OVERVIEW OF GEOPHYSICAL METHODS OF INVESTIGATION THROUGH A CASE STUDY – MASW & ERT
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Geophysical methods are popular way of identification of subsurface strata during reconnaissance survey before detailed geotechnical investigation. These methods are also useful to confirm if there are any major changes at the site where minimal borehole has been made for geotechnical investigation. This way the total investigation cost can be optimised to a great extent.
Most common geophysical methods are
This article presents results from a recently completed study for a mining project where Seismic and Electrical methods are used in conjunction to understand the subsurface soil layering.
Plot 1:
The profile (Figure 9a) obtained from the analysis of the records suggests a soft soil layer is present above the depth of 5 m with a VS of 200 – 300 m/s. Below 5 m depth, two distinctive features can be seen at both the ends of the profile, having VS in the range of 600 – 800 m/s. These features can be classified as dense soil or weathered rock or soft rock. These features seem to be up to 50 m from start and 100 m from start to end of profile. Between these two features (from 50 m to 100 m from start of line below the depth of 5 m) a softer layer of the soil has been found with the VS in the range of 400 – 600 m/s.
A profile obtained from ERT survey is also presented in Figure 9b. The section captures material interpreted as Iron rich clay and Alteration (due to weathering) as interpreted from the resistivity values. With detailed comparison of the results of MASW and ERT we can see that both of them agree with each other. While reading the two results it must be noted that ERT profile is starting from start of line and MASW profile starts at 23 m from start of line. Both the profiles show a softer section at the centre of line with rock or stiff soil type material at both the ends.
Plot 2:
The profile (Figure 10a) obtained from the test suggest a geomorphological formation where depth of softer soil cover in increasing with distance from hilly Nala in the close proximity. The top layer which has varying thickness of 5 – 20 m suggest a landfill created by engineered fill has shear wave velocity of 200 – 400 m/s. At the start of the profile up to 45 m from start a stiff soil or soft to medium type rock formation can also be interpreted at the depth of 5m. The sloping layer has shear wave velocity of 600 – 800 m/s which can be interpreted as stiff soil formation mixed with pebbles (characteristic material dumped by hilly Nalas) or soft rock.
Figure 10b represents an inverse model of resistivity section for traverse line of 240m. ERT line was located 15 m away from the MASW profile but they can be used for correlation purpose and validation. The ERT section also suggest a sloping geomorphological profile (although not with similar slope as observed in MASW). Also, a softer layer is observed in MASW around 75 m from the start of line. The ERT results also suggest a similar formation.
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Conclusion:
Two geophysical methods had been employed for the survey namely Multi – Channel Analysis of Surface Waves (MASW) and Electrical Resistivity Tomography (ERT).
The results of these two surveys are presented with interpretation and explanation. There is only one line where MASW and ERT has been performed over the same alignment i.e. Plot 1. For this line, the results of both the tests agree. Both the results show two rock or dense soil masses separated by stiff soil. Although other MASW line do not coincide with nearby ERT line (Plot 2) but still their results can be compared to some extent due to proximity.
This study shows the use of geophysical survey to identify detailed subsurface strata up to great length without borehole or trenching. If a geophysical survey is planned with care it can save lot of resources required for geotechnical investigations using boreholes.
Geophysical survey provides an approximate information of the site but the information can be obtained for larger area. This information can further be used to identify the critical areas for detailed and accurate geotechnical investigation thus optimizing the required number of boreholes.
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