10th Asian Regional Conference of IAEG (2015)

Four-dimensional electrical resistivity tomography of the groundwater in a landslide
zone
Zheng-yi FENG(1), Bo-Siang LIN(1), Hsin-Chang LIU(2), Chih-Tsai SUN(3),Wei-Chuan SONG(1)(1) National Chung Hsing University, Taiwan
E-mail: tonyfeng@nchu.edu.tw(2) National Chung Hsing University, Taiwan, (3) SUNRISE Geophysical Survey, Inc., Taiwan
Abstract
This study developed a four-dimensional electrical resistivity tomography (4-D ERT)
method and verified the method by field test. The 4-D ERT method is based on 3-D ERT
technique to evaluate the variation of the groundwater in a landslide due to rainfall and time
lapsing. It is required that fixed electrode array is arranged in a special layout in the field.
The 3-D ERT measures were performed from time to time for 8 times in about a year. The
method is helpful to engineers for understanding the spatial and temporal distribution ofgroundwater when performing landslide investigation.
Keywords: electrical resistivity tomography (ERT), groundwater, landslide
1.

Introduction

2.

In recent years, the monitoring of electricalresistivity (ER) has become an increasingly common
geological investigation technique. Zhang (2008)
monitored ER when studying the subterranean
structure of a mud volcano in Wusanto, Tainan
County, and the activity of the Qishan Fault. In thiswork, Zhang (2008) also incorporated thefour-dimensional (4D) concept. He measured the mudvolcano at four different times to examine how thesubterranean structure changes over time.

Methodology

We selected a landslide area as the research site.
The site was next to Meihua Elementary School inMeihua Village, Jianshi Township, Hsinchu County,Taiwan (see Fig. 1).

This study developed a 4D ERT approach inwhich time series of 3D electrical resistivity
tomography (3D ERT) were performed. The 3D ERT
utilizes a two-dimensional system to scan special
electrode arrays, collect data, and perform inverse
calculations (Geotomo Software, 2009).
Fig. 1 The study site in Hsinchu prefecture, Taiwan

We applied the 4D ERT to understand how
groundwater changes over time and associated with
rainfall in a landslide. The results obtained werecompared with the data from a report by Soil andWater Conservation Bureau, Taiwan (2013). This
technique can be used in landslide-prone areas orwhere landslides have already occurred. We hope togain a better understanding of groundwater changes
in landslides by studying temporal changes inelectrical resistivity in landslides.

Five test lines were laid out. The locations of thetest lines are shown in Fig. 2. Each of which included
16 electrode arrays were positioned five meters apart.
The layout of the electrodes and their number codes
are as follows: first test line (01-16), second line
(17-32), third line (22-48), fourth line (49-64), andfifth line (65-80).

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This can be attributed to severe weathering, fissuresand high permeability in the research site where is ina displaced landslide zone.
Test 8 (Fig. 8) preformed on 4 December 2013presented conditions drier than those obtained on 9September 2013 (Fig. 6). This is due to the fact thatno rainfall occurred in the week prior to the test date,thereby increasing ER.
In summary, groundwater was shown to drainaway from the colluvium of landslide zone within amatter of days, possibly due to highly fracturesmaterials and thereby very high permeability in thelandslide zone.
Fig. 2 Layout of the test lines and electrodes(numbered 1~80)
3.

4.

Based on the techniques developed for 3D ERT,
this study used fixed electrode arrays and time series
measurements to study the spatial and temporal
distribution of groundwater in a landslide area. The
research site, Meihua Village in Hsinchu, Taiwan, isclassified as a landslide prone area. Our primary
focus was on the changes in groundwater over time inthe landslide zone. Fixed electrodes arrays and test
lines were setup prior to the first test on January 25,
2013. Our results demonstrate that groundwaterdrains away from this landslide area within just a few
days after typhoon/rainfall, possibly due to highly
fissured and high permeability in the landslide zone.
The proposed 4D ERT testing method provides aindication of changes in groundwater caused byrainfall over time and could therefore be highly
valuable to engineers seeking to identify the locationand behavior of groundwater to assist in assessing therisk of landslide disasters.

Results and discussion

The 3D ERT was tested a total of eight times. The
test dates, and data file numberings are provided inTable 1.
Table 1 Information of the eight 3D ERT tests
Test no.
Test date
Data file no.
Test 1

2013/01/25

McOHM961

Test 2

2013/03/01

McOHM962

Test 3
Test 42013/03/15
2013/09/11

McOHM965
McOHM102

Test 5
Test 6
Test 7
Test 82013/09/25
2013/10/09
2013/11/06
2013/12/04

McOHM104
McOHM106
McOHM121
McOHM222

Conclusions

Acknowledgements

Among the eight 3D ERT test results, we selected
three test results to in Figures 3-8. The three tests are
Tests 1, 5 and 8 which are marked in bold in Table 1.
In addition, rainfall records and typhoon of the testsite were plotted in Fig. 9.
Our first ERT test was conducted on 25 January,
2013, the results of which were used as a reference tocompare with the other tests. There had been no
rainfall for one week prior to the test date of Test 1,
wherefore (Figs. 3 and 4) the shallow zone in theimages appears somewhat dry and ER is at
approximately 600 Ω ·m. This was adopted as an“initial value” in this study.
Typhoon Usagi September 20-23, 2013 brought
total rainfall of 89.8mm. Test 5 was conducted on
September 25, 2013, two days after themeteorological effect on Taiwan of Typhoon Usagi
had ended. However, the rainfall was very lightduring these two days, September 24-25, measuring
only 0.4mm. The similarity in the ER results in Figs.
4 and 6 indicate that most of the groundwater drained
away rapidly after the Typhoon Usagi in two days.

The authors acknowledge and express theirgratitude to the National Science Council for theirsupport of Project No. 101-2622-M-005-003-CC3,
and to the Qingshan Engineering Consultancy forproviding monitoring data.
ReferencesZhang, B.Y. (2008): Combining Three-Dimensional
Electrical Resistance Imaging Technology with
Ground-Penetrating Radar Data for Long-TermMonitoring of Subterranean Volcano Structure andQishan Fault Activity. A research report submitted
to National Science Council, Taiwan.
Soil and Water Conservation Bureau (2013):
Investigation Report on Landslide Potential in theMeihua Area (2/2). A report published by Soil andWater Conservation Bureau, Taiwan, R.O.C.
Geotomo Software (2009): RES3DINV - Rapid 3-D
Resistivity & IP inversion using the least-squares

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10th Asian Regional Conference of IAEG (2015)

method. User’s manual, published by Geotomo
Software, Penang, Malaysia.

Fig. 3 Test 1 (2013/01/25) 3D ERT Image

Fig. 5 Test 5 (2013/09/25) 3D ERT Image

Fig. 4 Test 1 (2013/01/25) - Vertical slice of 3D ERT
ImageFig. 6 Test 5 (2013/09/25) - Vertical slice of 3D ERT
Image

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10th Asian Regional Conference of IAEG (2015)

Fig. 7 Test 8 (2013/12/04) 3D ERT ImagFig. 8 Test 8 (2013/12/04) - Vertical slice of 3D ERTImageRainfall (mm/day)
500

Trami
8/20~8/23

450
400

422
Soulik
7/11~7/13

350
300

293

250

Usagi
9/19~9/22

200
178.2

150
10050

Fitow
10/4~10/7

1○0
2012/12/30

2○3○

2013/3/30

2013/6/30
Date

179.8

143

6 ○
85 ○
4 ○
7 ○○

82

59.2

102.6

2013/9/30

2013/12/31

Fig 9 Rainfall records and typhoon durations of the test site (Ref: Soil and Water Conservation Bureau, 2013);
1 ~○
8 . (Typhoons: Soulik; Trami; Usagi; Fitow)
Timings of the 8 tests performed were indicated as○4