Seismic velocities

Olivine is the principal mineral (in terms of mass) of the earth s upper mantle where it is present as a combination of the two main components forsterite (85-95 mol.%) and fayalite (5-15 mol.%). In the earth mantle the increase in seismic velocity with depth is believed to be due to the presence of denser modifications of common minerals. 

Accdg to Meyer (Ref 1, p 401), expls used in seismic exploration must be especially water-resistant, very powerful to give expl waves which will be readily detectable over large areas within die limits of the instruments used, and have a high detonation velocity. Blasting Gelatin and Gelatin Dynamites... 

Data reduction is done by a process called inversion. It is not possible to uniquely derive the structure of a body from first principles based on seismic data. Instead, a model of the structure must be assumed and then the predictions of the model are compared to the observations. The model is then adjusted until the predictions match the observations. The more precise the predictions, the better the model can be tested by the observations. Properties that can be investigated by helioseismic inversion include the density, pressure, sound speed, angular velocity, temperature, and composition. 

Oldham discovered that there are actually two kinds of seismic vibrations, one called P (or "primary," because it travels faster and arrives first) and the other called S (or "secondary," because of its later arrival at the same station). The compressional motion of the P waves can be transmitted through most substances, although the speed at which the wave moves decreases as the stiffness of the medium decreases. In contrast, the transverse motion of S waves cannot be transmitted through a liquid, because the loosely bonded molecules in a liquid slip past each other too easily. S waves are observed to disappear at the top of the core. Then, at a depth of approximately 5100 km, the P wave velocity abruptly increases and there is a hint of the reappearance of an S wave. From such observations, Danish geophysicist Inge Lehman hypothesized in 1936 that the core was stratified, with an outer liquid portion and an inner solid portion. The existence of molten metal at core pressures requires some light element to act as antifreeze. 

The velocities deduced for seismic waves emanating from the core revealed that it must consist of very dense material, but its composition remained a mystery. However, cosmochemical reasoning provided important constraints. The core comprises approximately... 

DuPont s Pyrocore. See Ref 7, p9 DuPont s Seismic Prospecting Explosives, such as Seismograph 60% High Velocity" Gelatin Nitramons WW, WW-EL, S Sc S-EL Seismogel A Seismex and Seismex PW (Ref... 

Seismex" is a high strength Ammonia Dynamite which provides the same seismic return as the gelatins but is not as water resistant and its detonation velocity is lower. It should not be exposed to severe conditions (See also Ref 63, p362)... 

Blasters can use this equation to estimate the peak particle velocity of a seismic wave or they can use the graph shown in Fig 2. For example Determine the typical peak particle velocity from a normally confined blast with a maximum charge-weight-per-delay-period of 400 lbs at a distance of 1000 ft from the receiving site. The scaled distance, R/W 4 = 1000/40044 = 50 corresponds to a peak particle velocity of 0.31 ips on the graph in Fig 2... 

It should be emphasized that the expression given in the above equation relating the peak particle velocity, charge-weight-per-delay-period, and distance provides typical values only for planning blasting projects in the absence of seismic data. For further detailed information on blasting situations where the above equation is not applicable, the measurement and interpretation of seismic waves, and techniques necessary to reduce blast vibration, the reader is referred to Refs 1,3 4 Effects of Seismic Waves on Structures. 

There is a paucity of reliable, consistent data for hydrate elastic properties. Since these properties depend on crystal structures, many of them can be estimated reliably. However, since 1998, there have been significant efforts to perform accurate measurements of these properties in order to help facilitate correct interpretation of sonic or seismic velocity field data obtained on hydrates in the natural environments. 

Two final points should be made regarding the hydrate pressure-temperature limits shown in Figures 7.11a,b. The intersection of the phase boundary with the geothermal gradient limits the lower stability depth of hydrates. In Section 7.4.2, it will be shown that this intersection usually coincides with the BSR, an approximate exploration tool, which is caused by a seismic velocity decrease from hydrates above, to gas below the reflector. 

Seismic Velocity Techniques and Bottom Simulating Reflections... 

Seismic velocity techniques for hydrate detection have two components (1) translation of seismic signals to velocity and (2) translation between velocity and detection of hydrates. The first component is beyond the scope of this monograph. However, a brief consideration will be given to advances in translating velocity to the detection of hydrates. 

Anomalously high seismic velocities (Vp > 2.0 km/s) directly above the BSR. 

Elastic constants of minerals are the key to understanding geophysical properties of the Earth s interior. Bulk modulus and rigidity parameters, for example, influence the velocities of seismic waves through the Earth. Numerous experi-... 

The olivine spinel phase transition Experimental phase equilibrium studies have confirmed deductions from seismic velocity data that below 400 km, olivine and pyroxene, the major constituents of Upper Mantle rocks, are transformed to denser polymorphs with the garnet, y-phase (spinel) and P-phase (wadsleyite) structures (fig. 9.2). In transformations involving olivine to the P- or y-phases, transition pressures... 

Seismic refraction techniques can measure the density, thickness, and depth of geologic layers using sound (acoustic) waves transmitted Into the subsurface. These sound waves travel at different velocities In various soils and rock and are also refracted (or bent) at the Interface between layers, thereby affecting their path of travel. The time required for the wave to complete this path Is measured, permitting determination of the number of layers at the site as well as the sound velocity and depth of each layer. The wave velocity In each layer Is related to layer properties such as density and hardness. 

The seismic refraction method Is based on several Important assumptions (1) layer acoustic velocities Increase with depth (2) sufficient velocity contrast exists between layers to discriminate between different strata of interests (3) and layers must be thick enough to permit detection. 

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