scholarly journals Integrated reflection and refraction processing of an ultra-shallow seismic survey

2015 ◽  
Vol 2015 (1) ◽  
pp. 1-5
Author(s):  
Alan Meulenbroek
Keyword(s):  
Seismic Survey ◽  
Reflection And Refraction ◽  
Shallow Seismic

Improving surface‐wave group velocity measurements by energy reassignment

Geophysics ◽  
2003 ◽  
Vol 68 (2) ◽  
pp. 677-684 ◽  
Author(s):  
Helle A. Pedersen ◽  
Jérôme I. Mars ◽  
Pierre‐Olivier Amblard
Keyword(s):  
Surface Waves ◽  
Dispersion Curve ◽  
Group Velocity ◽  
Shear Wave ◽  
Group Velocity Dispersion ◽  
Seismic Survey ◽  
Earth Model ◽  
Velocity Measurements ◽  
Time Frequency ◽  
Shallow Seismic

Surface waves are increasingly used for shallow seismic surveys—in particular, in acoustic logging, environmental, and engineering applications. These waves are dispersive, and their dispersion curves are used to obtain shear velocity profiles with depth. The main obstacle to their more widespread use is the complexity of the associated data processing and interpretation of the results. Our objective is to show that energy reassignment in the time–frequency domain helps improve the precision of group velocity measurements of surface waves. To show this, full‐waveform seismograms with added white noise for a shallow flat‐layered earth model are analyzed by classic and reassigned multiple filter analysis (MFA). Classic MFA gives the expected smeared image of the group velocity dispersion curve, while the reassigned curve gives a very well‐constrained, narrow dispersion curve. Systematic errors from spectral fall‐off are largely corrected by the reassignment procedure. The subsequent inversion of the dispersion curve to obtain the shear‐wave velocity with depth is carried out through a procedure combining linearized inversion with a nonlinear Monte Carlo inversion. The diminished uncertainty obtained after reassignment introduces significantly better constraints on the earth model than by inverting the output of classic MFA. The reassignment is finally carried out on data from a shallow seismic survey in northern Belgium, with the aim of determining the shear‐wave velocities for seismic risk assessment. The reassignment is very stable in this case as well. The use of reassignment can make dispersion measurements highly automated, thereby facilitating the use of surface waves for shallow surveys.

High‐resolution shallow‐seismic experiments in sand, Part II: Velocities in shallow unconsolidated sand

Geophysics ◽  
1998 ◽  
Vol 63 (4) ◽  
pp. 1234-1240 ◽  
Author(s):  
Ran Bachrach ◽  
Jack Dvorkin ◽  
Amos Nur
Keyword(s):  
High Resolution ◽  
Velocity Profile ◽  
Contact Theory ◽  
Velocity Versus ◽  
Measured Velocity ◽  
Reflection And Refraction ◽  
Shallow Seismic ◽  
Unconsolidated Sand ◽  
Gassmann’S Equation ◽  
Vertical Velocity Profile

We conducted a shallow high‐resolution seismic reflection and refraction experiment on a sandy beach. The depth of investigation was about 2 m. We interpret the data using the Hertz‐Mindlin contact theory combined with Gassmann’s equation. These were used to obtain the vertical velocity profile. Then the profile was computed from seismic data using the turning‐rays approximation. The normal moveout (NMO) velocity at the depth of 2 m matches the velocity profile. As a result, we developed a method to invert measured velocity from first arrivals, i.e., velocity versus distance into velocity versus depth using only one adjustable parameter. This parameter contains all the information about the internal structure and elasticity of the sand. The lowest velocity observed was about 40 m/s. It is noteworthy that the theoretical lower bound for velocity in dry sand with air is as low as 13 m/s. We find that modeling sand as a quartz sphere pack does not quantitatively agree with the measured data. However, the theoretical functional form proves to be useful for the inversion.

Seismic investigations offshore South-East Greenland

1998 ◽  
pp. 145-151
Author(s):  
John R. Hopper ◽  
Dan Lizarralde ◽  
Hans Christian Larsen
Keyword(s):  
North Atlantic ◽  
Tectonic Evolution ◽  
Seismic Survey ◽  
East Greenland ◽  
The North ◽  
Shallow Seismic ◽  
Site Survey ◽  
Drilling Operations ◽  
Greenland Margin ◽  
Cooperative Venture

NOTE: This article was published in a former series of GEUS Bulletin. Please use the original series name when citing this article, for example: Hopper, J. R., Lizarralde, D., & Larsen, H. C. (1998). Seismic investigations offshore South-East Greenland. Geology of Greenland Survey Bulletin, 180, 145-151. https://doi.org/10.34194/ggub.v180.5098 _______________ A high-resolution, shallow-seismic survey off the SouthEast Greenland coast was carried out during August and September 1997 aboard the R/V Dana of the Danish Ministry of Agriculture and Fisheries. This seismic survey supports two large ongoing regional research projects. The Danish Lithosphere Centre (DLC) is involved in a number of investigations to understand the tectonic evolution of the North Atlantic region since the early Tertiary, and a consortium of scientists from the Geological Survey of Denmark and Greenland (GEUS) and the Free University of Amsterdam (VU) are engaged in palaeo-oceanographic studies of climate change since the Neogene. The survey was thus a cooperative venture where ship time was shared between the participating research institutes. This report focuses on the DLC component of the cruise, which primarily involved the acquisition of site-survey data to be used in the planning and execution of drilling operations scheduled for 1998. These drilling operations are aimed at understanding the voluminous volcanic activity that accompanied continental rifting and the formation of the South-East Greenland margin. The GEUS/VU component of the cruise is summarised elsewhere in this volume (Kuijpers et al. 1998, this volume).

Crustal structure from a marine seismic survey off the west coast of Canada

1979 ◽  
Vol 16 (6) ◽  
pp. 1265-1280 ◽  
Author(s):  
Ron M. Clowes ◽  
Stanislav Knize
Keyword(s):  
Crustal Structure ◽  
West Coast ◽  
Seismic Survey ◽  
Low Velocity ◽  
Reflected Waves ◽  
Reflection And Refraction ◽  
Imaginary Line ◽  
Marine Seismic ◽  
Oil Company ◽  
Refracted Waves

A marine seismic system for recording near-vertical incidence to wide-angle reflected waves and refracted waves has been used to obtain detailed crustal structure off Canada's west coast. Profiles about 20 km in length were recorded in three regions: the Hudson '70 survey area near 51 °N, 133 °W; west of Queen Charlotte Sound; and in northern Cascadia basin, west of central Vancouver Island. In the first area, the interpretation was completely consistent with the Hudson '70 study, but more detail was provided for the upper crust. About 0.6 km of sediments with velocity 2.4 km/s overly layers 2A and 2B with velocities of 4.0 and 5.5 km/s and thicknesses of 1.1 and 1.5 km respectively. The oceanic layer has a velocity of 6.8 km/s. Off Queen Charlotte Sound, the sediments vary in thickness from 3–3.5 km and are divided into an upper sequence with low velocities (2.1 and 2.8 km/s) and a lower sequence with higher velocities (about 4.2 km/s). Basaltic basement beneath the sediments has a velocity of 5.85 km/s. The seismic data indicate that sediment deposition has been complex, possibly interspersed with thin basalt sills derived from a nearby spreading centre. On the basis of these and other data, Winona basin is proposed to extend northwestward as far as an imaginary line drawn landward from the trough between the Dellwood Knolls. In order to test this proposal and delineate in detail the total sedimentary section, high resolution reflection studies with greater than 2 s of subbottom penetration are required. In Cascadia basin, reflection and refraction interpretations gave consistent results. The entire sedimentary sequence has low velocity values (≤2.6 km/s) and is about 1.8 km thick. A thin layer (0.4–0.7 km) of basaltic basement with velocity ~5.1 km/s lies below the sediments, and in turn is underlain by a 2 km layer with velocity ~6.1 km/s. A near-vertical incidence profile recorded in this study and a stacked record section provided by an oil company show reflections to subbottom depths of ~4.5 km, corresponding to the top of layer 3. The latter is laterally variable and poorly defined. Reflections from within layer 2 are recorded and some may be related to flows of basalt during crustal formation.

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