RU2503037C1 - Method of evaluating geologic structure of top layers of bottom - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: parameters of bottom sediments are obtained based on experimental measurements of the spatial interference structure of an acoustic field in a given area and subsequent comparison thereof with results of solving a wave equation with given boundaries, parameters of which vary within a given range during mathematical estimations. Bottom parameters are obtained as a result of the best match of experimental data with data of the solution of the wave equation.

EFFECT: high accuracy of probing data.

Description

To create various kinds of offshore structures (mining platforms, pipelines, moorings), it is necessary to know the structure and parameters of bottom sediments in the places of their intended location. As a rule, the structure of bottom sediments is obtained by trial drilling to a depth of bottom sediments of up to several tens of meters. Drilling itself is quite laborious and requires material and time costs.

The characteristics of the geological structure of the upper layers of the bottom, such as the density and stiffness of the soil, can be obtained by acoustic methods by measuring the reflection level of sound energy from bottom sediments by directing radiation and receiving pulsed signals.

To increase the depth of penetration of sound energy into the soil, it is necessary to use as low as possible frequencies from units to tens of Hertz. Creating a directional emitter for such a frequency range is quite difficult, since it must have dimensions larger than the wavelength, and this is several tens of meters.

At the same time, the spatial characteristics of the sound field in water at these frequencies, even with continuous point radiation, are determined by the characteristics of bottom sediments, so that by measuring the interference structure of the sound field in the water space, it is possible to evaluate the characteristics of the layered structure of the bottom (number of layers, sizes, density and stiffness of each layer). The very interference structure of the field in the water layer for low frequencies is determined by the sum of the so-called normal waves (A.P. Stashkevich, “Acoustics of the Sea”, “Sudostroenie” publishing house, Leningrad, 1966).

The parameters of normal waves (propagation velocity, amplitude and attenuation) can be determined by positioning the emitter and receiver at a certain distance from each other in the aquatic environment and the subsequent uniform change in the distance between them by towing the emitter or receiver horizontally.

The receiver constantly records the changing interference structure of the wave field in the medium, according to which, using known algorithms, parameters of normal waves of the acoustic field, such as propagation velocity, amplitude and attenuation, are extracted.

Then they solve the wave equation with boundary conditions, setting the parameters of several layers of the bottom lying on each other as a boundary, such as the thickness of the layer, its density, the speed of propagation and attenuation of sound in them. As a result of solving the wave equation, the parameters of normal waves are obtained. These parameters are compared with the parameters obtained in the experiment. The parameters of the geological structure of the bottom are obtained with the best agreement between the experimental data and the calculation data when the bottom parameters are varied in them.

The change in the distance Δr between the emitter and the receiver should be such that it fits one period of the interference structure of the sound field for a given radiation frequency. This condition must satisfy the following relation

, $$\Delta r>\frac{\mathrm{one}}{f}\cdot \frac{C_g\cdot C_{\mathrm{at}}}{C_g-C_{\mathrm{at}}}$$

,

where f is the radiation frequency, C _g is the smallest estimated speed of sound propagation in the bottom soil, C _in is the signal propagation speed in water.

Information sources:

1. RF patent No. 2392643.

2. RF patent No. 2072535.

3. RF patent No. 2072534.

Claims (2)

1. A method for assessing the geological structure of the upper layers of the bottom by measuring the propagation parameters of low-frequency acoustic signals in the marine environment, associated with the emission and reception of acoustic harmonic vibrations in the aquatic environment, characterized in that, in order to obtain data on the geological structure of bottom sediments and their parameters, produce an acoustic signal into the aquatic environment and receive it at a certain horizontal distance from the source, which constantly varies uniformly in time along the line connecting horizontally the source and the receiver so that the receiver constantly records the changing interference structure of the wave field in the medium, according to which, using known algorithms, the parameters of normal waves of the acoustic field, such as the phase propagation velocity, amplitude and attenuation, are extracted and then the wave equation with boundary conditions is solved by setting in the form of a boundary the parameters of several layers lying on each other, such as the layer thickness, its density, the speed of propagation and attenuation of sound, the calculation results in the form of the parameters of normal waves is compared with the data of the experimental results, constantly changing the values of the parameters of the layers in the calculations, the parameters of the geological structure of the upper sediments of the bottom are obtained with the best coincidence of the calculation data with the experimental results. 2. The method for assessing the geological structure of the upper layers of the bottom according to claim 1, characterized in that the distance between the emitter and the receiver changes either when the emitter is towed at a uniform speed with a stationary receiver, or when the receiver is towed with a fixed emitter, and the distance Δr between they must satisfy inequality
$$\Delta r>\frac{\mathrm{one}}{f}\cdot \frac{C_g\cdot C_{\mathrm{at}}}{C_g-C_{\mathrm{at}}}$$

,
where f is the radiation frequency, C _g is the smallest estimated speed of sound propagation in the bottom soil, C _in is the signal propagation speed in water.