Method of evaluating geologic structure of top layers of bottom

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.

To create a different kind of offshore structures (oil platforms, pipelines, terminals) need to know the structure and characteristics of bottom sediments in areas of their intended placement. Generally, the structure of the sediment obtained by test drilling to a depth of sediments up to several tens of meters. Self drilling is cumbersome and requires time and money.

Characteristics of the geological structure of the upper layers of the seabed, such as density and stiffness of the soil, you can get acoustic methods by measuring the level of reflection of sound energy from the sediment by directional radiation and reception of pulsed signals.

To increase the depth of penetration of the sound energy in the soil you want to use as low as possible frequencies from units to tens of Hertz. A directional emitter for that range of frequencies is quite difficult, as it must have dimensions larger than the wavelength, as it is several tens of meters.

At the same time the characteristics of the sound field in water for space at these frequencies, even when continuous scatter radiation are determined by the characteristics of bottom sediments, so that by measuring the interference structure of a sound field in water can be estimated characteristics of the layered structure of the s bottom (the number of layers, the size, density and hardness of each layer). The very same interference pattern in the water layer for the low frequencies is determined by the amount of so-called normal waves (A.P. Stashkevich "Acoustics of the sea", publishing house "Sudostroenie, Leningrad, 1966).

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

The receiver continuously monitors the changing interference structure of the wave field in the medium in which using known algorithms match parameters of the normal waves of the acoustic field, such as speed of propagation, amplitude and damping.

Then solve the wave equation with boundary conditions, setting the boundaries of the several parameters lying one on each of the bottom layers, such as layer thickness, density, velocity of propagation and attenuation of sound. The solution of the wave equation have the options of normal waves. These parameters are compared with the parameters obtained in the experiment. The parameters of the geological structure of the seabed get in better agreement between the experimental data receovery variations in the parameters of the seabed.

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

$$\Delta r>\frac{1}{f}\cdot \frac{C_g\cdot C_{\mathrm{in}}}{C_g-C_{\mathrm{in}}}$$,

where f is the radiation frequency, C_gthe smallest estimated propagation velocity of sound in the ground floor, C_in- the speed of signal propagation in the water.

Sources of information:

1. RF patent №2392643.

2. RF patent №2072535.

3. RF patent №2072534.

1. Method of assessment of the geological structure of the upper layers of the seabed by measuring parameters of the distribution of low-frequency acoustic signals in the marine environment associated with radiation and reception of acoustic harmonic oscillations in an aqueous medium, characterized in that, in order to obtain data on the geological structure of sediments and their parameters, produce radiation in the aquatic environment of the acoustic signal and its reception at some distance from the source horizontally, which is always uniformly varies with time along the line connecting p is the horizontal source and receiver so that the receiver continuously monitors the changing interference structure of the wave field in the medium in which using known algorithms match parameters of the normal waves of the acoustic field, such as the phase velocity of propagation, amplitude and attenuation, and then solve the wave equation with boundary conditions, setting the boundaries of the several parameters lying one on top of the other layers, such as layer thickness, density, velocity of propagation and attenuation of sound, the results of the calculations as parameters of normal waves are compared with the data of experimental results, constantly changing parameter values of the layers in the calculations, the parameters of the geological structure of the upper sediments of the bottom get in the best the coincidence of these calculations with experimental results.

2. Method of assessment of the geological structure of the upper layers of the seabed according to claim 1, characterized in that the change of the distance between emitter and receiver occurs or when towing emitter with a uniform speed with a fixed receiver or towing receiver with a fixed emitter, while the change in the distance Δr between them must satisfy the inequality
$$\Delta r>\frac{1}{f}\cdot \frac{C_g\cdot C_{\mathrm{in}}}{C_g-C_{\mathrm{in}}}$$,
where f is the radiation frequency, C_gthe smallest estimated propagation velocity of sound in the ground floor, C_in- the speed of signal propagation in water.