L. Dresen

Wave propagation in physical models of micromorphic media

РезюмеРaсnросmрaненuе сеŭсмuческuх волн в мuкроморфноŭ сре¶rt;е, nо mеорuu о¶rt;ноŭ uз возможных мо¶rt;елеŭ сре¶rt;ы в очaге землеmрясенuя, uзучaеmся nуmём ¶rt;вухмерного ульmрaзвукового мо¶rt;елuровaнuя. Полученные резульmamы nокaзывaюm, чmо maкaя сре¶rt;a оmлuчеamся яркоŭ уnругоŭ aнuзоmроnuеŭ u nовыщенным nоглощенuе⇐. Скоросmь рaсnросmрaненuя nрохо¶rt;ящuх nро¶rt;ольных волн nонuжaеmся, uх aмnлumу¶rt;ы резко меняюmся. Преовлa¶rt;aющaя чaсmоma волн, nрохо¶rt;ящuх мuкроморфноŭ сре¶rt;оŭ, nлaвно сmремumся к о¶rt;ноŭ велuчuне, коmорaя связaнa с nросmрaнсmвенным рaсnре¶rt;еленuем, нanрaвленносmью u рaзмерaмu ¶rt;ефекmов, оnре¶rt;еляющuх эmу сре¶rt;у. Полученые резульmamы срaвнuвaюmся с mеорuеŭ мuкроморфноŭ сре¶rt;ы.SummaryThe propagation of seismic waves in a micromorphic body, which is supposed to be one of the possible models of the medium in the earthquake focus, is studied by means of the two-dimensional ultrasonic model technique. The results obtained indicate that such a medium is characterized by distinct elastic anisotropy and by increased attenuation. The propagation velocity of longitudinal waves decreases and their amplitudes change considerably. The prevailing frequency of the waves passing through the micromorphic medium tends smoothly to one value which depends on the space distribution, orientation and dimensions of the elements characterizing such a medium. The obtained results are confronted with the theory of a micromorphic medium.

The influence of local irregularities on the propagation of Love waves

Abstract The propagation of Love waves in models with local irregularities (a canyon or an alluvial valley) related to general waste deposit problems in environmental geophysics is numerically investigated. The investigation is based on a direct boundary element method for calculating wave fields and on a Greens function technique for evaluating transmission, reflection and ‘diffraction’ coefficients for Love waves, incident on a local surface or subsurface irregularity in layered media. The present model studies reveal that the influence of local irregularities on the propagation of Love waves depends both on the geometry and material properties of the local irregularities and on the dynamic parameters of the incident Love waves. Local irregularities lead to strong scattering of Love waves whose wavelengths are comparable to the characteristic sizes of the irregularities. For Love waves having wavelengths which are very large compared to the characteristic sizes of irregularities the influence of these irregularities are negligible. In addition, Love waves incident on a local irregularity always undergo mode conversion. Part of the energy of the incident Love wave will covert to other Love wave modes and body waves. With respect to the application of surface waves in environmental geophysics, two results may be stressed: (i) Though the conversion of Love waves to body waves has been qualitatively anticipated in the past, its enormous strength quantified by the diffraction coefficients is interesting; (ii) Love waves are more sensitive to local velocity changes than to local density changes. For in situ measurements, this result leads to the conclusion that density is a less relevant quantity, although it may better characterize the material inside a waste deposit than velocity.

Modellseismische Untersuchungen an 2- und 3-dimensionalen Testmodellen, Teil 1

SummaryFollowing the recommendation of the European Seismological Commission (Copenhagen 1966) to carry out comparative measurements on simple seismic models with a prescribed velocity-depth distribution (test models), an attempt was made to compare results obtained with different model techniques. In the present paper, model investigations with the first two proposed test models carried out by 2- and 3-dimensional model techniques are described. Seismogrammes, travel-time curves and amplitude-distance curves of the models investigated are given. A quantitative comparison of the results shows a satisfactory agreement.

Transmitted waves in the vicinity of the interface beyond the critical point

SummaryThe space (time) delay of transmitted waves in the vicinity of the interface beyond the critical point is investigated on two-dimensional seismic models of a low velocity channel; results of laboratory experiments are compared with the conclusions of the theory. The effect of the time delay in field measurements is demonstrated by means of two typical geological formations.