Simulation of scattered seismic surface waves on mountainous onshore areas: Understanding the “ground roll energy cone”

2021 ◽  
Vol 40 (8) ◽  
pp. 601-609
Author(s):  
Ivan Javier Sánchez-Galvis ◽  
Jheyston Serrano ◽  
Daniel A. Sierra ◽  
William Agudelo
Keyword(s):  
Surface Waves ◽  
Synthetic Data ◽  
Real Data ◽  
Small Scale ◽  
Low Velocity ◽  
Near Surface ◽  
Seismic Surface Waves ◽  
Ground Roll ◽  
Low Velocity Layer ◽  
Velocity Layer

The accurate simulation of seismic surface waves on complex land areas requires elastic models with realistic near-surface parameters. The SEAM Phase II Foothills model, proposed by the SEG Advanced Modeling (SEAM) Corporation, is one of the most comprehensive efforts undertaken by the geophysics community to understand complex seismic wave propagation in foothills areas. However, while this model includes a rough topography, alluvial sediments, and complex geologic structures, synthetic data from the SEAM consortium do not reproduce the qualitative characteristics of the scattering energy that is generally interpreted as the “ground roll energy cone” on shot records of real data. To simulate the scattering, a near-surface elastic model in mountainous areas ideally must include the following three elements: (1) rough topography and bedrock, (2) low-velocity layer, and (3) small-scale heterogeneities (size approximately Rayleigh wavelength). The SEAM Foothills model only includes element (1) and, to a lesser extent, element (2). We represent a heterogeneous near surface as a random medium with two parameters: the average size of the heterogeneities and fractional fluctuation. A parametric analysis shows the influence of each parameter on the synthetic data and how similar it is compared to real data acquired in a foothills area in Colombia. We perform the analysis in the shot gather panel and dispersion image. Our study shows that it is necessary to include the low-velocity layer and small-scale distributed heterogeneities in the shallow part of the SEAM model to get synthetic data with realistic scattered surface-wave energy.

Normal modes of continental surface waves

1958 ◽  
Vol 48 (1) ◽  
pp. 33-49 ◽  
Author(s):  
Jack Oliver ◽  
Maurice Ewing
Keyword(s):  
Surface Waves ◽  
Normal Modes ◽  
Wave Dispersion ◽  
Low Velocity ◽  
Surface Wave Dispersion ◽  
Mode Dispersion ◽  
Seismic Surface Waves ◽  
Low Velocity Layer ◽  
Rayleigh Mode ◽  
Velocity Layer

Abstract When the path between epicenter and station traverses only continental structure, the dispersion of the entire train of directly arriving seismic surface waves can be explained as the result of normal mode propagation in a crust-mantle system in which the velocity increases in some manner with depth within the crust. At least four modes, the Rayleigh mode, Sezawa's M2 mode, and the first two Love waves, may appear prominently on the seismogram. The characteristics of the higher-mode dispersion curves permit the explanation of the Lg phase of Press and Ewing, B䳨's Lg1 and Lg2, and, in some cases, Caloi's Sa without recourse to a low-velocity layer in the crust or mantle. Speculation on changes in these curves for less simplified models indicates that the remaining cases of Sa as well as Leet's C or coupled wave may be explained by classical theory. The occurrence of the higher-mode waves is widespread; they are found on the four continents for which data are available. Higher-mode data, particularly when combined with information from the fundamental modes, make surface-wave dispersion, previously a useful tool, a much more potent method for the study of crustal structure.

RELATION OF SEISMIC CORRECTIONS TO SURFACE GEOLOGY

Geophysics ◽  
1952 ◽  
Vol 17 (2) ◽  
pp. 218-228 ◽  
Author(s):  
H. M. Thralls ◽  
R. W. Mossman
Keyword(s):  
Seismic Data ◽  
Reference Plane ◽  
Illinois Basin ◽  
Low Velocity ◽  
Near Surface ◽  
Low Velocity Layer ◽  
Surface Geology ◽  
Velocity Layer ◽  
Layer Problem

The arbitrary application of any set type of near‐surface corrections to seismic data can lead to erroneous results. The determination of the type of correction to be used must be based, in part, on the type of formations present in the near‐surface. Case studies are offered to illustrate conditions arising in areas of youthful and mature topography. Specifically, they deal with a complex low velocity layer problem in a river valley, a pre‐glacial topography in the Illinois Basin, a problem arising in a mature topography in Kansas, and a youthful topography in central Wyoming. In such cases, the use of a “floating” elevation reference plane is advocated for the “Correction Zone” lying immediately below the surface.

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.

Leaking modes in a crust with a surface layer

1971 ◽  
Vol 61 (1) ◽  
pp. 93-107 ◽  
Author(s):  
Anton M. Dainty
Keyword(s):  
Dispersion Curves ◽  
Source Depth ◽  
Low Velocity ◽  
Surface Source ◽  
Low Frequencies ◽  
Near Surface ◽  
High Frequencies ◽  
Explosive Source ◽  
Low Velocity Layer ◽  
Velocity Layer

abstract Dispersion curves, attenuation functions and excitation functions for an explosive source at depth for four different models of the crust are presented for the leaking modes P(+, −), P(−, +) and π1(−, +). One of the objectives of the calculations was to determine the effect of a surface, low-velocity layer on the dispersion curves and attenuation functions. For the mode P(+, −) (the fundamental leaking mode), the differences are slight, while more pronounced differences are found for the other modes. The variation of the excitation function with depth of the source has been studied. For the modes P(+, −), P(−, +) low frequencies are enhanced and high frequencies suppressed for one of the models as the source depth increases. According to this study, a source deep in the crust should be a more efficient exciter of the mode P(+, −) (the most commonly seen mode) than a near-surface source.

Test of the upper mantle low velocity layer in Siberia with surface waves

2006 ◽  
Vol 416 (1-4) ◽  
pp. 113-131 ◽  
Author(s):  
Antonella Pontevivo ◽  
Hans Thybo
Keyword(s):  
Surface Waves ◽  
Upper Mantle ◽  
Low Velocity ◽  
Low Velocity Layer ◽  
Velocity Layer

Surface waves confined to a thin low-velocity layer

1976 ◽  
Vol 6 (5) ◽  
pp. 539-546 ◽  
Author(s):  
I. A. Molotkov ◽  
S. S. Sardarov
Keyword(s):  
Surface Waves ◽  
Low Velocity ◽  
Low Velocity Layer ◽  
Velocity Layer

scholarly journals TWO‐DIMENSIONAL MODEL SEISMOLOGY

Geophysics ◽  
1954 ◽  
Vol 19 (2) ◽  
pp. 202-219 ◽  
Author(s):  
Jack Oliver ◽  
Frank Press ◽  
Maurice Ewing
Keyword(s):  
High Velocity ◽  
Rayleigh Waves ◽  
Low Cost ◽  
Shear Velocity ◽  
Small Scale ◽  
Two Dimensional ◽  
Low Velocity ◽  
Low Velocity Layer ◽  
High Velocity Layer ◽  
Velocity Layer

The solutions of many problems in seismology may be obtained by means of ultrasonic pulses propagating in small scale models. Thin sheets, serving as two dimensional models, are particularly advantageous because of their low cost, availability, ease of fabrication into various configurations, lower energy requirements, and appropriate dilatational‐to‐shear‐velocity ratios. Four examples are presented: flexural waves in a sheet, Rayleigh waves in a low velocity layer overlying a semi‐infinite high velocity layer, Rayleigh waves in a high velocity layer overlying a semi‐infinite low velocity layer, and body and surface waves in a disk.

scholarly journals Seismic LAB or LID? The Baltic Shield case

2013 ◽  
Vol 5 (1) ◽  
pp. 699-736
Author(s):  
M. Grad ◽  
T. Tiira ◽  
S. Olsson ◽  
K. Komminaho
Keyword(s):  
Baltic Shield ◽  
Barents Sea ◽  
Reference Model ◽  
Original Data ◽  
Moho Depth ◽  
Small Scale ◽  
Low Velocity ◽  
Low Velocity Layer ◽  
The Baltic ◽  
Velocity Layer

Abstract. The problem of the asthenosphere for old Precambrian cratons, including East European Craton and its part – the Baltic Shield, is still discussed. To study the seismic lithosphere-asthenosphere boundary (LAB) beneath the Baltic Shield we used records of 9 local events with magnitudes in the range 2.7–5.9. The relatively big number of seismic stations in the Baltic Shield with a station spacing of 30–100 km permits for relatively dense recordings, and is sufficient in lithospheric scale. For modelling of the lower lithosphere and asthenosphere, the original data were corrected for topography and the Moho depth for each event and each station location, using a reference model with a 46 km thick crust. Observed P and S arrivals are significantly earlier than those predicted by the iasp91 model, which clearly indicates that lithospheric P and S velocities beneath the Baltic Shield are higher than in the global iasp91 model. For two northern events at Spitsbergen and Novaya Zemlya we observe a low velocity layer, 60–70 km thick asthenosphere, and the LAB beneath Barents Sea was found at depth of about 200 km. Sections for other events show continous first arrivals of P waves with no evidence for "shadow zone" in the whole range of registration, which could be interpreted as absence of asthenosphere beneath the central part of the Baltic Shield, or that LAB in this area occurs deeper (>200 km). The relatively thin low velocity layer found beneath southern Sweden, 15 km below the Moho, could be interpreted as small scale lithospheric inhomogeneities, rather than asthenosphere. Differentiation of the lid velocity beneath the Baltic Shield could be interpreted as regional inhomogeneity. It could also be interpreted as anisotropy of the Baltic Shield lithosphere, with fast velocity close to the east-west direction, and slow velocity close to the south-north direction.

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