A TECHNIQUE FOR SOLVING THE LOW‐VELOCITY LAYER PROBLEM

Geophysics ◽  
1963 ◽  
Vol 28 (5) ◽  
pp. 869-876 ◽  
Author(s):  
Jay F. Thompson
Keyword(s):  
Energy Source ◽  
Surface Energy ◽  
Seismic Data ◽  
Low Velocity ◽  
Weight Drop ◽  
Low Velocity Layer ◽  
Velocity Layer ◽  
Layer Problem

The advent of surface‐energy source methods and the increasing use of shallow‐hole shot patterns to obtain seismic data have complicated the determination of datum corrections. This is particularly true when spreads are offset from the station. At the expense of seismic party time, the data usually are acquired by shooting uphole surveys or special refraction profiles. This paper describes a refraction technique which uses a weight‐drop energy source and special recording instruments to obtain this vital information.

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.

Wave‐equation velocity replacement of the low‐velocity layer for overthrust‐belt data

Geophysics ◽  
1995 ◽  
Vol 60 (2) ◽  
pp. 573-579 ◽  
Author(s):  
William A. Schneider ◽  
Lindy D. Phillip ◽  
Ernest F. Paal
Keyword(s):  
Wave Equation ◽  
Seismic Data ◽  
Water Layer ◽  
Estimation Procedure ◽  
Low Velocity ◽  
Near Surface ◽  
Replacement Procedure ◽  
Low Velocity Layer ◽  
Overthrust Belt ◽  
Velocity Layer

Seismic land data are commonly plagued by nonhyperbolic distortions induced by a variable near‐surface, low‐velocity layer (LVL). First‐arrival refraction analysis is conventionally employed to estimate the LVL geometry and velocities. Then vertical static time shifts are used to replace the LVL velocities with the more uniform, faster velocities that characterize the underlying refracting layer. This methodology has earned a good reputation as a geophysical data processing tool; however, velocity replacement with static shifts assumes that no ray bending occurred at the LVL base and that waves propagated vertically through the LVL (even though conventional refraction analysis methods, which are used to derive LVL models from seismic data, are less restrictive). These assumptions often are inadequate in thick, complex LVL situations, where resulting errors may considerably hamper a statics‐based velocity replacement procedure. Wave‐equation datuming may be used to perform LVL velocity replacement when statics are inadequate. This method extrapolates the seismic data from the surface to the LVL base with the LVL velocities. Then it extrapolates the data from the LVL base to an arbitrary datum, with the replacement velocity field. The marine analog of such a procedure has been well documented in the geophysical literature, where the object is to remove distortions caused by an irregular water layer. Application of wave‐equation datuming to land data is more difficult because of certain common characteristics of land data (irregular shooting, large data gaps, and crooked line geometry, combined with lower signal/noise) and because the LVL estimation procedure is considerably more difficult. We demonstrate wave‐equation velocity replacement on land data from a western U.S. overthrust belt. The LVL in this region was particularly thick and complicated and ideal for a wave‐theoretical velocity‐replacement procedure. Standard refraction analysis techniques were employed to estimate the LVL, then wave‐equation datuming was used to perform the velocity replacement. Our derived LVL model was not perfect, so some imaging errors were expected because wave‐equation datuming is highly dependent upon the LVL model. Nevertheless, our results show that wave‐equation datuming generally allowed better shallow reflector imaging than could be achieved with conventional statics processing.

scholarly journals Pervasive low-velocity layer atop the 410-km discontinuity beneath the northwest Pacific subduction zone: Implications for rheology and geodynamics

2021 ◽  
Vol 554 ◽  
pp. 116642
Author(s):  
Guangjie Han ◽  
Juan Li ◽  
Guangrui Guo ◽  
Walter D. Mooney ◽  
Shun-ichiro Karato ◽  
...  
Keyword(s):  
Subduction Zone ◽  
Northwest Pacific ◽  
Low Velocity ◽  
Low Velocity Layer ◽  
Velocity Layer

Focal mechanisms of earthquakes related to the 29 November 1975 Kalapana, Hawaii, earthquake: The effect of structure models

1981 ◽  
Vol 71 (3) ◽  
pp. 713-729 ◽  
Author(s):  
R. S. Crosson ◽  
E. T. Endo
Keyword(s):  
Main Shock ◽  
Horizontal Layer ◽  
Focal Mechanisms ◽  
Local Network ◽  
The South ◽  
Low Velocity ◽  
Tectonic Model ◽  
Low Velocity Layer ◽  
Using Data ◽  
Velocity Layer

abstract Initial focal mechanism determinations for the 29 November 1975 Kalapana, Hawaii, earthquake indicated discrepancy between the mechanism determined from teleseismic data by Ando and the mechanism determined using data from the local U.S. Geological Survey network surrounding the epicenter region. The resolution of this difference is crucial to correctly understand this earthquake, as well as to understand the tectonics of the south flank of Kilauea volcano. When a model with a low-velocity layer at the base of the crust is used for projection back to the focal sphere for the local network mechanisms, the discrepancy vanishes. To further investigate this result, focal mechanisms were determined using several contrasting models for a set of well-recorded earthquakes. A large number of these earthquakes have mechanisms identical to the main shock when the low-velocity layer model is used. Dispersion of P and T axes is also minimized by use of this model. A low-angle slip direction, favored for the main shock and typical of most other solutions, exhibits remarkable stability normal to the east rift zone of Kilauea. Our results suggest a tectonic model, similar in nature to that proposed by Ando, in which the south flank of Kilauea consists of a mobile block of crust which is relatively free to move laterally on a low-strength zone at about 10 km depth. Forceful injection of magma along the rift zones provides the loading stress which is released by catastrophic failure in the weak, horizontal layer in a cycle of perhaps 100 yr.

Uphole and low velocity layer (LVL) surveys

Birddog ◽  
1995 ◽  
pp. 156-172
Author(s):  
John Scott Roy
Keyword(s):  
Low Velocity ◽  
Low Velocity Layer ◽  
Velocity Layer

LOVE WAVE MODELLING AND INVERSION FOR LOW VELOCITY LAYER CASES

2006 ◽  
Author(s):  
Jamhir Safani ◽  
Adam O’Neill ◽  
Toshifumi Matsuoka
Keyword(s):  
Love Wave ◽  
Low Velocity ◽  
Wave Modelling ◽  
Low Velocity Layer ◽  
And Inversion ◽  
Velocity Layer

Sea-floor evolution: rare-earth evidence

1971 ◽  
Vol 268 (1192) ◽  
pp. 663-706 ◽  
Keyword(s):  
Rare Earth ◽  
Mantle Source ◽  
Upper Mantle ◽  
Gulf Of Aden ◽  
Low Velocity ◽  
East Pacific ◽  
Mid Ocean Ridge ◽  
Low Velocity Layer ◽  
Ocean Ridge ◽  
Velocity Layer

A systematic survey of rare-earth (r.e.) abundances in submarine tholeiitic basalts along mid-oceanic ridges has been made by neutron activation analysis. The r.e. fractionation patterns are remarkably uniform along each mid-oceanic ridge and from one ridge to another (Juan de Fuca Ridge, East Pacific and Chile Rise, Pacific-Antarctic, Mid-Indian and Carlsberg Ridge, Gulf of Aden, Red Sea Trough and Reykjanes Ridge). The patterns are all depleted in light r.e. except for three samples (Gulf of Aden and Mid-Indian Ridge) which are unfractionated relative to chondrites. They contrast markedly with tholeiitic plateau basalt which are shown to be related to the early volcanic phases associated with continental drift. Tholeiitic plateau basalts are light r.e. enriched as are most continental rocks. Mid-ocean ridge basalts are also distinguishable from spatially related oceanic shield volcanoes of tholeiitic composition (Red Sea Trough-Jebel Teir Is., East Pacific Rise-Culpepper Island). Thus on a r.e. basis there are tholeiites within tholeiites. The r.e. difference between mid-ocean ridge tholeiites and tholeiitic plateau basalts can be related to distinct thermal and tectonic régimes and consequently magmatic modes and rates of intrusions from the low velocity layer in the upper mantle. The difference between continental and oceanic volcanism appears to be triggered by: (1) presence or absence of a moving continental lithosphere over the low velocity layer, and (2) whether or not major rifts tap the low velocity layer through the lithosphere. Fractional crystallization during ascent of melts before eruption at the ridge crest does not affect appreciably the relative r.e. patterns. R.e. in mid-ocean ridge basalts appear to intrinsically reflect their distribution in the upper mantle source, i.e. the low velocity layer. Based on secondary order r.e. variation of mid-ocean ridge basalts: (1) If fractional crystallization is invoked for the small r.e. variations, up to approximately 50 % extraction of olivine and Ca-poor orthopyroxene in various combinations can be tolerated. However, only limited amount of plagioclase or Ca-rich clinopyroxene can be extracted, the former because of its effect on the abundance of Eu abundance and the latter because of its effect on the [La/Sm] e.f. ratio, alternatively. (2) If partial melting during ascent is invoked, and a minimum of 10% melting is assumed, the permissible degree of melting of originally a lherzolite upper mantle may vary between 10 and 30% . It is not possible to establish readily to what extent these two processes have been operative as they cannot be distinguished on the basis of r.e. data only. However, there is evidence indicating that both have been operative and are responsible for the small r.e. variations observed in mid-ocean ridge basalts. An attempt to correlate second order r.e. variations along or across mid-oceanic ridges with spreading rate, age, or distance from ridge crests has been made but the results are inconclusive. No r.e. secular variation of the oceanic crust is apparent. R.e. average ridge to ridge variations are attributed to small lateral inhomogeneities of the source of basalts in the low velocity layer, and to a certain extent, to its past history. The remarkable r.e. uniformity of mid-oceanic ridge tholeiites requires a unique and simple volcanic process to be operative. It calls for upward migration of melt or slush from a relatively homogeneous source in the mantle—the low velocity layer, followed by further partial melting during ascent. The model, although consistent with geophysics, may have to be reconciled with some evidence from experimental petrology. Models for r.e. composition of the upper mantle source of ridge basalt, formation of layers 2 and 3, and the moho-discontinuity, are also presented.

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