Investigation of the Low Velocity Layer using Shallow Seismic Refraction Survey in Magadi Basin, Kenya

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.

Download Full-text

Upper crustal structure of southern Queen Charlotte Basin from sonobuoy refraction studies

Canadian Journal of Earth Sciences ◽

10.1139/e85-178 ◽

1985 ◽

Vol 22 (11) ◽

pp. 1696-1710 ◽

Cited By ~ 2

Author(s):

Ron M. Clowes ◽

Ewa Gens-Lenartowicz

Keyword(s):

Seismic Refraction ◽

Petroleum Exploration ◽

Low Velocity ◽

Crustal Rocks ◽

Air Gun ◽

Hydrocarbon Sources ◽

Low Velocity Layer ◽

Wrangellia Terrane ◽

Seismic Sections ◽

Velocity Layer

The Queen Charlotte Basin, which lies on the west coast of Canada, was the location of a petroleum exploration program in the 1960's. Recently published tectonic models indicate that estimates of hydrocarbon potential require re-evaluation, and a renewed exploration interest has been expressed. In 1981, a seismic refraction survey of the upper crust using radio-telemetering sonobuoys and a 27 L air-gun source was carried out. A particular objective of the study was to determine the existence and depth extent of any sedimentary layer, hypothesized on the basis of other studies, beneath or within the Tertiary Masset volcanics in which some of the exploration wells had terminated. Three reversed profiles and one unreversed profile up to 40 km long were recorded. Interpretation of the data made use of the travel-time and amplitude information of the seismic sections by comparison with theoretical sections computed by two-dimensional ray tracing and a new asymptotic ray theory synthetic seismogram algorithm.Consistent with the earlier industry results, sediment thicknesses vary considerably throughout the southern Queen Charlotte Basin. The Tertiary Masset volcanics appear to be pervasive throughout the study area, with thicknesses varying from less than 1 km to greater than 3 km. On three of the four profiles a low-velocity layer, interpreted as Mesozoic sediments or sediments interbedded with volcanics, was found to lie beneath the volcanics. Thicknesses ranged from about 1 km to zero at a pinchout. The lowermost layer of all models is considered to be crustal rocks and is identified with the top of Wrangellia, an allochthonous terrane proposed to underlie the southern Queen Charlotte Basin. Along the profile for which no low-velocity layer sediments were interpreted, the Wrangellia terrane forms a dome rising to within 2 km of the surface. Other recent studies suggest that hydrocarbon sources could be associated with the Mesozoic rocks of Wrangellia and with any sediments underlying the Tertiary lavas, as well as with Tertiary marine sediments above the volcanics. Thus further exploration is warranted.

Download Full-text

Comparison of the Characteristics of Low Velocity Layer (LVL) in the Mangrove Swamp and in the Upper Flood Plain Environments in the Niger Delta, using Seismic Refraction Methods

Journal of Geology & Geophysics ◽

10.4172/2381-8719.1000248 ◽

2016 ◽

Vol 5 (4) ◽

Cited By ~ 1

Author(s):

Uko ET ◽

Emudianughe JE

Keyword(s):

Niger Delta ◽

Seismic Refraction ◽

Flood Plain ◽

Low Velocity ◽

Mangrove Swamp ◽

Low Velocity Layer ◽

Velocity Layer

Download Full-text

High‐resolution shallow‐seismic experiments in sand, Part I: Water table, fluid flow, and saturation

Geophysics ◽

10.1190/1.1444423 ◽

1998 ◽

Vol 63 (4) ◽

pp. 1225-1233 ◽

Cited By ~ 86

Author(s):

Ran Bachrach ◽

Amos Nur

Keyword(s):

High Resolution ◽

Water Table ◽

High Tide ◽

Saturated Sand ◽

Low Velocity ◽

Near Surface ◽

Shallow Seismic ◽

Low Velocity Layer ◽

Dry Sand ◽

Velocity Layer

A high‐resolution, very shallow seismic reflection and refraction experiment was conducted to investigate the seismic response of groundwater level changes in beach sand in situ. A fixed 10-m-long receiver array was used for repeated seismic profiling. Direct measurements of water level in a monitoring well and moisture content in the sand were taken as well. The water table in the well changed by about 1 m in slightly delayed response to the nearby ocean tides. In contrast, inversion of the seismic data yielded a totally different picture. The reflection from the water table at high tide appeared at a later time than the reflection at low tide. This unexpected discrepancy can be reconciled using Gassmann’s equation: a low‐velocity layer must exist between the near‐surface dry sand and the deeper and much faster fully saturated sand. This low‐velocity layer coincides with the newly saturated zone and is caused by a combination of the sand’s high density (close to that of fully saturated sand), and its high compressibility (close to that of dry sand). This low‐velocity zone causes a velocity pulldown for the high‐frequency reflections, and causes a high‐tide reflection to appear later in time than low‐tide reflection. The calculated velocities in the dry layer show changes with time that correlate with sand dryness, as predicted by the temporal changes of the sand’s density due to changing water/air ratio. The results show that near‐surface velocities in sand are sensitive to partial saturation in the transition zone between dry and saturated sand. We were able to extract the saturation of the first layer and the depth to the water table from the seismic velocities. The high‐resolution reflections monitored the flow process that occurred in the sand during the tides, and provided a real‐time image of the hydrological process.

Download Full-text

Does CO2 cause partial melting in the low-velocity layer of the mantle?: Comment and reply

Geology ◽

10.1130/0091-7613(1976)4<787:dccpmi>2.0.co;2 ◽

1976 ◽

Vol 4 (12) ◽

pp. 787 ◽

Cited By ~ 8

Author(s):

David H. Eggler

Keyword(s):

Partial Melting ◽

Low Velocity ◽

Low Velocity Layer ◽

Velocity Layer

Download Full-text

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

Earth and Planetary Science Letters ◽

10.1016/j.epsl.2020.116642 ◽

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

Download Full-text

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

Bulletin of the Seismological Society of America ◽

10.1785/bssa0710030713 ◽

1981 ◽

Vol 71 (3) ◽

pp. 713-729 ◽

Cited By ~ 1

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.

Download Full-text