Low-Frequency Seismic Sounding method (LFS) is based on the effect of increased low-frequency energy (1-10Hz) in the spectrum of microseisms over the hydrocarbon pools, which was discovered in 1989. In 2002-2009, the effect was studied by JSC "Gradient" on over 100 areas in course of the research studies and exploration operations. Testing of the technology based on such a vast amount of data in different geological conditions has confirmed the theoretical understanding of the nature of anomaly associated with multiple reflections (resonance) of seismic waves between the surface and the oil and gas deposit (Fig. 1).
Fig. 1. The model of the resonance of seismic waves between the surface and oil reservoir
Abnormally high rate of wave reflection from the oil-saturated reservoir can be explained based on the ratio of the basic physical properties of water and oil, as well as taking into account that exactly the low-frequency component of the waves is being significantly reflected. Compressibility of oil is 2-5 times higher than compressibility of the water and surrounding rocks. Compressibility of water differs for only 10-20% as compared with the compressibility of surrounding rocks. The compressibility is affected by the gas factor which considerably increases it. Viscosity of oil is two orders of magnitude greater than that of the water and surrounding rocks. Under the low-frequency influence, the forces of viscosity within the deposit weaken and, as a result, the deposit is becoming easily deformable by the infrasonic wave. This has an impact on decreasing of its acoustic impedance and, as a result, promotes significant reflection coefficient (Fig. 2). Thus, the reflection coefficient of the oil and gas deposits depends on the frequency of seismic waves and increases with its decrease. The intensity of anomaly depends on the thickness of oil reservoir and its physical properties - viscosity, compressibility and oil saturation.
Fig. 2. Impedance frequency dependence for oil and water