Effects of near‐surface waveguides on shallow high‐resolution seismic refraction and reflection data

Robertsson, Johan O. A.; Holliger, Klaus; Green, A. G.; Pugin, A.; Iaco, R. De

doi:10.1029/96gl00384

Cited by 70 publications

(28 citation statements)

References 9 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…Depending on the depth to the water table, heterogeneity of the vadose zone, and the velocity contrast across the water table, additional sources of high-amplitude, low-velocity coherent noise can be generated which interfere with deeper, primary reflections. This includes guided waves trapped between the surface and the saturated zone (Robertsson et al, 1996), interbed multiples where significant impedance contrasts exist within the vadose zone, and converted modes generated at the water table (Figures 2 and 3). Each of these noise signals travels at velocities lower than the highest P-wave velocity above the saturated zone, so that for v sat /v dr y 1, interference with FIG.…”

Section: Coherent Noisementioning

confidence: 98%

Depth characterization of shallow aquifers with seismic reflection, Part I—The failure of NMO velocity analysis and quantitative error prediction

Bradford

2002

GEOPHYSICS

View full text Add to dashboard Cite

As seismic reflection data become more prevalent as input for quantitative environmental and engineering studies, there is a growing need to assess and improve the accuracy of reflection processing methodologies. It is common for compressional-wave velocities to increase by a factor of four or more where shallow, unconsolidated sediments change from a dry or partially watersaturated regime to full saturation. While this degree of velocity contrast is rare in conventional seismology, it is a common scenario in shallow environments and leads to significant problems when trying to record and interpret reflections within about the first 30 m below the water table. The problem is compounded in shallow reflection studies where problems primarily associated with surface-related noise limit the range of offsets we can use to record reflected energy. For offset-to-depth ratios typically required to record reflections originating in this zone, the assumptions of NMO velocity analysis are violated, leading to very large errors in depth and layer thickness estimates if the Dix equation is assumed valid. For a broad range of velocity profiles, saturated layer thickness will be overestimated by a minimum of 10% if the boundary of interest is <30 m below the water table. The error increases rapidly as the boundary shallows and can be very large (>100%) if the saturated layer is <10 m thick. This degree of error has a significant and negative impact if quantitative interpretations of aquifer geometry are used in aquifer evaluation such as predictive groundwater flow modeling or total resource estimates.

show abstract

Section: Coherent Noisementioning

confidence: 98%

Depth characterization of shallow aquifers with seismic reflection, Part I—The failure of NMO velocity analysis and quantitative error prediction

Bradford

2002

GEOPHYSICS

View full text Add to dashboard Cite

show abstract

“…In many source gathers, strong low-frequency guided phases are also present, representing trapped energy within the near-surface low-velocity layer [e.g., Robertsson et al, 1996]. Strong reflections are visible only within the optimum reflection windows.…”

Section: Prestack Processingmentioning

confidence: 99%

Ultrahigh‐resolution seismic reflection imaging of the Alpine Fault, New Zealand

et al. 2009

Self Cite

View full text Add to dashboard Cite

[1] High-resolution seismic reflection surveys across active fault zones are capable of supplying key structural information required for assessments of seismic hazard and risk.We have recorded a 360 m long ultrahigh-resolution seismic reflection profile across the Alpine Fault in New Zealand. The Alpine Fault, a continental transform that juxtaposes major tectonic plates, is capable of generating large (M > 7.8) damaging earthquakes. Our seismic profile across a northern section of the fault targets fault zone structures in Holocene to late Pleistocene sediments and underlying Triassic and Paleozoic basement units from 3.5 to 150 m depth. Since ultrashallow seismic data are strongly influenced by near-surface heterogeneity and source-generated noise, an innovative processing sequence and nonstandard processing parameters are required to produce detailed information on the complex alluvial, glaciofluvial and glaciolacustrine sediments and shallow to steep dipping fault-related features. We present high-quality images of structures and deformation within the fault zone that extend and complement interpretations based on shallow paleoseismic and ground-penetrating radar studies. Our images demonstrate that the Alpine Fault dips 75°-80°to the southeast through the Quaternary sediments, and there is evidence that it continues to dip steeply between the shallow basement units. We interpret characteristic curved basement surfaces on either side of the Alpine Fault and deformation in the footwall as consequences of normal drag generated by the reverse-slip components of displacement on the fault. The fault dip and apparent $35 m vertical offset of the late Pleistocene erosional basement surface across the Alpine Fault yield a provisional dip-slip rate of 2.0 ± 0.6 mm/yr. The more significant dextral-slip rate cannot be determined from our seismic profile.

show abstract

“…Only a few full-scale 3D seismic-reflection investigations conducted in crystalline environments have been published to date; these studies were acquired almost exclusively in conjunction with the mining industry for purposes of detecting and delineating ore deposits ͑e.g., Milkereit et al, 2000;Adam et al, 2003͒. Crystalline environments generally are characterized by a highly variable, low-velocity soil layer overlying a rather homogeneous, high-velocity rock mass exhibiting small vertical and lateral velocity changes. The low-velocity layer introduces significant static shifts, and the prominent bedrock interface is responsible for various mode conversions that contaminate shot gathers as high-amplitude, source-generated noise ͑Robertsson et al, 1996a; Robertsson et al, 1996b;Holliger and Robertsson, 1998͒. The source-generated noise can be misinterpreted as shallow reflections; thus, during processing it is essential to conduct a careful separation of shallow reflections from direct and refracted P-and SV-waves, guided phases, and surManuscript received by the Editor 15 May 2007; published online 31 October 2007. face waves.…”

Section: Introductionmentioning

confidence: 99%

Shallow 3D seismic-reflection imaging of fracture zones in crystalline rock

2007

View full text Add to dashboard Cite

A limited 3D seismic-reflection data set was used to map fracture zones in crystalline rock for a nuclear waste disposal site study. Seismic-reflection data simultaneously recorded along two roughly perpendicular profiles ͑1850 and 1060 m long͒ and with a 400ϫ 400-m receiver array centered at the intersection of the lines sampled a 680ϫ 960-m 2 area in three dimensions. High levels of source-generated noise required a processing sequence involving surface-consistent deconvolution, which effectively increased the strength of reflected signals, and a linear -p filtering scheme to suppress any remaining direct SV-wave energy. A flexible-binning scheme significantly balanced and increased the CMP fold, but the offset and azimuth distributions remain irregular; a wide azimuth range and offsets Ͻ400 m are concentrated in the center of the survey area although long offsets ͑Ͼ800 m͒ are only found at the edges of the site. Three-dimensional dip moveout and 3D poststack migration were necessary to image events with conflicting dips up to about 40°. Despite the irregular acquisition geometry and the high level of sourcegenerated noise, we obtained images rich in structural detail. Seven continuous to semicontinuous reflection events were traced through the final data volume to a maximum depth of around 750 m. Previous 2D seismic-reflection studies and borehole data indicate that fracture zones are the most likely cause of the reflections.

show abstract

Effects of near‐surface waveguides on shallow high‐resolution seismic refraction and reflection data

Cited by 70 publications

References 9 publications

Depth characterization of shallow aquifers with seismic reflection, Part I—The failure of NMO velocity analysis and quantitative error prediction

Depth characterization of shallow aquifers with seismic reflection, Part I—The failure of NMO velocity analysis and quantitative error prediction

Ultrahigh‐resolution seismic reflection imaging of the Alpine Fault, New Zealand

Shallow 3D seismic-reflection imaging of fracture zones in crystalline rock

Contact Info

Product

Resources

About