scholarly journals Some improvements in subbasalt imaging using pre-stack depth migration

2010 ◽  
Vol 2 (1) ◽  
pp. 1-17 ◽  
Author(s):  
I. Flecha ◽  
R. Carbonell ◽  
R. W. Hobbs ◽  
H. Zeyen
Keyword(s):  
High Velocity ◽  
Velocity Model ◽  
Original Model ◽  
Depth Migration ◽  
Reflection Data ◽  
Long Offset ◽  
Synthetic Test ◽  
Complex Models ◽  
High Velocity Layer ◽  
Velocity Layer

Abstract. Subbasalt imaging can be improved by carefully applying pre-stack depth migration. Pre-stack depth migration requires a detailed velocity model and an accurate traveltime calculation. Ray tracing methods are fast but, often fail in calculating traveltimes in complex models, specially, when they feature high velocity contrasts. Finitte difference solutions of the eikonal are more stable and can produce a traveltime field for the whole model avoiding shadow zones. A synthetic test was carried out to check the performance of a new pre-stack depth migration algorithm in a model that features a high velocity layer surrounded by lower velocities. The results reasonably reproduce the original model. The same scheme was used to process long-offset reflection data from the Faroe Shelf where conventional techniques (stack) were insufficient to assess the structure under a basalt layer. Pre-stack depth migration produced an improved image which recovered the main features in the stacked section and allowed to identify some subbasalt coherent events.

scholarly journals Some improvements in subbasalt imaging using pre-stack depth migration

Solid Earth ◽  
2011 ◽  
Vol 2 (1) ◽  
pp. 1-7 ◽  
Author(s):  
I. Flecha ◽  
R. Carbonell ◽  
R. W. Hobbs ◽  
H. Zeyen
Keyword(s):  
High Velocity ◽  
Velocity Model ◽  
Original Model ◽  
Depth Migration ◽  
Reflection Data ◽  
Long Offset ◽  
Synthetic Test ◽  
Complex Models ◽  
High Velocity Layer ◽  
Velocity Layer

Abstract. Subbasalt imaging can be improved by carefully applying pre-stack depth migration. Pre-stack depth migration requires a detailed velocity model and an accurate traveltime calculation. Ray tracing methods are fast but, often fail in calculating traveltimes in complex models, specially, when they feature high velocity contrasts. Finitte difference solutions of the eikonal are more stable and can produce a traveltime field for the whole model avoiding shadow zones. A synthetic test was carried out to check the performance of a new pre-stack depth migration algorithm in a model that features a high velocity layer surrounded by lower velocities. The results reasonably reproduce the original model. The same scheme was used to process long-offset reflection data from the Faroe Shelf where conventional techniques (stack) were insufficient to assess the structure under a basalt layer. Pre-stack depth migration produced an improved image which recovered the main features in the stacked section and allowed to identify some subbasalt coherent events.

An indirect seismic method for determining the thickness of a low‐velocity layer underlying a high‐velocity layer

Geophysics ◽  
1981 ◽  
Vol 46 (7) ◽  
pp. 1003-1008 ◽  
Author(s):  
K. L. Kaila ◽  
H. C. Tewari ◽  
V. G. Krishna
Keyword(s):  
High Velocity ◽  
Low Velocity ◽  
Field Case ◽  
Reflection Data ◽  
First Arrival ◽  
Low Velocity Layer ◽  
Velocity Of Propagation ◽  
High Velocity Layer ◽  
Refraction Data ◽  
Velocity Layer

We present an indirect method for determining the thickness of a low‐velocity layer (LVL) underlying a high‐velocity layer (HVL) in seismic prospecting. Comparison of the average velocity‐depth function determined from the first arrival refraction data with that obtained from reflection data in the same region, especially below the LVL, makes it possible to recognize the presence of the LVL and to estimate its probable thickness. The applicability of the method has been demonstrated in a field case where the presence of an LVL is indicated by geologic evidence. It has been shown that thickness estimates of an LVL and an HVL can be made reliably in situations where the velocity in the LVL can be accurately estimated from nearby exposures or in a drilled well. For the field case analyzed, a thickness of 0.75 km was estimated for an LVL (probably Mesozoic sediments) underlying a 0.25 km thick HVL (probably basalt). The velocity of propagation in the LVL was taken from seismic data on nearby exposed Mesozoics as 4.0 km/sec, and the velocity of the HVL is 5.4 km/sec, based on the refraction data. In areas where the velocity in the LVL cannot be inferred accurately, an upper limit of this velocity can be obtained which permits estimation of the maximum possible thickness of the LVL. In the field example presented, we show that the velocity in the LVL cannot exceed 4.17 km/sec.

Wide-area imaging from OBS multiples

Geophysics ◽  
2009 ◽  
Vol 74 (6) ◽  
pp. Q41-Q47 ◽  
Author(s):  
Ranjan Dash ◽  
George Spence ◽  
Roy Hyndman ◽  
Sergio Grion ◽  
Yi Wang ◽  
...  
Keyword(s):  
Single Channel ◽  
Water Layer ◽  
Velocity Model ◽  
Imaging Method ◽  
Ocean Bottom ◽  
Ocean Bottom Seismometer ◽  
Conventional Imaging ◽  
Depth Migration ◽  
Reflection Data ◽  
Obs Data

The subseafloor structure offshore western Canada was imaged using first-order water-layer multiples from ocean-bottom seismometer (OBS) data and the results were compared to conventional imaging using primary reflections. This multiple-migration (mirror-imaging) method uses the downgoing pressure wavefield just above the seafloor, which is devoid of any primary reflections but consists of receiver-side ghosts of these primary reflections. The mirror-imaging method employs a primaries-only Kirchhoff prestack depth migration algorithm to image the receiver ghosts. The additional travel path of the multiples through the water layer is accounted for by a simple manipulation of the velocity model and processing datum: the receivers lie not on the seabed but on a sea surface twice as high as the true water column. Migration results show that the multiple-migrated image provides a much broader illumination of the subsurface than is possible for conventional imaging using the primaries, especially for the very shallow reflections and sparse OBS spacing. The resulting image from mirror imaging has illumination comparable to the vertical incidence surface streamer (single-channel) reflection data.

Imaging beneath a high‐velocity layer using converted waves

Geophysics ◽  
1992 ◽  
Vol 57 (11) ◽  
pp. 1444-1452 ◽  
Author(s):  
Guy W. Purnell
Keyword(s):  
High Velocity ◽  
Energy Partitioning ◽  
P Wave ◽  
Converted Wave ◽  
P Waves ◽  
Lower Velocity ◽  
S Waves ◽  
High Velocity Layer ◽  
The Waves ◽  
Velocity Layer

High‐velocity layers (HVLs) often hinder seismic imaging of deeper reflectors using conventional techniques. A major factor is often the unusual energy partitioning of waves incident at an HVL boundary from lower‐velocity material. Using elastic physical modeling, I demonstrate that one effect of this factor is to limit the range of dips beneath an HVL that can be imaged using unconverted P‐wave arrivals. At the same time, however, partitioning may also result in P‐waves outside the HVL coupling efficiently with S‐waves inside. By exploiting some of the waves that convert upon transmission into and/or out of the physical‐model HVL, I am able to image a much broader range of underlying dips. This is accomplished by acoustic migration tailored (via the migration velocities used) for selected families of converted‐wave arrivals.

Surface‐to‐borehole illumination of a high‐velocity layer using marine VSP

1994 ◽  
Author(s):  
Colin MacBeth ◽  
Enru Liu ◽  
Mark Boyd ◽  
Karen Sweeney
Keyword(s):  
High Velocity ◽  
High Velocity Layer ◽  
Velocity Layer

Elastic wave propagation along layers in two-dimensional models

1963 ◽  
Vol 53 (3) ◽  
pp. 593-618
Author(s):  
D. K. Chowdhury ◽  
Peter Dehlinger
Keyword(s):  
Layer Thickness ◽  
High Velocity ◽  
Wave Length ◽  
Two Dimensional ◽  
Layer Model ◽  
Low Velocity ◽  
Long Period ◽  
Low Velocity Layer ◽  
High Velocity Layer ◽  
Velocity Layer

Abstract Propagation of direct waves and dispersive long-period waves along a layered system was investigated experimentally by means of two-dimensional ultrasonic models. Velocities of direct and head waves were measured within layers or in a medium adjacent to layers as functions of layer thickness to wave length or source-from-interface distance to wave length. Amplitudes of direct longitudinal, direct shear, and long-period waves were measured on three profiles, each perpendicular to the layers. Three models were used: the first consisted of a low-velocity layer between two thick sheets; the second of a high-velocity layer between two sheets; the third of six alternating high- and low-velocity layers between two sheets. The source was a wave train, simulating a wave from a seismic explosion. The frequency was varied so as to obtain different ratios of layer thickness to wave length. In the single low-velocity layer model the direct longitudinal wave contained a larger amplitude than the dispersive long-period wave in the layer at offset distance of 6 to 10 times the layer thickness. In the single high-velocity layer model the direct longitudinal wave was attenuated rapidly and the amplitudes of the long-period waves were negligigble. In the multilayered model, direct waves had negligible amplitudes at the corresponding distances; nearly all of the energy was in the dispersive long-period waves. In this model the low-velocity layer carried 1 1/2 to 3 times the amplitude observed in the high-velocity layers, whether the source was located in the high- or low-velocity layers. Dispersion of the long-period waves in the multilayered model was pronounced within the low-velocity layers and weak in the high-velocity layers, when the source was either in a high- or low-velocity layer. Dispersion was anomalous when the source was in a low-velocity layer and normal when in a high-velocity layer.

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