most events are compatible with a stress tensor dominated by downdip tension, however, we observe several pairs of earthquakes with nearly antiparallel slip implying that the stress regime is heterogeneous and/or faults are extremely weak. The location of these events near the abrupt downdip termination of seismicity and the low-velocity zone suggest that they are caused by weakening via grain-size and volume reduction associated with eclogitization of the lower crustal gabbro layer.

Tunnel signature prediction for a cross‐borehole seismic survey

Geophysics ◽  
1995 ◽  
Vol 60 (1) ◽  
pp. 76-86 ◽  
Author(s):  
Richard D. Rechtien ◽  
Roy J. Greenfield ◽  
Robert F. Ballard
Keyword(s):  
Cross Section ◽  
Field Data ◽  
Rectangular Cross Section ◽  
Circular Cross Section ◽  
Data Interpretation ◽  
P Wave ◽  
Seismic Survey ◽  
S Waves ◽  
Borehole Seismic ◽  
Tunnel Size

Seismic location of tunnels or voids with a cross‐borehole survey is examined with field data and theory. The field data were taken at a site with a 2.2-m high by 2.7-m wide, roughly rectangular cross‐section tunnel, using a newly developed 1 to 5 kHz system employing a P‐wave sparker source. The synthetic records were obtained using a 2.5-D boundary‐valued solution for an explosive point source near a cylindrical void, and the solution was evaluated with the method of steepest descent. The synthetic waveforms compared well to the field data; both showed a maximum reduction of amplitude in the tunnel shadow of 8 dB and a maximum first arrival delay of 0.1 ms. Additional theoretical modeling was used to examine the variations of the received signals with tunnel size and frequency and showed amplitude reduction increased with frequency and tunnel size. Calculations showed that S‐waves scattered from the tunnel are more than 20 dB smaller than the primary P‐wave on hydrophones and more than 12 dB smaller on particle velocity sensors and so could be difficult to see in field data. The close comparison of synthetic waveforms to the field data indicate that the cylindrical model can be used to model data for roughly square cross‐section tunnels or voids, as well as for circular cross‐section tunnels, and thus is useful for data interpretation and survey planning.

scholarly journals New insights into Mt. Vesuvius hydrothermal system and its dynamic based on a critical review of seismic tomography and geochemical features

2013 ◽  
Vol 56 (4) ◽  
Author(s):  
Edoardo Del Pezzo ◽  
Giovanni Chiodini ◽  
Stefano Caliro ◽  
Francesca Bianco ◽  
Rosario Avino
Keyword(s):  
Hydrothermal System ◽  
Seismic Velocity ◽  
Seismic Energy ◽  
P Wave ◽  
Depth Range ◽  
Low Velocity ◽  
High Attenuation ◽  
Amplitude Spectra ◽  
Wave Travel ◽  
Attenuation Values

<p>The seismic velocity and attenuation tomography images, calculated inverting respectively P-wave travel times and amplitude spectra of local VT quakes at Mt. Vesuvius have been reviewed and graphically represented using a new software recently developed using Mathematica<span><sup>8TM</sup></span>. The 3-D plots of the interpolated velocity and attenuation fields obtained through this software evidence low-velocity volumes associated with high attenuation anomalies in the depth range from about 1 km to 3 km below the sea level. The heterogeneity in the distribution of the velocity and attenuation values increases in the volume centred around the crater axis and laterally extended about 4 km, where the geochemical interpretation of the data from fumarole emissions reveals the presence of a hydrothermal system with temperatures as high as 400-450°C roughly in the same depth range (1.5 km to 4 km). The zone where the hydrothermal system is space-confined possibly hosted the residual magma erupted by Mt. Vesuvius during the recent eruptions, and is the site where most of the seismic energy release has occurred since the last 1944 eruption.</p>

scholarly journals Inside Mt. Vesuvius: a new method to look at the seismic (velocity and attenuation) tomographic imaging

2013 ◽  
Vol 56 (4) ◽  
Author(s):  
Edoardo Del Pezzo ◽  
Francesca Bianco
Keyword(s):  
Graphical Representation ◽  
Seismic Velocity ◽  
P Wave ◽  
Low Velocity ◽  
Geological Structures ◽  
High Attenuation ◽  
Amplitude Spectra ◽  
Vertical Slice ◽  
3D Volume ◽  
Color Scales

<p>New velocity and attenuation images of the geological structures below Mt. Vesuvius have been obtained using the programming facilities as well as the enhanced graphical power of Mathematica<span><sup>8TM</sup></span>. The velocity and attenuation space distributions, already calculated inverting respectively P-wave travel times and amplitude spectra of local VT quakes, are first optimally interpolated and then graphically represented in a new Mathematica<span><sup>8TM</sup></span> code notebook (a powerful computational document with more facilities than a simple code) developed by the present authors. The notebook aims at interactively and friendly representing 3D volume distributions of velocity and attenuation parameters. The user can easily obtain vertical sections (N-S, E-W, NE-SW and NW-SE oriented) and define color scales to represent velocity or attenuation variations or prefer iso-surface plots to represent the pattern of peculiar geological structures. The use of dynamic graphical representation, allowing the sliding of any (horizontal and/or vertical) slice through the volume under study, gives an unusual and powerful vision of any small velocity or attenuation anomaly. The (open source) code, coupled with the friendly use of internal routines of Mathematica, allows to adapt the graphical representation to any user necessity. The method appears to be particularly adapt to represent attenuation images, where the space variations of the parameters are strong with respect to their average. The 3-D plots of the interpolated velocity and attenuation fields enhance the image of Mt. Vesuvius structure, evidencing low-velocity associated with high attenuation anomalies which appeared unfocused in the plots reported by Scarpa et al. [2002] and De Siena et al. [2009].</p>

Travel times and amplitudes of S waves from nuclear explosions in Nevada

1969 ◽  
Vol 59 (1) ◽  
pp. 385-398 ◽  
Author(s):  
Otto W. Nuttli
Keyword(s):  
Stress Field ◽  
Body Wave ◽  
P Wave ◽  
Travel Times ◽  
S Wave ◽  
Low Velocity ◽  
Time Curve ◽  
Regional Stress ◽  
S Waves ◽  
Regional Stress Field

Abstract The underground Nevada explosions HALF-BEAK and GREELEY were unique in creating relatively large amplitude and long-period body S waves which could be detected at teleseismic distances. Observations of the travel times of these S waves provide a surface focus travel-time curve which in its major features is similar to a curve calculated from the upper mantle velocity model of Ibrahim and Nuttli (1967). This model includes a low-velocity channel at a depth of 150 to 200 km and regions of rapidly increasing velocity beginning at depths of 400 and 750 km. Observations of the S wave amplitudes suggest that a discontinuous increase in velocity occurs at 400 km, whereas at 750 km the velocity is continuous but the velocity gradient discontinuous. Body wave magnitudes calculated from S amplitudes are 5.3 ± 0.2 for GREELEY and 4.9 ± 0.2 for HALF-BEAK. These are about one unit less than body wave magnitudes from P amplitudes as reported by others. The shape and orientation of the radiation pattern of SH for both explosions are consistent with the Rayleigh and P-wave amplitude distribution of BILBY as given by Toksoz and Clermont (1967). This suggests that the regional stress field is the same at all three sites, and that the direction of cracking as well as the strain energy release in the elastic zone outside the cavity is determined by the regional stress field.

Investigation of multiple reflections and wave conversion by means of a vertical wave test (vertical seismic profiling) in southern Mississippi

Geophysics ◽  
1982 ◽  
Vol 47 (7) ◽  
pp. 977-1000 ◽  
Author(s):  
C. C. Lash
Keyword(s):  
Wave Train ◽  
Low Frequency ◽  
P Wave ◽  
Multiple Reflections ◽  
Low Velocity ◽  
P Waves ◽  
S Waves ◽  
Seismic Profiling ◽  
Vertical Seismic Profiling ◽  
Sonic Log

A vertical wave test employing the vertical seismic profiling (VSP) technique in southern Mississippi confirmed suspicions that apparent multiple reflections might include converted waves as well as multiply reflected compressional waves. Both compressional (P) and shear (S) waves generated near the source were observed to travel to great depths, and P‐to‐S conversions were apparent in deep zones as well as shallow. P‐wave reflections were observed in agreement with predictions from synthetic records based on the sonic log. Up‐traveling P‐waves were reflected a short distance below the surface, at the base of the low‐velocity layer, and were followed as down‐traveling P‐waves to 200 ft depth by means of a vertical spread. Below 2000 ft and following the first P wave train, the predominate energy appeared to be down‐traveling P‐waves which could not be traced back to the reflection of up‐traveling P‐waves. The continuity of wavelets indicated instead that the strong down‐traveling S‐waves generated near the source produced P‐waves by S‐to‐P conversion somewhere in the zone between 800 and 1400 ft. The interference on the recordings made with an individual seismometer, or a small group of seismometers, using dynamite shots as the source was generally of a low‐frequency nature, so that the signal‐to‐noise (S/N) ratio was improved by the use of a high passband filter. The interference was greatly reduced without the need for a filter on recordings in which the source was a distributed charge of 100 ft length. The distributed charge produced much less shear‐wave energy in the P reflection band, demonstrating that the interference encountered when using a concentrated charge source was the consequence of the generation of S‐waves near the source. The distributed charges were previously chosen as a means for effectively eliminating secondary (ghost) reflections, an unwanted form of multiple reflections.

scholarly journals Fault-zone waves observed at the southern Joshua Tree earthquake rupture zone

1994 ◽  
Vol 84 (3) ◽  
pp. 761-767
Author(s):  
S. E. Hough ◽  
Y. Ben-Zion ◽  
P. Leary
Keyword(s):  
Fault Zone ◽  
Wave Train ◽  
Spectral Characteristics ◽  
P Wave ◽  
S Wave ◽  
Low Velocity ◽  
Zone Structure ◽  
High Attenuation ◽  
Vertical Fault ◽  
Synthetic Waveform

Abstract Waveform and spectral characteristics of several aftershocks of the M 6.1 22 April 1992 Joshua Tree earthquake recorded at stations just north of the Indio Hills in the Coachella Valley can be interpreted in terms of waves propagating within narrow, low-velocity, high-attenuation, vertical zones. Evidence for our interpretation consists of: (1) emergent P arrivals prior to and opposite in polarity to the impulsive direct phase; these arrivals can be modeled as headwaves indicative of a transfault velocity contrast; (2) spectral peaks in the S wave train that can be interpreted as internally reflected, low-velocity fault-zone wave energy; and (3) spatial selectivity of event-station pairs at which these data are observed, suggesting a long, narrow geologic structure. The observed waveforms are modeled using the analytical solution of Ben-Zion and Aki (1990) for a plane-parallel layered fault-zone structure. Synthetic waveform fits to the observed data indicate the presence of NS-trending vertical fault-zone layers characterized by a thickness of 50 to 100 m, a velocity decrease of 10 to 15% relative to the surrounding rock, and a P-wave quality factor in the range 25 to 50.

scholarly journals A study of the Northwestern Pacific upper mantle

1965 ◽  
Vol 55 (5) ◽  
pp. 925-939
Author(s):  
Daniel A. Walker
Keyword(s):  
Upper Mantle ◽  
Fundamental Problem ◽  
Body Wave ◽  
Lower Boundary ◽  
P Wave ◽  
Northwestern Pacific ◽  
Travel Times ◽  
Low Velocity ◽  
S Waves ◽  
Wave Channel

abstract A fundamental problem of earthquake seismology is the occurrence of the upper mantle low-velocity channel. This study is intended to examine its existence in the upper mantle below the Northwestern Pacific on the basis of body-wave arrivals at a bottom-mounted hydrophone near Wake Island. A comparison of the observed travel times and the Jeffreys-Bullen travel times shows an extreme anomaly in the 21- to 33-degree range for both P and S waves. Assumed linear paths suggest a P-wave-channel upper boundary between 165 km and 185 km, and a lower boundary between 290 km and 542 km. Travel times for P and S waves indicate that the velocities in the channel remain constant at 8.1 km/sec and 4.65 km/sec respectively.

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