The detection of oil reservoirs in the subsurface is an important problem of exploration geophysics. For the solution of this problem seismic crosshole measurements can be used. The anomalous behavior of a wave elastic field in an oil-saturated seam in comparison with this field in a dry seam can be used as the theoretical background for solving this problem. At the beginning of the 1960's it was proposed to regard a liquid layer sandwiched between two elastic half-spaces as a simple model of a fluid-filled collector (see [1]). It was shown that in this model a slow, weakly attenuated wave exists for arbitrary sets of parameters of the media. The properties of this slow wave heavily depend on the rigidity of the half-spaces surrounding the liquid layer. This wave is absent if the layer is solid. Therefore, the existence of a slow wave is an indicator of the saturation of a collector. The absence at that time of any reliable experimental data about wave dynamics in an oil-saturated layer and a strong idealization of the model proposed did not allow one to draw any conclusions concerning the physical realization in nature of the theoretical prediction. However, in the 1980's the interest in this problem arose again in connection with the investigations of a low-frequency oscillation appearing during volcanic eruption [2]. Theoretical studies had found a slow wave arising in a magma flow, and it was shown that the oscillation observed earlier during volcanic activity depends on the motion of this slow wave [3]. Recently, slow waves have been observed in a number of cases during crosshole measurements in the Tyumen oil region. These slow waves are propagating in oil-saturated layers and have rather intensive amplitudes. A typical acoustic signal of a slow wave will be presented below. All these facts show that the extensive study of wave propagation from a point source in a liquid layer surrounded by elastic half-spaces is an urgent problem. Such a consideration will also facilitate further passage to more complicated models of an oil-saturated seam.In a cylindrical coordinate system (r, z, 0) we consider a liquid layer (t) (0 < z < h) sandwiched between two half-spaces (0) (z < 0) and (2) (z > h). We denote by a~ -1 (i = 0, 1, 2), b~ -1 (i = 0, 2) the velocities of the longitudinal and transverse waves in media 0, 1, 2, respectively, and by pi (i = 0, 1, 2) the densities of the media, which are related with the Lain6 constants )~i, #i as follows:A point source of the type "center of dilatation" is located at the point (0, 0, -H) of the medium and is described by the Heaviside step time function. In medium (0)

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Conversion of a slow wave propagating in a fluid layer inside an elastic medium into a tube wave propagating in a borehole that intersects this layer is considered. It is shown that the total field of the latter wave consists of three summands of different physical nature. Seismograms of the slow and tube waves are presented. Bibliography: 5 titles.It is known that in a fluid layer located inside an elastic medium, a low-velocity (slow) wave arises. The spectrum of the slow wave begins with the null frequency [1][2]. Lately, interest in slow waves has increased in connection with different geophysical applications (volcanic activity, wave propagat ion in oil-bearing rocks [3]) related to such waves. A slow wave is a surface wave, the amplitude of which decreases exponentially in both directions away from the fluid layer. The energy of the propagating wave is partially trapped by the elastic medium, and, for sufficiently low frequencies, this part of the energy can be significant. At its incidence on a borehole intersecting the fluid layer, the slow wave excites intricate interference oscillations in the borehole, which are related to the response of the elastic medium with a fluid-fiUed cylindrical cavity to the incident perturbation. The investigation of the nature of this response is the purpose of this paper. This problem is solved in the frequency approximation for X >> a, where X is wavelength, a is the radius of the borehole. Earlier, the problem of the excitation of a tube wave in a fluid-filled borehole by the Rayleigh wave propagating along the free surface of an elastic half-space was considered in [4][5]. The problem of conversion of a slow wave into a tube wave is more complicated because of the dispersion of the phase velocity of the slow wave. In this case, we are unable to provide the solution in the time domain in explicit form, and the inverse Fourier transform must be applied. Fig. 1 and consists of a fluid layer (1) (-2 h-< z < h) sandwiched between two identical elastic half-spaces (2) h h (z > 5-, z < -5")" The velocities of the longitudinal and transverse waves in the corresponding medium are denoted by ai (i = 1, 2) and bi (i = 2); the densities of the media are denoted by pi (i = 1,2), and the Lam6 constants are denoted by Ai (i = 1, 2) and/_q (i = 2). A point source of the center of dilatation type is located at the point r = 0, z = H, and its dependence on time is described by the Heaviside step function. The potential of the field of displacements induced by such a source is determined by the expression In equality (1), the "+" ("-") sign in the exponent is taken for z < H (z > H). The branch cuts on the q plane are drawn from the branch points ~i7~ -1 into the left half-plane in parallel to the real axis, and the branches of the radical c~2 are specified by the condition arg c~2 = 0 for 77 > 0. Construction of the solution for a slow wave The model of the medium is presented inFor this problem, it is convenient to split the total field into its symmetric and antisymmetric (about the pl...

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It is shown that in a cracked layer sandwiched between two elastic half-spaces, a slow wave propagates. For low frequencies the velocity of this wave is much less than its velocity in an infinite fluid. The dispersion and amplitude characteristics of the slow wave are studied as a function of the porosity and frequency. Bibliography': 5 titles.In the authors' paper [1], a slow wave propagating in a fluid layer placed in an elastic medium was investigated. It was shown that a number of phenomena observed on crosshole sounding of the media with oil collectors can be explained by the initiation of a slow wave. Such waves, for example, were observed during volcanic activity accompanied by the formation of magma-filled cracks [2]. The simulation of cracked collectors by a fluid layer is based on an overly simplified idealization, and it requires further development for the case of more realistic models. One of these models was proposed by L. Molotkov in [3], where an effective model of a cracked medium with finite porosity was constructed with the help of the method of matrix averaging.The interference waves initiated in a cracked layer are considered in [4], where new types of waves are described. For small frequencies, these waves have velocities close to the velocity of a plate wave.In the present paper, we show that in a cracked layer sandwiched between two elastic half-spaces, a slow wave with velocity dependent on the porosity of the layer exists.We consider a medium composed of alternating fluid and elastic solid layers having densities pl and p2, respectively; v is the velocity of the sound wave in a fluid layer, up, vs are the velocities of the longitudinal and transversal waves in a solid layer, and hi and h2 are the thicknesses of fluid and solid layers. The medium composed of such layers is characterized by the porosity el equal to the ratio hl/(hl + h2) of the width of a fluid layer to the total width of the period. In [4], it was shown that in such a medium, three waves propagate along the cracks with velocities equal to el e2plv p2 VpThe waves "vith velocities vl and v2 are called quasilongitudinal and plate waves, respectively. with velocity v3 is the slowest wave with velocity heavily dependent on the porosity. If el = 0, then v3 but in the case el > 0.1, the velocity v3 has a value close to the sound speed v in an infinite fluid. values of v3 for a small porosity are determined by the relationThe wave