An ultrashallow SH‐wave seismic reflection experiment on a subsurface ground model

Geophysics ◽  
2001 ◽  
Vol 66 (4) ◽  
pp. 1097-1104 ◽  
Author(s):  
G. P. Deidda ◽  
R. Balia
Keyword(s):  
Water Table ◽  
Seismic Reflection ◽  
Cost Effective ◽  
Minimal Amount ◽  
Filling Material ◽  
Ground Model ◽  
Sh Wave ◽  
Low Velocity ◽  
Near Surface ◽  
Concrete Layer

An SH‐wave seismic reflection experiment was conducted to evaluate the feasibility and cost effectiveness of reflection imaging ultrashallow targets commonly encountered in engineering, groundwater, and environmental investigations. It was carried out on a purpose‐built subsurface ground model consisting of a concrete layer, at a depth from 2.85–5 m, and a low‐velocity overburden (<80 and 150 m/s for S‐ and P‐waves, respectively), constituted of filling material, with the water table 2.60 m deep. High‐quality CDP data, acquired by using a 10‐kg sledgehammer and newly designed horizontal detectors, allowed us to obtain an extremely detailed stacked section with a minimal amount of processing. Uncertainty in determining the depth and horizontal dimensions of the concrete model was estimated to be 0.2 and 0.3 m, respectively; however, the dominant frequencies lower than 150 Hz, the low‐transmission coefficient at the upper interface, and the relatively high velocity (900 m/s) of the concrete layer prevented us from resolving the layer thickness. The experiment demonstrates that when overburden materials exhibit low velocities (a common condition in near surface), the SH‐wave seismic reflection method is a reliable, detailed, and cost‐effective technique to image ultrashallow targets, even in disturbed material and below the water table.

Shear-wave seismic reflection studies of unconsolidated sediments in the near surface

Geophysics ◽  
2010 ◽  
Vol 75 (2) ◽  
pp. B59-B66 ◽  
Author(s):  
Seth S. Haines ◽  
Karl J. Ellefsen
Keyword(s):  
Water Table ◽  
Seismic Reflection ◽  
Love Wave ◽  
Unconsolidated Sediments ◽  
Sh Wave ◽  
Sh Waves ◽  
Near Surface ◽  
Strong Reflection ◽  
Bedrock Surface ◽  
Sand And Gravel

We have successfully applied of SH-wave seismic reflection methods to two different near-surface problems targeting unconsolidated sediments. At the former Fort Ord, where the water table is approximately [Formula: see text] deep, we imaged aeolian and marine aquifer and aquitard stratigraphy to a depth of approximately [Formula: see text]. We identified reflections from sand/clay and sand/silt interfaces and we mapped these interfaces along our transects. At an aggregate study site in Indiana, where the water table is at a depth of [Formula: see text], we imaged stratigraphy in alluvial sand and gravel, and observe a strong reflection from the [Formula: see text]-deep bedrock surface. In both cases, we exploited the high resolution potential of SH waves, their insensitivity to water content, and the possibility of reducing Love wave contamination by working along a roadway. We accomplished our results using only sledgehammer sources and simple data processing flows.

An example of extreme near-surface variability in shallow seismic reflection data

2016 ◽  
Vol 4 (3) ◽  
pp. SH1-SH9
Author(s):  
Steven D. Sloan ◽  
J. Tyler Schwenk ◽  
Robert H. Stevens
Keyword(s):  
Seismic Reflection ◽  
Field Data ◽  
Low Velocity ◽  
Seismic Reflection Data ◽  
Near Surface ◽  
Shallow Subsurface ◽  
Reflection Data ◽  
Shallow Seismic ◽  
Low Velocity Layer ◽  
Velocity Layer

Variability of material properties in the shallow subsurface presents challenges for near-surface geophysical methods and exploration-scale applications. As the depth of investigation decreases, denser sampling is required, especially of the near offsets, to accurately characterize the shallow subsurface. We have developed a field data example using high-resolution shallow seismic reflection data to demonstrate how quickly near-surface properties can change over short distances and the effects on field data and processed sections. The addition of a relatively thin, 20 cm thick, low-velocity layer can lead to masked reflections and an inability to map shallow reflectors. Short receiver intervals, on the order of 10 cm, were necessary to identify the cause of the diminished data quality and would have gone unknown using larger, more conventional station spacing. Combined analysis of first arrivals, surface waves, and reflections aided in determining the effects and extent of a low-velocity layer that inhibited the identification and constructive stacking of the reflection from a shallow water table using normal-moveout-based processing methods. Our results also highlight the benefits of using unprocessed gathers to pragmatically guide processing and interpretation of seismic data.

The Application of Seismic Reflection Techniques for Subsurface Mapping in the Precambrian Shield near Flin Flon, Manitoba

1975 ◽  
Vol 12 (12) ◽  
pp. 2036-2047 ◽  
Author(s):  
Z. Hajnal ◽  
Mel R. Stauffer
Keyword(s):  
Seismic Reflection ◽  
Complex Structure ◽  
Volcanic Rocks ◽  
Field Studies ◽  
Low Velocity ◽  
Near Surface ◽  
Precambrian Shield ◽  
Rock Types ◽  
Flin Flon ◽  
Subsurface Mapping

Some of the conditions necessary for the use of seismic reflection techniques for subsurface mapping in Precambrian Shield terranes have been determined from field studies carried out near Flin Flon, Manitoba.In areas of unconsolidated overburden, geoflex-type surface energy sources provide sufficient energy. However, in outcrop regions, boreholes have to be drilled to a minimum depth of 1.5 m, preferably in patterns of 2–6 holes. Explosives with a higher detonation velocity than those presently available would be useful.A near-surface, low velocity layer was discovered on top of all examined rock types, and appears to be the result of open fractures in the rock. The thickness of this layer varies from 20–44 m in the rocks studied.A velocity contrast of 783.3 m/s exists between the Amisk volcanic rocks and Missi sedimentary rocks, making reflection mapping possible. Seismic events which were interpreted as reflections were identified, near contacts between these formations in the subsurface. A fault contact between Amisk and Missi rocks has been mapped to a depth of about 1.6 km, and a normal stratigraphic contact between the Amisk and Missi Groups has been mapped to a depth of about 0.25 km.Because of the complex structure in most Precambrian Shield terranes, it is necessary to locate seismic lines carefully with respect to the geological features being studied. In particular, it is best to keep the line within a rock unit that has constant velocity throughout, and to use short lines, so that a limited number of structures are intersected.

Traveltime tomographic inversion with simultaneous static corrections — Well worth the effort

Geophysics ◽  
2009 ◽  
Vol 74 (6) ◽  
pp. WCB25-WCB33 ◽  
Author(s):  
Ari Tryggvason ◽  
Cedric Schmelzbach ◽  
Christopher Juhlin
Keyword(s):  
Seismic Reflection ◽  
Real Data ◽  
Seismic Velocities ◽  
Low Velocity ◽  
Data Set ◽  
Detailed Model ◽  
Seismic Reflection Data ◽  
Near Surface ◽  
First Arrival ◽  
Static Corrections

We have developed a first-arrival traveltime inversion scheme that jointly solves for seismic velocities and source and receiver static-time terms. The static-time terms are included to compensate for varying time delays introduced by the near-surface low-velocity layer that is too thin to be resolved by tomography. Results on a real data set consisting of picked first-arrival times from a seismic-reflection 2D/3D experiment in a crystalline environment show that the tomography static-time terms are very similar in values and distribution to refraction-static corrections computed using standard refraction-statics software. When applied to 3D seismic-reflection data, tomography static-time terms produce similar or more coherent seismic-reflection images compared to the images using corrections from standard refraction-static software. Furthermore, the method provides a much more detailed model of the near-surface bedrock velocity than standard software when the static-time terms are included in the inversion. Low-velocity zones in this model correlate with other geologic and geophysical data, suggesting that our method results in a reliable model. In addition to generally being required in seismic-reflection imaging, static corrections are also necessary in traveltime tomography to obtain high-fidelity velocity images of the subsurface.

scholarly journals Common-reflection-surface imaging of shallow and ultrashallow reflectors

Geophysics ◽  
2012 ◽  
Vol 77 (4) ◽  
pp. B177-B185 ◽  
Author(s):  
Gian Piero Deidda ◽  
Enzo Battaglia ◽  
Zeno Heilmann
Keyword(s):  
Wave Reflection ◽  
High Sensitivity ◽  
P Wave ◽  
Data Sets ◽  
Velocity Analysis ◽  
Sh Wave ◽  
Low Velocity ◽  
Near Surface ◽  
Significant Step ◽  
Common Reflection Surface

We analyzed the feasibility of the common-reflection-surface (CRS) stack for near-surface surveys as an alternative to the conventional common midpoint (CMP) stacking procedure. The data-driven, less user-interactive CRS method could be more cost efficient for shallow surveys, where the high sensitivity to velocity analysis makes data processing a critical step. We compared the results for two field data sets collected to image shallow and ultrashallow reflectors: an example of shallow P-wave reflection for targets in the first few hundred meters, and an example of SH-wave reflection for targets in the first 10 m. By processing the shallow P-wave records using the CMP method, we imaged several nearly horizontal reflectors with onsets from 60 to about 250 ms. The CRS stack produced a stacked section more suited for a subsurface interpretation, without any preliminary formal and time-consuming velocity analysis, because the imaged reflectors possessed greater coherency and lateral continuity. With CMP processing of the SH-wave records, we imaged a dipping bedrock interface below four horizontal reflectors in unconsolidated, very low velocity sediments. The vertical and lateral resolution was very high, despite the very shallow depth: the image showed the pinchout of two layers at less than 10 m depth. The numerous traces used by the CRS stack improved the continuity of the shallowest reflector, but the deepest overburden reflectors appear unresolved, with not well-imaged pinchouts. Using the kinematic wavefield attributes determined for each stacking operation, we retrieved velocity fields fitting the stacking velocities we had estimated in the CMP processing. The use of CRS stack could be a significant step ahead to increase the acceptance of the seismic reflection method as a routine investigation method in shallow and ultrashallow seismics.

High‐resolution shallow‐seismic experiments in sand, Part I: Water table, fluid flow, and saturation

Geophysics ◽  
1998 ◽  
Vol 63 (4) ◽  
pp. 1225-1233 ◽  
Author(s):  
Ran Bachrach ◽  
Amos Nur
Keyword(s):  
High Resolution ◽  
Water Table ◽  
High Tide ◽  
Saturated Sand ◽  
Low Velocity ◽  
Near Surface ◽  
Shallow Seismic ◽  
Low Velocity Layer ◽  
Dry Sand ◽  
Velocity Layer

A high‐resolution, very shallow seismic reflection and refraction experiment was conducted to investigate the seismic response of groundwater level changes in beach sand in situ. A fixed 10-m-long receiver array was used for repeated seismic profiling. Direct measurements of water level in a monitoring well and moisture content in the sand were taken as well. The water table in the well changed by about 1 m in slightly delayed response to the nearby ocean tides. In contrast, inversion of the seismic data yielded a totally different picture. The reflection from the water table at high tide appeared at a later time than the reflection at low tide. This unexpected discrepancy can be reconciled using Gassmann’s equation: a low‐velocity layer must exist between the near‐surface dry sand and the deeper and much faster fully saturated sand. This low‐velocity layer coincides with the newly saturated zone and is caused by a combination of the sand’s high density (close to that of fully saturated sand), and its high compressibility (close to that of dry sand). This low‐velocity zone causes a velocity pulldown for the high‐frequency reflections, and causes a high‐tide reflection to appear later in time than low‐tide reflection. The calculated velocities in the dry layer show changes with time that correlate with sand dryness, as predicted by the temporal changes of the sand’s density due to changing water/air ratio. The results show that near‐surface velocities in sand are sensitive to partial saturation in the transition zone between dry and saturated sand. We were able to extract the saturation of the first layer and the depth to the water table from the seismic velocities. The high‐resolution reflections monitored the flow process that occurred in the sand during the tides, and provided a real‐time image of the hydrological process.

Coal seismic surveying over near-surface basalts: Experience from Central Queensland, Australia

Geophysics ◽  
2014 ◽  
Vol 79 (2) ◽  
pp. B109-B122 ◽  
Author(s):  
Binzhong Zhou ◽  
Peter Hatherly ◽  
Troy Peters ◽  
Weijia Sun
Keyword(s):  
Surface Waves ◽  
Seismic Reflection ◽  
Unconsolidated Sediments ◽  
Successful Outcome ◽  
Low Velocity ◽  
Prestack Depth Migration ◽  
Seismic Line ◽  
Near Surface ◽  
Depth Migration ◽  
Seismic Surveying

Seismic reflection surveying in basalt-covered areas often fails to image underlying reflectors. To gain insights into the nature of the problem and obtain potential solutions, we have conducted experimental 2D seismic reflection and offset VSP surveys at two coal mines in the Bowen Basin of Australia. At the first mine, the basalt is relatively deep (114 m) and relatively thin (20 m). Conventional seismic acquisition and processing of a 2D seismic line provide poor results. However, upgoing reflections from layers below the basalt are clearly evident in the VSP survey and prestack depth migration is able to improve the continuity of the reflectors beneath the basalt. At the second mine, the 360 m wide basalt is at a depth of 40 m and has a thickness of about 40 m. It is fresh and unweathered and consists of multiple flows which are interlayered with unconsolidated sediments. Long-offset data acquisition combined with prestack depth migration was expected to produce satisfactory results but this is not the case. The associated VSP survey suggests that the problems at this mine are due to (1) the generation of complex downgoing and upgoing wave-fields within the basalt and (2) significant scattering of surface waves from outside the basalt at the margins of the basalt. Another problem is that the target coal seams are at about 300 m depth and the muting required to remove refraction events limited the effectiveness of the prestack depth migration. Reducing the strength of the surface waves through selection of an appropriate source and placement of shots at the base of the low-velocity zone (as had been the case at the first mine) will therefore improve the chances for a successful outcome. A Vibroseis survey subsequently undertaken at the second mine, which produced shot records with reduced surface waves, shows this to be the case.

Fracture-Tolerant and Shear-Resistant Interlayers for Mitigation of Reflective Cracking

2019 ◽  
Vol 2673 (10) ◽  
pp. 35-46
Author(s):  
Cristian Cocconcelli ◽  
Bongsuk Park ◽  
Jian Zou ◽  
George Lopp ◽  
Reynaldo Roque
Keyword(s):  
Asphalt Pavement ◽  
Aggregate Size ◽  
Cost Effective ◽  
Design Guidelines ◽  
Reflective Cracking ◽  
Rubber Membrane ◽  
Near Surface ◽  
Field Evidence ◽  
Cracking Performance ◽  
Interface Cracking

Reflective cracking is frequently reported as the most common distress affecting resurfaced pavements. An asphalt rubber membrane interlayer (ARMI) approach has been traditionally used in Florida to mitigate reflective cracking. However, recent field evidence has raised doubts about the effectiveness of the ARMI when placed near the surface, indicating questionable benefits to reflective cracking and increased instability rutting potential. The main purpose of this research was to develop guidelines for an effective alternative to the ARMI for mitigation of near-surface reflective cracking in overlays on asphalt pavement. Fourteen interlayer mixtures, covering three aggregate types widely used in Florida, and two nominal maximum aggregate sizes (NMAS) were designed according to key characteristics identified for mitigation of reflective cracking, that is, sufficient gradation coarseness and high asphalt content. The dominant aggregate size range—interstitial component (DASR-IC) model was used for the design of all mixture gradations. A composite specimen interface cracking (CSIC) test was employed to evaluate reflective cracking performance of interlayer systems. In addition, asphalt pavement analyzer (APA) tests were performed to determine whether the interlayer mixtures had sufficient rutting resistance. The results indicated that interlayer mixtures designed with lower compaction effort, reduced design air voids, and coarser gradation led to more cost-effective fracture-tolerant and shear-resistant (FTSR) interlayers. Therefore, preliminary design guidelines including minimum effective film thickness and maximum DASR porosity requirements were proposed for 9.5-mm NMAS (35 µm and 50%) and 4.75-mm NMAS FTSR mixtures (20 µm and 60%) to mitigate near-surface reflective cracking.

Sign in / Sign up

Export Citation Format

Share Document