• Earthquake and Geophysics Division

1. Electrical Resistivity Tomography (ERT)

Electrical resistivity method is a non-invasive geophysical method to infer subsurface geological interfaces based on measurement on the surface. Resistivity method takes advantage of the differences in earth material resistance that responds to the application of electric current through the material body and measurement of corresponding potential difference. Different types of soil compositions and rock formations have different resistivity (inverse of conductivity). ERT method is widely used for groundwater resources mapping, site investigation for engineering purposes, landslide mapping and active fault delineation.

2. Seismic Refraction Tomography (SRT)

Seismic refraction is a geophysical technique governed by Snell's Law. The method is widely applied in the field of near-surface investigations such as engineering geology, geotechnical engineering and exploration geophysics. Refraction profiles are performed using geophones spaced at an appropriate intervals along a straight line; a seismograph records the signals generated by active sources including hammar, blasting, seismic gun or vibroseis. The refraction method is effective in resolving geologic interfaces with sufficient velocity contrast.

3. Ground Penetrating RADAR (GPR) test

Ground-penetrating RADAR (GPR) is a geophysical method that uses radar pulses to image the subsurface. The method is non-destructive in nature and uses electromagnetic radiation in the microwave band (UHF/VHF frequencies) of the radio spectrum, and detects the reflected signals from subsurface structures. GPR method can be applied to study rocks, soil, ice, fresh water, pavements and structures. In the right conditions, practitioners can use GPR to detect subsurface objects, changes in material properties, and voids and cracks. GPR uses high-frequency radio waves, usually in the range 10 MHz to 2.6 GHz. A GPR transmitter emits electromagnetic energy into the ground. When the energy encounters a buried object or a boundary between materials having different permittivities, it may be reflected or refracted or scattered back to the surface. The method is considered to be more effective at shallow depth.

4. Microtremor Survey

Unlike seismic refraction method, passive source is used for microtremor survey. It is one of the emerging new methods applied in the field of applied geophysics and engineering. Subsurface mapping can also be done using micro-seismic survey which uses the signals generated by natural sources. The set of seismic equipment necessary for the survey can detect the shear waves velocities. The different soil and rock composition will have distinct characteristics which will have different values of shear wave velocity. This velocity value can be correlated and interpreted in the form of subsurface geological strata. This method is even helpful in understanding the level of hazard and risk of earthquake events in the area.

5. Earthquake information

Earthquake and Geophysics division under Department of Geology and Mines has established the National Earthquake Monitoring Network (NEMN), which currently has 14 source monitoring stations, 3 continuous GPS stations and 20 intensity meters in each district across the country. The division is currently installing 205 intensity meters in various Gewog as well. The division disseminates earthquake source and intensity information, and also prepares the country?s seismicity maps.

6. Hazard Maps

Seismic Hazard is the hazard associated with potential earthquakes in a particular area and a Seismic Hazard Map shows the distribution of earthquake shaking levels that have a certain probability of occurring. Hazard maps are constructed by considering:

a. Active Faults

b. Past earthquakes and possible future earthquakes

c. Near-subsurface soil conditions

These maps are created to provide the most accurate and detailed information possible to assist engineers in designing buildings, bridges, highways, and utilities that will withstand shaking from earthquakes.

7. Drone Mapping

Drone mapping is the practice involves acquiring hundreds of aerial images and then ?stitching? them together digitally with specialized mapping software to make a larger more accurate composite image.