SmartSolo seismic sensors have been used on many engineering geophysical projects where microtremor survey methods (MSM) have been successfully applied. As a highly effective technique, it is worth taking a closer look at MSM, to more clearly understand what it is, how it works, and what advantages there are using this approach.
What is a microtremor, and what makes it stand out beyond other exploratory methods?
Microtremors are a complex vibration phenomenon comprising body wave and surface wave components, where the surface wave energy is more than 70% of the total wave energy。A microtremor survey typically uses an array of seismometers to detect the ‘noise’ from the ground, with the most common array shapes being circular, triangular, cross, or straight line. The dispersion characteristics of the Rayleigh wave can be obtained from the acquired microtremor data, and then dispersion and H/V curves can be calculated and inverted. As a result, the shear wave velocity model and predicted lithology of the formation can be obtained, enabling shallow structures to be revealed and studied.
In more recent times, SmartSolo sensor products have been used to complete multiple urban micro seismic surveys, and there is still considerable demand for this type of project. Compared with other engineering geophysical methods, it offers the following advantages:
- Inherently safe, and will not damage any structures above or below ground;
- No operation restrictions, or concerns about interfering with the locale;
- Enables highly efficient data acquisition productivity at low cost;
- Very small footprint and low impact on the environment;
Some example microtremor projects:
Cavity detection beneath Airport apron:
When tunneling under an airport, there is the possibility of delamination gaps occurring between the concrete or tarmac pavement and the shield structure used while excavating. In this example, the main purpose of the microtremor survey is to investigate whether any hidden cavities were created under the airport apron or runway, and to determine their specific location and size. Sometimes referred to as lagging settlement, it is often necessary to monitor the near surface stability and changes after a tunnel boring operation passes beneath, to detect any dangerous holes caused by sand and soil loss.
Road safety risk investigations
1、Dynamic monitoring of substandard road foundations and subsurface stratigraphic changes;
2、Locate and investigate hidden holes in the ground below the road surface;
3、Monitor the delamination caused by sand and stone loss below the road surface;
When tunneling beneath or near to inner-city roads and buildings, the geological risk can be assessed and quantified using microtremor survey methods. With no drilling, no dust, and no noise, excavation and tunnel shield construction risk can be thoroughly understood with little or no interference to the ongoing local traffic or business activities.
The detection of structurally weak geology:
“A karst exploration during subway construction in Shenzhen”
The prediction of a karst cave by microtremor survey methods was found to closely agree with the results of drilling, and helped to form meaningful geological information in the space between drill samples.
With the help of microtremor survey technology and its ability to accurately locate and visualize the shallow cave structures, tunnel excavation is able to proceed and successfully navigate the karst area.
The exploration becomes a process of checking that the results from the microtremor data are closely consistent with the findings from a matrix of drill samples, and provides good confirmation of the karst cave locations.
Detection and positioning of municipal pipelines:
- Locate and investigate the size and direction of the pipeline.
- Determine the absolute position and relative positioning of the pipeline.
This form of engineering geophysical survey effort is very much result-oriented, and of course it is essential to use appropriate seismic processing and interpretation methods. For efficient data acquisition of this type, reliable high-quality seismographs are necessary, with small footprint, flexible deployment, and ease of operation. SmartSolo is an excellent choice for microtremor applications, and DTCC look forward to assisting with more projects using this valuable technique.