Background seismic vibrations are present at any given point in the geological environment. Their sources are processes that cause deformation of the Earth structure. These processes occur both on the surface and inside the Earth. If we construct data processing so as to extract vertical-directed longitudinal waves from this continuous seismic background noise, we can obtain their spectral characteristic, the structure of which will reflect the amplitude-frequency characteristic of the medium at the observation point.
High-sensitivity low-frequency three-component devices, with ultra low level of intrinsic noise, are used to register background seismic vibrations. Such seismometers are 10-100 times more sensitive than standard seismic equipment and therefore can reliably record low-frequency signals.
The oil and gas deposit in the geological environment will absorb the background vibrations. Absorption in the oil and gas reservoir is frequency-dependent and reaches the highest values at low frequencies (1-5 Hz). This is related to the frequency dependence of the forces of viscous internal friction between the fluid and the frame of the reservoir. Absorption reduces the interference effect from the boundaries of the oil and gas reservoir and causes abnormal reflection in the low-frequency range. Anomalous reflection from the oil and gas deposit leads to a change in the AFC of the medium above the reservoir. Therefore, analyzing the spectra in the low-frequency range, one can draw conclusions about the presence of hydrocarbons in the environment.
To interpret the accumulated spectrum, a basic velocity model based on seismic survey, VSP and acoustic velocity logging is used for numeric simulation of the propagation of vertically directed P-waves. The oil and gas deposit in the model is defined as a layer with reduced speed and increased absorption. Matching the properties of the deposit in the model with the existing data achieves the likeness of the model spectrum with the real one.
The LFS technology was successfully tested offshore with the use of submersible and re-surfacing nodes. A hermetically sealed device hosts a three-component low-frequency seismometer, a low-noise amplifier, a recorder, a battery, a horizontal level stabilization system, and a gyroscope. The devices are recovered via an automatic re-surfacing system, which unfastens the ballast, leaving it on to the seabed to eventually turn into environmentally harmless sand. The method allows the user to quickly install and remove the devices from depths of up to 6000m.