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Comparison between the findings of electrical resistivity methods and

microtremor array measurement (MAM) at two locations in Kathmandu

Valley

Surendra Raj Pant

Engineer-Geophysicist

Geophysical Research & Consultancy Service (P) Ltd

Lalitpur, Tel: +977 1 5553617; E-mail: grcs25@yahoo.com

Introduction

In recent years electrical resistivity methods have gained wide acceptance in geological and

engineering community of Nepal. Electrical resistivity methods have been applied not only for

groundwater and mineral exploration (conventional application) but also for geotechnical site

investigation. However, interpretation of the electrical methods in engineering projects is not always

appreciated by engineers and geologists. Experience and care are needed for the interpretation of the

electrical results applied for geotechnical site investigations. Furthermore, results are highly

dependent on the lithological types, their thickness and electrical conductivities. Principle of

equivalence (several models for the same data set) has strong effect in electrical methods than in

other methods of geophysical investigation. Moreover, it fails to provide mechanical properties

which are so important for geotechnical site characterization. At the same time there is impression

created by site investigation guidelines of Government of Nepal that electrical method is panacea for

every problem related with subsurface investigation. In long run this misconception will degrade our

capacity to understand the limitations and advantages of the methods and finally a message will be

sent to the international community of geoscientists and engineers that in Nepal there is no reliable

source of geophysical service. However, application of geophysical methods by knowing its

limitations and advantages and supplementing with other geophysical methods that can provide

geotechnical parameters helps to improve the final product of geophysical mission in site

characterization. By using seismic shear wave techniques one can calculate geotechnical parameters

of the subsurface. Among several methods of data acquisition for shear wave velocity (SWV)

passive source methods (recordings of microtremors) are widely accepted and applied for site

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investigation by international community of geoscientists and engineers. The reason behind this

acceptance is the low cost for data acquisition.

Comparison between electrical resistivity tomography (ERT) and microtremor array

measurement (MAM) results in Sano Khokana, Lalitpur

Initially electrical resistivity tomography (ERT) survey has been carried out to test subsurface for the

feasibility study of groundwater to fulfill the daily requirement of water for orphanage (Children’s

Home) in Sano Khokana. ERT survey was carried out in March 2010. The area was also previously

studied by vertical electrical soundings (VES) by Water Supply & Sewerage Corporation. In 2012

water well has been drilled in the premises of Children’s Home. The exact depth of drilling was not

documented because of the disputes with the locals the contractor left the site without completion of

the whole procedure. Verbally it was informed that they found some harder formation to be potential

for groundwater exploration related with the high resistivity zone at depth around 90 m. In April

2013 active source MASW and passive source MAM tests have been conducted. Active source has

limited depth of penetration whereas passive source has greater depth of penetration. The area under

investigation is calm so only low frequency microtremors that carry information from depth are

expected to be recorded. The site of MAM test was about 50 m towards the east from Children’s

Home. Since the geological setup of the area is not highly variable (no high lateral heterogeneity is

expected) the investigations carried out within this short distances can be compared. Here are the

interpretations of ERT and MAM results.

Electrical Resistivity tomography(ERT) in Sanokhokana (near Children’s Home)

Figure 1 show that near surface higher resistivity zone is related with the loose and less moist surface

layer. Its thickness varies between 10 m to 20 m along the profile. The variation of resistivity in

depth in this area is related with the variation of lithology. Higher resistivity zone in the intermediate

depth (depth to the top varies between 30 m to 40 m along the profile) is related with the onset of

coarser material. High resistivity zone which is continuous at deeper part (varies between depths 80

m to 90 m) is highly karstified bedrock (voids likely to be filled with mud).

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Figure 1. Electrical resistivity tomogram (Children’s Home, Sano Khokana)

Microtremor Array Measurement (MAM)

Microtremors are low amplitude ambient vibrations whose frequencies are more than 1 Hz and

microseisms are those whose frequencies of vibrations are less than 1 Hz. In both cases vibrations

are predominated by surface waves (mainly Rayleigh type wave). Rayleigh wave has dispersion

characteristics (dependence of phase velocity with frequency). The sources of microtremors are

related with both manmade activity (traffic, industry, etc) and natural activity (wind, river, sea

waves, etc). The principle of the surface wave techniques is that low frequency has longer wave

lengths and stresses material of greater depth and carry information from greater depth as well and

high frequency has shorter wavelength and stresses material at shallow depth and they carry

information from shallower depth. Since microtremors are predominated by low frequencies and

they are useful to explore deeper part of subsurface where active source methods are not cost

effective.

In Sano Khokana MAM recordings have been carried out along a line of length 88 m. The sensors

were low frequency geophones of natural frequency 4.5Hz. The interval between the geophones was

8 m. The recordings have been carried out in 12 channels. Total no of records were 23 and the

duration of each record was 32 seconds. The digitization of the records has been carried out at an

interval of 2 milli seconds.

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Figure 2. Dispersion image of microtremor array measurement (MAM) survey near Children’s

Home, Sano Khokana Site, geophone interval 8 m

The SPAC (SPatial Auto Correlation) function has been executed to extract the fundamental mode

from dispersion image (see Figure 2). Dispersion image shows that dispersion curve related with

fundamental mode, as low as 2 Hz can be extracted. The blue straight lines in the dispersion image

are the limiting boundaries. The lower line is the limit of aliasing effect and the upper line is for the

resolution of array used. Extracted dispersion curve (see Figure 3) were modeled to derive the

subsurface model of the SWV (see Figure 4).

0.0

0.5

1.0

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8.5

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Fre

qu

enc

y (H

z)

0 200 400 600 800 1000

Phase velocity (m/sec) Source= 0.0m

Dispersion curve : 123.dat-145.dat

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Figure 3. Dispersion curve extracted from dispersion image in Figure 2 at Sano Khokana Site.

Circles are the data and solid line is the theoretically calculated curve for the inverted model.

Similar to ERT model interpretation MAM model (see Figure 4) also indicate a four-layered

subsurface. First layer which extends between 0 m to 16 m is very loose clay and silt dominated

layer. The second layer which has higher SWV (300 m/s -350 m/s) is the indication of the

lithological change from clay domination to sand/gravel domination. This zone is indicated by

intermediate to high resistivity zone in ERT section. The third zone is SWV inversion zone (low

SWV zone which drops from 350 m/s to 325 m/s)) related with the change in lithology. In ERT

section the third SWV zone identified in MAM is indicated by low resistivity zone. The depth to the

top of the low resistivity zone varies between 40 m to 60 m and continues to the depths of 85 m to 90

m depths. The model obtained from MAM indicates that the possibility of the onset of high SWV

zone which is likely to be related with the high resistivity zone in the bottom part of the ERT section

stats at depth of about 80 m.

600

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50

0

Ph

as

e v

elo

city

(m/s

ec

)

0 2 4 6 8 10

Frequency (Hz)

Dispersion curve : 123.dat-145.dat

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Figure 4. Subsurface SWV obtained from the modeling of dispersion curve at Sano Khokana Site.

Microtremor array measurement (MAM) and vertical electrical sounding (VES) in premises of

Tribhuvan University, Kirtipur

The area of MAM test is very quiet and no microtremors of higher frequencies related with traffic

activity are expected to have influence in the record. By recording low frequencies noise it was a

hope to explore deeper part of that area. However due to the presence of thick loose clay layer the

depth information was limited to almost half of that what was expected. MAM test site is located at

about 50 m towards the east from the building of Central Department of Geology and the center of

the VES is located about 25 m towards the south-west from the start point of the MAM profile. The

center of the VES is at the location of groundwater well. Investigation by a deep VES was carried

out in 1996. The aim of VES was to probe the depth for the availability of groundwater and for the

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De

pth

(m)

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S-wave velocity (m/s)

S-wave velocity model (inverted): 123.dat-145.dat

Average Vs 30m = 198.6 m/sec

1623.2

1566.9

14411.1

16115.8

23321.0

31626.7

350

32.9

349

39.6

333

46.7

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54.4

328

62.6

337

71.2

358

80.4

388

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recommendation of the drilling depth. Field data with modeled theoretical curve are presented on the

left and inverted model is presented on the right of Figure 5. The layers parameters are presented in

Table 1. The result indicates that the lake clay extends to the depth of 153 m. This depth is the top of

the river deposits (aquifer) predominated by sand. And it is expected to be hard or more competent

formation of the area. In 1997 a borehole for groundwater development was drilled. The borehole log

shows that the onset of the silt dominated layer is at depth of 155 m and the sand dominated layer

starts at depth of 165 m. Close to the surface few meters of silt and silty clay was found and below

this all the way was lake clay down to the depth of 155 m. Very low resistivity of the lake deposit

indicates that the lake clay is likely to be loose.

Figure 5. Vertical electrical sounding that has been carried out in the premises of Tribhuvan

University for location of deep groundwater well (south east of Chemistry Department and South of

Environmental Study Department) at Kirtipur Site

Table 1. Layer’s parameters derived from the modeling of vertical electrical sounding (VES) at the

premises of Tribhuvan University, Kirtipur

Layer No. 1 2 3

Resistivity, Ohm.m 23 (silty clay) 7 (lake clay) 275 (predominantly sand )

Depth, m 9.5 153

1 10 100 1000

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1000

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ITY

(o

hm

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1000

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pth

(m

)

RESISTIVITY (ohm-m)

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Figure 6. Dispersion image of microtremor array measurement (MAM) survey in the premises of

Tribhuvan University, Kirtipur, geophone interval was 10 m and recording was carried out in 24

channels at Kirtipur Site

In January 4, 2014 MAM test was carried out along a profile of length 230 m long. Because of the

presence of terrace the profile has some degree of undulation. The geophone spacing was 10 m and

the geophone natural frequency was 4.5 Hz. To capture the low frequency microtremor the record

length was set to 60 seconds. Twenty five records have been used for SPAC analysis. Selection of

long profile length and long duration of record was the hope to capture the low frequency wave that

has long wavelength and carries information from the deeper part possibly below the lake deposit.

The dispersion image in Figure 6 shows that the fundamental modes of vibrations are towards the

lower frequency side (<3.5 Hz) and dispersion curve can be extracted between 1 Hz to 3.5 Hz. The

shift towards the low frequency side is the indication of very loose subsurface (low SWV). The

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qu

en

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(H

z)

0 50 100 150 200 250 300 350 400

Phase velocity (m/sec) Source= 0.0m

Dispersion curve : 1026.dat-1050.dat

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dispersion curve was modeled (see Figure 7) and the inverted model (see Figure 8) indicates that the

SWV of the subsurface is less than 300 m/s throughout the investigation. The maximum SWV

observed is 279 m/s and the minimum observed SWV is about 125 m/s. The model also indicates

that as the depth increases the consolidation of the lake clay also increases to some degree. This

smooth increase in SWV with depth is also the indication that there is no change in lithology within

the investigated depth. Compared to Sano Khokana the depth of penetration in this site with profile

length 230 m is considered to be much lower. Equivalent depth information was attained by 88 m

long profile in Sano Khokana whereas in Kirtipur site it was attained by 230 m long profile. In both

sites the depth information is controlled by wavelength which in turn is related with the SWV of

subsurface material.

Figure 7. Dispersion curve extracted from dispersion image in Figure 6 at Kirtipur Site. Circles are

the data and black continuous line is the theoretically calculated curve for the inverted model.

400

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50

0

Ph

as

e v

elo

city

(m/s

ec

)

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Frequency (Hz)

Dispersion curve : TU60seconds-record.rst

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Figure 8. Subsurface SWV obtained from the modeling of dispersion curve at Kirtipur Site

Conclusions

In this research comparison between subsurface information of electrical resistivity and microtremor

array measurements (MAM) has been carried out. Lithological interpretation of both methods is

comparable. Although the resistivity method can provide litholgical information it is unable to

provide parameters related with the strength of the material. However, shear wave velocity (SWV)

has direct relationship with the strength parameters of the material and SWV can be cost effectively

measured by MAM.

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De

pth

(m)

0 100 200 300 400

S-wave velocity (m/s)

S-wave velocity model (inverted): TU60seconds-record.rst

Average Vs 30m = 140.0 m/sec

1243.2

1236.9

12411.1

12815.8

14321.0

16626.7

184

32.9

199

39.6

215

46.7

232

54.4

249

62.6

257

71.2

267

80.4

279

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Comparison of the results of VES and MAM in Kirtipur Site have revealed that VES has mapped the

lake clay as a single layer whereas MAM indicates progressive increase in SWV which is likely to be

related with the increase in consolidation with depth.

Depth information attained by MAM is highly site dependent. Comparison of results of Sano

Khokana Site and Kirtipur Site reveal that the depth of information obtained in both sites were

equivalent despite much longer layout of the profile (almost three times in Kirtipur Site than in Sano

Khokana Site). This fact helps to make useful conclusion that the controlling factor for the depth of

information in MAM mainly depends on SWV of the material and to lesser extent the length of the

profile. Length of the profile is important for the determination of effects of aliasing and maximum

depth of resolution of the array used in the measurement.