John T. Kuo

Partially molten middle crust beneath southern Tibet : Synthesis of project INDEPTH results

INDEPTH geophysical and geological observations imply that a partially molten midcrustal layer exists beneath southern Tibet. This partially molten layer has been produced by crustal thickening and behaves as a fluid on the time scale of Himalayan deformation. It is confined on the south by the structurally imbricated Indian crust underlying the Tethyan and High Himalaya and is underlain, apparently, by a stiff Indian mantle lid. The results suggest that during Neogene time the underthrusting Indian crust has acted as a plunger, displacing the molten middle crust to the north while at the same time contributing to this layer by melting and ductile flow. Viewed broadly, the Neogene evolution of the Himalaya is essentially a record of the southward extrusion of the partially molten middle crust underlying southern Tibet.

Kirchhoff elastic wave migration for the case noncoincident source and receiver

In taking into account both compressional (P) and shear (S) waves, more geologic information can likely be extracted from the seismic data. The presence of shear and converted shear waves in both land and marine seismic data recordings calls for the development of elastic wave‐migration methods. The migration method presently developed consists of simultaneous migration of P- and S-waves for offset seismic data based on the Kirchhoff‐Helmholtz type integrals for elastic waves. A new principle of simultaneously migrating both P- and S-waves is introduced. The present method, named the Kirchhoff elastic wave migration, has been tested using the 2-D synthetic surface data calculated from several elastic models of a dipping layer (including a horizontal layer), a composite dipping and horizontal layer, and two layers over a half‐space. The results of these tests not only assure the feasibility of this migration scheme, but also demonstrate that enhanced images in the migrated sections are well formed. Moreove...

INDEPTH (International Deep Profiling of Tibet and the Himalaya) multichannel seismic reflection data : Description and availability

Project INDEPTH (International Deep Profiling of Tibet and the Himalaya) has collected over 300 km of multichannel, deep seismic reflection data using explosive sources as part of a multidisciplinary effort to image the structure of the crust and uppermost mantle of the Tibetan plateau. The reflection profiles lie within the Yadong-Gulu rift and were acquired in the summers of 1992 and 1994. Data processing utilized typical industry tools, and a new method was used to migrate the data. Both unmigrated and migrated sections are presented here in large format to facilitate further interpretations.

Internal friction in rocks

This paper intends to show that the low‐amplitude effects, in particular the attenuation proportional to the frequency from very low frequencies up to the kilohertz range, are connected with the motion of dislocations in cracks. The measurements have been made of the resonant frequencies and the internal friction of spherical rock speciments, including Solenhofen limestone, Indiana limestone, Westerley Granite, Friedensville sphalerite, galena, magnetite, and olivine of small grain sizes. The different types of stresses are thus easily determined through the various modes of motion. On the basis of dislocation motion in cracks, it is shown that the ratio of the radial Q to the torsional Q is about 0.8 and is nearly independent of the crystal structure. This result is in agreement with the measurements, which vary from about 0.6 to 0.9. From longitudinal and torsional measurements of cylinders, the torsional Q’s are nearly twice those for the longitudinal mode. The model used also agrees with the variation...

Real data results of Kirchhoff elastic wave migration

Although Kirchhoff integral migration has attracted considerable attention for seismic data processing since the early 1970s, it, like all other seismic migration methods, is only applicable to compressional (P) waves. Because of a recent surge of interest in shear (S) waves, Kuo and Dai (1984) developed the Kirchhoff elastic (P and S) wave migration (KEWM) formulation and migration principle for the case of source and receiver noncoincidence. They obtained encouraging results using two‐dimensional (2-D) synthetic surface data from various geometric elastic models, including a dipping layer, a composite dipping and horizontal layer, and two layers over a half‐space.

A method for computation of velocity profiles by inversion of large-offset records

This paper describes a new method for recovering velocity profiles utilizing both phase and amplitude information including wide‐angle arrivals, post‐ and precritical reflections. This method is based on a double spatial transformation with a minimization procedure. The first transformation is slant stacking of the observed wave field (seismogram). The second is projecting the slant stacked wave field into the domain of horizontal slowness p and depth z. In this domain the inverse problem is reduced to finding the critical path p=V-1(z) where V(z) is the true velocity of the compressional waves. A numerical algorithm based on a minimization technique is used to find the critical path, which is equivalent to the set of turning points of the critically reflected rays. When this path is found, then the following criteria are satisfied: (1) most of the energy is concentrated away from the precritical region; (2) the computed reflection coefficients reach their maximum on this path; and (3) for horizontally st...

Apparatus for examining structures using stimulated acoustic emission

Abstract : A vibrator is held in contact with a structure undergoing testing, such as an airplane wing. Vibrations are distributed through the structure and structural defects such as fissures will respond by emitting an acoustical signal. The acoustic signal may occur over a wide frequency range. A pick-up having a crystal transducer also makes contact with the surface of the structure to detect acoustic emissions. The crystal has a wide band response capable of detecting structural defect acoustic emissions. The pick-up has electrical leads connected thereto so that the acoustic emissions are converted to electrical signals which may be recorded or viewed on an oscilloscope. Movement of the pick-up across the surface of the structure will help determine the location of the structural defect. (Author)

Internal Friction in Westerley Granite: Relation to Dislocation Theory

Compressional wave‐velocity and attenuation measurements have been made for fine‐grained Westerley granite over a frequency range from 25 kHz to 2.5 MHz. The internal friction Q−1 varies by a factor of 10 over this frequency range, increasing from 0.80×10−2 around 25 kHz to 6.5×10−2 at 2.5 MHz. The measurements are consistent with the low‐ and high‐frequency components associated with kink motion of dislocations. The constants evaluated from the measurements are consistent with values obtained for polycrystalline metals.

Transient time-domain electromagnetics

For externally imposed electric and magnetic sources, the electromagnetic (EM) field equations may be derived from the symmetrical Maxwells equations. Two variational principles for the initial boundary-value problem of electromagnetics are derived. The first one of the Gurtin type is deduced from the integro-differential equations equivalent to the EM field equations, while the second one is a simplified version of the first variational principle.The variational field equations are placed in a form suitable for a finite-element formulation in space. An explicit central-differences scheme is then applied to the finite-element variational equation to yield a recursive relation for time integration.The results from a Newmont electromagnetic pulse (EMP) survey done in the Mutooroo prospect in Australia are verified numerically. The finite-element model which simulates the Mutooroo survey assumes a thin vertical dike embedded in conductive host rock that is overlain by a thick layer of conductive overburden. The effect of overburden on the time-domain EM response appears first at the transmitter, continues to build up, eventually splits into two parts, and migrates away from the transmitter. As such, the effect of overburden is being migrated away, the effect of the deep-seated dike and the host rock on the response emerges and decays. The calculated EMP anomalies due to the dike agree with the corresponding field survey results in their shapes and magnitudes. The zero crossover point of the antisymmetrical vertical magnetic component and the anomaly peak of the symmetrical horizontal magnetic component are located immediately above the dike on the surface, the magnitudes of which are quantitatively comparable. The electric field decays more slowly than the magnetic field and could provide additional information in the exploration of mineral deposits.The agreement between the finite-element solution and the field results thus provides a means of interpreting time-domain EM survey data.

Group and Phase Velocities for Rayleigh Waves of Period Greater than 380 Seconds

Recent theoretical and experimental investigations of the periods of free spheroidal oscillation of the earth have shown good agreement. These data are used to determine group and phase velocities for Rayleigh waves of period greater than 380 seconds. The velocities so obtained are compared with those determined from analysis of progressive waves. It is concluded that group and phase velocities determined by either of the two methods are in agreement.