LOFRES

The project was based on an innovative method utilizing global and local seismic noise. The frequency of the analyzed signals ranges from 0.1 to 3 Hz, and with cultural noise, up to 30 Hz. The investigation can cover depths from several tens to several thousand meters.

Prior to the project's implementation, Poland lacked a system enabling such studies as well as established methodological foundations. The project team made a fundamental contribution to the development of the measurement system and methodology.

The LOFRES system enables automated, continuous vibration measurements in the low-frequency range using appropriately configured measurement modules and a cooperating data acquisition center.

• development of the concept of a maximally automated system for measuring vibration velocity in field conditions with an appropriate dynamic range in the low-frequency range, as well as the recording, processing, and interpretation of large datasets,
• development of theory, measurement process modeling, measurement methodology, as well as processing, interpretation, and calibration algorithms,
• practical verification of the model, theory, and methodology in the planned research work,
• development of system software,
• creation and testing of a technology demonstrator capable of monitoring near-surface layers using seismic interferometry or microseismic sounding,
• adaptation of the system to perform measurements in various configurations depending on the needs and the applied method,
• implementation of an optimal version with mobile autonomous stations equipped with single- or three-component sensors and stationary reference stations,
• the measurement and reference stations were equipped with battery power, a GPS receiver for precise time base synchronization and determining sensor positions during measurements,
• the system was equipped with storage media and radio transmission transmitters for sending data via a mobile server and GSM LTE modems,
• minimization of measurement session costs by cyclically relocating mobile stations to different grid nodes, enabling multiple reuses of equipment,
• possibility of permanent field installation of the system for continuous real-time tracking of changes (e.g., in mining to monitor stress changes and deformation).

Studies conducted at various test sites led to the formulation of the following conclusions:

• seismic interferometry allows identifying the degree of rock mass disturbance caused by mining operations based on the shear wave velocity field,
• when designing the measurement layout, the frequency characteristics and directivity of seismic noise in the survey area must be investigated,
• analysis of relative coda wave velocity variations obtained by seismic interferometry provides valuable information about the degree of rock mass disturbance,
• using all components (Z, N, E) reduces estimation errors of relative coda wave velocity changes associated with the inhomogeneity of the seismic noise wavefield,
• under conditions of strong seismic noise, the results of the MASW method are less useful compared to seismic interferometry or refraction, despite the use of various filtering techniques,
• the seismic interferometry methodology enables determining the S-wave velocity field in the geological medium, which is of significant importance for engineering applications,
• the proposed processing and interpretation methodology proved effective under the measurement conditions of the conducted studies,
• the results indicate that seismic interferometry can be applied to investigate the structure and elastic properties of the near-surface geological medium and to analyze spatio-temporal changes associated with landslide movement.

Knowledge of the structure of the geological medium, and particularly the degree of its disturbance, is essential for the foundation of various types of large-area structures, as well as tunnels, airports, transportation corridors, embankments, hydrotechnical structures and reservoirs, or residential developments.

In areas of active and completed mining operations, the system supports the assessment of surface hazards resulting from continuous and discontinuous deformations, enabling the selection of an optimal revitalization method.

The system can be used for:

• assessing the degree of rock mass disturbance in areas exposed to induced seismicity and tectonic tremors,
• monitoring the depletion degree of various deposits (crude oil, gas, shale, rock salt, sulfur by underground melting, etc.).

The advantage of the developed low-frequency passive tomography system over the conventionally used active tomography is that there is no need to use explosive charges or expensive vehicle-mounted exciters to generate the probing wave. There is no associated damage to the ground surface. The measurement is virtually non-invasive (limited exclusively to sensor installation). The system enables continuous measurement in mining operation areas.

Those primarily interested in it include:

• construction companies carrying out the construction of large-area structures,
• exploration geophysics companies,
• the mining industry – for tremor hazard assessment and tomography of the mining longwall area.

The LOFRES system consists of hardware and software components used for processing and interpreting seismic data.

MS Methodology (Microseismic Sounding)

It requires the use of at least 2 measurement stations. One reference station (base) remains stationary and does not change its position. The remaining ones are mobile and can be moved to different measurement points. Recording at each point is carried out for a period of at least one hour in order to achieve stationarity of the record.

IS Methodology (Seismic Interferometry)

It does not require stationarity of the seismic wavefield, so a reference base is not necessary as in the MS method. However, deployment at all measurement points is required, which necessitates having a larger number of sensors.