An account of the meeting for informal discussion held on Friday 14 November 1969

1971 ◽  
Vol 268 (1192) ◽  
pp. 733-736 ◽  
Keyword(s):  
Standard Deviation ◽  
Oceanic Crust ◽  
Deep Sea ◽  
Seismic Velocity ◽  
Seismic Refraction ◽  
Ocean Crust ◽  
Crust Structure ◽  
Rock Types ◽  
Informal Discussion

The meeting for informal discussion began with the short papers, by Green, O’Hara and Walker, which are printed in this volume and were offered and discussed under the heading ‘Petrogenesis’. These were followed by a discussion under the general heading ‘Ocean crust structure and ophiolites’ on which I have notes on 39 contributions. No written contributions were received and the account which follows is a personal one based on these notes; it has not been checked with individual contributors and if any are misrepresented I offer my apologies. Introducing the discussion on the oceanic crust Matthews stressed the need to reconcile the relatively uniformly layered picture of the crust given by seismic refraction measurements, which is well established at least on the ocean basins, with the much less strongly layered assemblage of rock types revealed by petrologists. In particular we have to take note of the surprising uniformity of velocity in layer 3 which has a worldwide average of 6.69 km s -1 with a standard deviation of only 0.26 km s -1 (Raitt 1963). It would be of great interest to have many more determinations of seismic velocity on specimens of deep-sea amphibolites and greenschists. Matthews presented a cartoon showing a possible view of the formation and composition of the oceanic crust. This cartoon, modified in the light of some of the subsequent comments, is shown in figure 1. It was successful in provoking discussion.

scholarly journals Structure and evolution of the Jan Mayen Microcontinent

2020 ◽  
Author(s):  
Anke Dannowski ◽  
Michael Schnabel ◽  
Udo Barckhausen ◽  
Dieter Franke ◽  
Martin Thorwart ◽  
...  
Keyword(s):  
Continental Crust ◽  
Oceanic Crust ◽  
Seismic Velocity ◽  
Seismic Refraction ◽  
Magnetic Anomalies ◽  
High Amplitude ◽  
Total Width ◽  
Ocean Bottom ◽  
East Greenland ◽  
The North

<p>The Jan Mayen Ridge (JMR) is a 150-km-long and 10–30 km wide seafloor expression in N-S direction in the centre of the North Atlantic and part of the Jan Mayen Microcontinent (JMMC). Previous studies show that the eastern flank of the JMR was formed during the breakup of the Norway Basin along today’s Aegir Ridge, prior to magnetic anomaly C23 (~50 Ma). The western margin of the JMMC is conjugate to East Greenland. Rifting gradually propagated northward, likely from Chron C21 (~46 Ma) onward. Fan-shaped magnetic anomalies in the Norway Basin suggest that the JMMC must have rotated counter-clockwise. The JMR is likely underlain by continental crust. Volcanic flows have been observed within the sediments in the Jan Mayen Basin (JMB). While a relatively uniform upper crust was observed throughout the JMMC, the thickness of the lower continental crust varies significantly from up to 15 km below the JMR down to almost zero thickness towards the western part of the JMB. However, the character of the lower crust and the development of the conjugate East Greenland – JMMC margins during Oligocene are still disputed.</p><p>Here, we investigate the crustal structure of the JMMC using a new 265-km-long seismic refraction line crossing the JMMC at 69.7°N in E-W direction, which was acquired on board of RV Maria S. Merian during cruise MSM67. The profile consists of 30 ocean bottom seismometers (OBS) with a spacing of 9.5 km. The dataset was complemented by on-board gravity measurements and a magnetometer array towed behind the vessel during shooting. The line extends from oceanic crust in the Norway Basin, across the microcontinent and into oceanic crust that formed at the presently active mid-oceanic Kolbeinsey Ridge. The magnetic profile shows old seafloor spreading anomalies in the east (likely anomaly 24, ~52 Ma), then low amplitude magnetic anomalies in the central portion of the profile, which are typical for many plutonic continental rocks. On the western part of the profile, high amplitude anomalies of younger oceanic crust (likely anomalies C5C trough C6, ~19–16 Ma) are recognized near the western termination of the JMB. The seismic velocity distribution and crustal thickness vary strongly along the profile, with velocities typical for oceanic crust at either end of the profile and a thickened crust (12–13 km) underneath the JMR. This suggests that the JMMC consists of thinned continental crust with a total width of 100 km.</p>

Experiments in the use of directed line sources in refraction profiling Arckaringa Basin - 1971

1971 ◽  
Vol 2 (4) ◽  
pp. 1
Author(s):  
B.E. Milton
Keyword(s):  
Seismic Velocity ◽  
Seismic Refraction ◽  
Ground Surface ◽  
South Australia ◽  
Trade Mark ◽  
Seismic Survey ◽  
Hole Drilling ◽  
Near Surface ◽  
Directed Line ◽  
Rock Types

During investigations of the Arckaringa Basin, four types of pre-Permian dolomite were intersected in stratigraphic wells and these can be distinguished by differences in density and seismic velocity. Knowledge of the limits of each dolomite is of importance in establishing the history of the Basin, and a seismic survey was carried out in May and June, 1971 to determine whether seismic refraction profiling could be used to map these rock types. To increase mobility and reduce costs, experiments were conducted using 'Geoflex' (trade mark of Imperial Chemical Industries Ltd.), a detonating cord, as an energy source. The Geoflex was ploughed to a depth of about 18 inches below the ground surface before being detonated. The results obtained in six widely separated localities were mostly fair to good. The quality of the primary and secondary refraction events appears to be largely independent of the size of the charge and is related to the entry angle of the input signal. This is determined by the velocity of the Geoflex and the surface and near-surface materials, and the length of the Geoflex array. Results show that over a wide area of the western Great Artesian Basin in South Australia, a directed line source can be used to obtain good quality refraction data at a cost considerably lower than that resulting from the use of conventional shot hole drilling and dynamite or other explosives.

Integration of Multi-geophysical Approaches to Identify Potential Pathways of Heavy Metals Contamination - A Case Study in Zeida, Morocco

2020 ◽  
Vol 25 (3) ◽  
pp. 415-423
Author(s):  
Ahmed Lachhab ◽  
El Mehdi Benyassine ◽  
Mohamed Rouai ◽  
Abdelilah Dekayir ◽  
Jean C. Parisot ◽  
...  
Keyword(s):  
Heavy Metals ◽  
Seismic Velocity ◽  
Seismic Refraction ◽  
Geophysical Methods ◽  
Low Frequency ◽  
Electrical Current ◽  
P Wave ◽  
Low Resistivity ◽  
Refraction Tomography ◽  
Geophysical Surveys

The tailings of Zeida's abandoned mine are found near the city of Midelt, in the middle of the high Moulouya watershed between the Middle and the High Atlas of Morocco. The tailings occupy an area of about 100 ha and are stored either in large mining pit lakes with clay-marl substratum or directly on a heavily fractured granite bedrock. The high contents of lead and arsenic in these tailings have transformed them into sources of pollution that disperse by wind, runoff, and seepage to the aquifer through faults and fractures. In this work, the main goal is to identify the pathways of contaminated water with heavy metals and arsenic to the local aquifers, water ponds, and Moulouya River. For this reason, geophysical surveys including electrical resistivity tomography (ERT), seismic refraction tomography (SRT) and very low-frequency electromagnetic (VLF-EM) methods were carried out over the tailings, and directly on the substratum outside the tailings. The result obtained from combining these methods has shown that pollutants were funneled through fractures, faults, and subsurface paleochannels and contaminated the hydrological system connecting groundwater, ponds, and the river. The ERT profiles have successfully shown the location of fractures, some of which extend throughout the upper formation to depths reaching the granite. The ERT was not successful in identifying fractures directly beneath the tailings due to their low resistivity which inhibits electrical current from propagating deeper. The seismic refraction surveys have provided valuable details on the local geology, and clearly identified the thickness of the tailings and explicitly marked the boundary between the Triassic formation and the granite. It also aided in the identification of paleochannels. The tailings materials were easily identified by both their low resistivity and low P-wave velocity values. Also, both resistivity and seismic velocity values rapidly increased beneath the tailings due to the compaction of the material and lack of moisture and have proven to be effective in identifying the upper limit of the granite. Faults were found to lie along the bottom of paleochannels, which suggest that the locations of these channels were caused by these same faults. The VLF-EM surveys have shown tilt angle anomalies over fractured areas which were also evinced by low resistivity area in ERT profiles. Finally, this study showed that the three geophysical methods were complementary and in good agreement in revealing the pathways of contamination from the tailings to the local aquifer, nearby ponds and Moulouya River.

The relationship between Vs, Vp, density and depth based on PS-logging data at K-NET and KiK-net sites

2021 ◽  
Author(s):  
Fumiaki Nagashima ◽  
Hiroshi Kawase
Keyword(s):  
Standard Deviation ◽  
Seismic Velocity ◽  
Saturated Soil ◽  
P Wave ◽  
Soil Types ◽  
Depth Range ◽  
Velocity Inversion ◽  
Velocity Models ◽  
Regression Curves ◽  
Ps Logging

Summary P-wave velocity (Vp) is an important parameter for constructing seismic velocity models of the subsurface structures by using microtremors and earthquake ground motions or any other geophysical exploration data. In order to reflect the ground survey information in Japan to the Vp structure, we investigated the relationships among Vs, Vp, and depth by using PS-logging data at all K-NET and KiK-net sites. Vp values are concentrated at around 500 m/s and 1,500 m/s when Vs is lower than 1,000 m/s, where these concentrated areas show two distinctive characteristics of unsaturated and saturated soil, respectively. Many Vp values in the layer shallower than 4 m are around 500 m/s, which suggests the dominance of unsaturated soil, while many Vp values in the layer deeper than 4 m are larger than 1,500 m/s, which suggests the dominance of saturated soil there. We also investigated those relationships for different soil types at K-NET sites. Although each soil type has its own depth range, all soil types show similar relationships among Vs, Vp, and depth. Then, considering the depth profile of Vp, we divided the dataset into two by the depth, which is shallower or deeper than 4 m, and calculated the geometrical mean of Vp and the geometrical standard deviation in every Vs bins of 200 m/s. Finally, we obtained the regression curves for the average and standard deviation of Vp estimated from Vs to get the Vp conversion functions from Vs, which can be applied to a wide Vs range. We also obtained the regression curves for two datasets with Vp lower and higher than 1,200 m/s. These regression curves can be applied when the groundwater level is known. In addition, we obtained the regression curves for density from Vs or Vp. An example of the application for those relationships in the velocity inversion is shown.

scholarly journals Detailed shallow structure seismic refraction investigation, with the application of different processing techniques

2001 ◽  
Vol 34 (4) ◽  
pp. 1309
Author(s):  
Τ. ΠΑΠΑΔΟΠΟΥΛΟΣ ◽  
Π. ΚΑΜΠΟΥΡΗΣ ◽  
Ι. ΑΛΕΞΟΠΟΥΛΟΣ
Keyword(s):  
Seismic Velocity ◽  
Seismic Refraction ◽  
Seismic Sources ◽  
Subsurface Structure ◽  
Seismic Refraction Data ◽  
Processing Techniques ◽  
National Road ◽  
Gradual Variation ◽  
Refraction Data ◽  
Shallow Structure

A comparative study of conventional and modern processing techniques of seismic refraction data is examined in this paper, for shallow structure investigation in the framework of a geotechnical research. The techniques used here were applied for the detection of narrow and low seismic velocity zones along the bedrock in the 10.5th Km of the new national road Igoumenitsa-Ioannina. The results were comparable and only slight deviations were observed due mainly to different algorithm procedures applied on data and the resolution provided by each technique. It is pointed out that the non linear tomography seismic refraction technique, overcomes the conventional ones since by increasing the number of seismic sources and considering the gradual variation of seismic velocity with depth, a better resolution and image reconstruction for the subsurface structure is obtained.

PcP from the nuclear explosion BILBY September 13, 1963

1965 ◽  
Vol 55 (2) ◽  
pp. 441-461
Author(s):  
Goetz G. R. Buchbinder
Keyword(s):  
Standard Deviation ◽  
Seismic Velocity ◽  
Nuclear Explosion ◽  
Amplitude Ratios ◽  
The Core ◽  
Core Boundary

Abstract The core-reflected phase, PcP, from the BILBY event, received at stations between 19° and 88°, arrived early by an average of 1.80 seconds with respect to the Jeffries-Bullen tables. The standard deviation of these data was 0.77 seconds. The corresponding P phases were early by 1.34 seconds. The tables therefore need adjustments. If the core boundary is to be moved by more than 10 km from the value of 2898 km then the mantle seismic velocity immediately above the core must be changed also. The PcP/P amplitude ratios are nearly always much larger than those predicted theoretically.

Crustal structure of the southeast flank of Kilauea Volcano, Hawaii, from seismic refraction measurements

1980 ◽  
Vol 70 (4) ◽  
pp. 1149-1159
Author(s):  
John J. Zucca ◽  
David P. Hill
Keyword(s):  
Oceanic Crust ◽  
Crustal Structure ◽  
Seismic Refraction ◽  
Kilauea Volcano ◽  
Geological Survey ◽  
Pn Velocity ◽  
Kīlauea Volcano ◽  
Volcanic Pile ◽  
Rift Zones ◽  
The U.S

abstract In November 1976, the U.S. Geological Survey, in conjunction with the Hawaii Institute of Geophysics, established a 100-km-long seismic refraction line normal to the southeast coast of Hawaii across the submarine flank of Kilauea Volcano. Interpretation of the data suggests that the oceanic crust dips about 2° toward the island underneath the volcanic pile. The unreversed Pn velocity is 7.9 km/ sec with crustal velocities varying strongly along the profile. Profiles across the rift zones of Kilauea suggest that the velocity in the rifts is higher than the velocity in the surrounding extrusive rocks and that the velocity in the southwest rift (∼6.5 km/sec) is lower than the velocity in the east rift (∼7.0 km/sec). The rift boundaries seem to dip away from the rift such that a large part of the volcanic pile is composed of the higher velocity core of riftzone rock.

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