Electrical Resistivity Structure of Changbai Volcanic Mountain:

Results from Magnetotelluric Exploration Methods

Gengen Qiu

, Hui Fang, Jie Zhang, Fagen Pei, Qingyin Lv and Yan Peng

Institute of Geophysical and Geochemical Exploration, Chinese Academy of Geological Sciences, Langfang, Hebei, China

Email:qiugengen@igge.cn

Keywords:

Magnetotelluric, Changbai Mountain, magma chamber

Abstract: To address the foreign media’s comments that the Tianchi Volcano in Changbai Mountain is likely to erupt

in the near future, we collected a magnetotelluric (MT) observation profile approximately 120 km in length

in the area to detect the deep electrical structures of magma chamber. Because electromagnetic noise from

an unknown source is very strong in the research area, we used remote-referencing and robust processing,

rhoplus analysis, impedance tensor decomposition, and a mutual approximate calculating method between

apparent resistivity and phase for MT data processing. These techniques resulted in relatively reliable

electromagnetic response data in the strong electromagnetic noise area. The results show that there is a clear

magma channel beneath Tianchi Volcano, at a depth of approximately 10 km. An obvious abnormal body of

low resistance, where resistivity is less than 10 Ω•m connects with the magma channel at 7 km deep and

north of the existing crater, suggesting a developing magma chamber in the shallow surface. There are two

nearly vertical low resistivity zones at 7-17 km in depth, one between stations C07-C09 and the other

between stations C04-C05, that connect directly to the low resistance body directly; these low resistivity

zones are likely active fault zones. Crustal low resistivity anomaly bodies are widely developed at depths of

13 – 30 km about 20 km south of Tianchi Volcano, suggesting an active magma chamber. The MT sounding

method results indicate that Changbai Mountain has a high risk of volcanic eruption, and the MT method

performed well detecting the state of current volcanism.

1 INTRODUCTION

In 2012, Taniguchi Hiroshi stated that the

probability of a volcanic eruption at Changbai

Mountain was 99% over the next 20 years based on

a "3-11" earthquake that occurred in 2011. To

address the foreign scholars’ claims that Changbai

Mountain volcano may erupt in coming years,

China's Changbai Mountain Administrative

Committee official responded based on data from

the China Earthquake Administration, the

Seismological Bureau of Jilin Province. The Tianchi

Volcano monitoring station was established at

Changbai Mountain in 1997, and all observations

since then have indicated that Changbai Mountain

shows no active volcanism. Although the most

recent eruption of the Changbai Mountain volcano

remains unconfirmed, the active period of the

Northeast Asian geological plate has attracted the

attention of many scientists.

Changbai Mountain is located at the border of

northeast China and North Korea. The middle of the

crater is Tianchi Lake, with half belonging to China

and the remaining half belonging to North Korea.

The water surface is 2150 m above sea level and the

average water depth is 204 meters. It is the deepest

lake in China and the border lake between China and

Korea; its storage capacity is more than 2 billion

cubic meters. Tianchi Volcano at Changbai

Mountain is the most comprehensive Cenozoic

composite volcano preserved in China (Liu et al.,

1995). In geological structure, Changbai Mountain

is located above the deep subduction zone of the

Western Pacific Ocean and a series of network faults

composed of NE and NW faults are developed in the

area. There have been several eruptions of this

volcano in history, with three credible volcanic

eruptions in 1668, 1702 and 1903 (Jin and Xi,

1994). Changbai Mountain is one has some of the

most intense Quaternary volcanic activity in China

344

Qiu, G., Fang, H., Zhang, J., Pei, F., Lv, Q. and Peng, Y.

Electrical Resistivity Structure of Changbai Volcanic Mountain - Results from Magnetotelluric Exploration Methods.

In Proceedings of the International Workshop on Environment and Geoscience (IWEG 2018), pages 344-352

ISBN: 978-989-758-342-1

and is also one of the most dangerous volcanoes in

recent years in China (Liu et al., 1995).

Because magma has clear characteristics of low

resistance, magnetotelluric (MT) exploration in the

volcanic area can provide important constraints on

volcanism. The MT sounding method is sensitive to

low resistivity structures and is an important method

for deep exploration. It has been widely applied in

deep structure detection and resource exploration

(Jin et al., 2010; Wei, 2002). Tang Ji and others

completed five umbrella lines and one North-West

line around Changbai crater and nearby areas. Sixty-

one stations were installed for broadband

magnetotelluric sounding data, and their detection

results showed that at Changbai Mountain Lake and

the north and east regions, there is very low

resistivity at depths of about 12 km; the resistivity of

abnormal body is ten to dozens of Ω•m, indicating a

probable crustal magma chamber (Tang and Liu,

1997; Tang and Li, 2001; Tang et al., 1999). The

purpose of this study is to investigate underground

volcanic magma chamber system and electrical

characteristics of the crust and upper mantle to

provide a geophysical basis for hazard prediction

and disaster evaluation of volcanic eruptions.

2 DATA ACQUISITION

Figure 1: Distribution of MT stations at the Tianchi

Volcanic area at Changbaishan Mountain. The center of

the survey line is Tianchi Lake. The length of the

measuring line is approximately 120 km.

Changbai Mountain scenic area is a key 5A level

tourist attraction in China. With the development

and utilization of the scenic area, various large

electromagnetic interference sources are spread

across the study area. Therefore, data acquisition

and data processing were two major key technical

problems for this project. Based on prior working

characterizing the geological deep structures in the

Tianchi Volcanic Area of Changbai Mountain, the

area shows two dimensional structures at NW and

NNW (Tang et al., 1999; Li et al., 1995). There are

also three dimensional anomalous bodies

corresponding to the regional tectonic background.

Most of the work area is covered by virgin forest ,

vegetation flourishes, and the terrain is undulating;

therefore, it is impossible to arrange a straight line

survey. For the convenience of comparative analysis

to the results from previous studies, we designed one

north-easterly MT observational profile with a

length of 120 km (Figure 1). Forty-two stations were

arranged along the line for broadband MT and long-

period MT measurements at a spacing interval of

about 2.5 km. For the strong electromagnetic noise,

wide frequency and long period data were acquired

simultaneously, which improves the data quality of

the low frequency signal

Figure 2: Magnetotelluric sounding curves for station No.

C03. The closed points represent observation data

acquired using the wide-band magnetotelluric sounding

instrument MTU-5A/P, and open symbols represent the

observation data acquired using the long period

magnetotelluric sounding instrument LEMI417M.

Electrical Resistivity Structure of Changbai Volcanic Mountain - Results from Magnetotelluric Exploration Methods

345

Changbai Mountain scenic area is a key 5A level

tourist attraction in China. With the development

and utilization of the scenic area, various large

electromagnetic interference sources are spread

across the study area. Therefore, data acquisition

and data processing were two major key technical

problems for this project. Based on prior working

characterizing the geological deep structures in the

Tianchi Volcanic Area of Changbai Mountain, the

area shows two dimensional structures at NW and

NNW (Tang et al., 1999; Li et al., 1995). There are

also three dimensional anomalous bodies

corresponding to the regional tectonic background.

Most of the work area is covered by virgin forest ,

vegetation flourishes, and the terrain is undulating;

therefore, it is impossible to arrange a straight line

survey. For the convenience of comparative analysis

to the results from previous studies, we designed one

north-easterly MT observational profile with a

length of 120 km (Figure 1). Forty-two stations were

arranged along the line for broadband MT and long-

period MT measurements at a spacing interval of

about 2.5 km. For the strong electromagnetic noise,

wide frequency and long period data were acquired

simultaneously, which improves the data quality of

the low frequency signal.

At each station, an induction type MTU - 5p

broadband MT sounding instrument and flux-gate

type LEMI417M MT sounding instrument were

deployed for combined observations. Usually, the

low frequency signal obtained using the long period

instrument is better than the wide-band MT

sounding instrument (Ye et al., 2013). The

observational frequency range of the broadband MT

instrument is 320-1/1000Hz, and the range of the

long period MT sounding is 1/500-1/10000Hz. The

data acquisition time for each wide-band MT station

was 30 hours, while the data acquisition time for the

long period MT station was 90 hours. Figure 2

shows the MT sounding response data for station

No.C03, where both sets of instruments were

deployed. The curve quality in the low frequency

section, acquired by LEMI417M, is better than that

of MTU-5P. With the combined observations from

two sets of instruments at the same station, the data

quality in the low frequency section can be

improved.

3 DATA PROCESSING

During field data collection, we tested consistency

with two mechanisms and repeated measurements to

control data quality. Changbai Mountain is located

at the border of northeast China and North Korea,

where there is strong electromagnetic noise. Reliable

electromagnetic response data was obtained after

remote-reference and robust processing, rhoplus

analysis, impedance tensor decomposition and

applying a method for mutual approximate

calculations between apparent resistivity and phase.

3.1 Remote Reference Technology

(a)

(b)

Figure 3: Data from station No.C14 before (a) and

after (b) processing by remote reference technology.

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346

Remote reference technology is based on the

principle that the magnetic field signal changes little

at over a short time in a certain range, but the noise

between the measurement and reference stations is

generally unrelated over a certain distance.

Therefore, the method has the effect of suppressing

interference (Yan and Hu, 1998; Edwards and Hastie,

1997). Due to the strong electromagnetic noise in

the work area, we chose a flat terrain with little

noise 40 km away from the working area to build a

long-term reference station during field data

collection. All station data were processed by

magnetic remote reference technology, and the data

quality from most stations was improved after

reference processing. Figure 3 compares the results

collected at station No.C14 before and after

processing by remote reference technology. It is

clear that remote reference processing technology in

Changbai Volcanic Mountain has a certain

suppression effect on electromagnetic noise.

3.2 Static Shift Correction

The additional electric field produced by the

accumulation of charge around local inhomogeneous

bodies usually results in an offset in the overall

curves, which easily produces false geological

anomalies if the curves are uncorrected. In the case

of transverse homogeneous media, the high

frequency segments of two apparent resistivity

curves at one station should have consistent data

values according to the principle of adjacent station

similarity and regional similarity. The high

frequency section of the apparent resistivity curve of

an adjacent station should be almost identical (Chen

et al., 2004; Weidelt and Kaikkonen, 1994; deGroot-

Hedlin, 1991; Zhang et al., 2015). Using a unified

analysis of all stations in the work area, a manual

static shift correction was made to the apparent

resistivity curve based on the static shift having a

great influence on apparent resistivity and little

influence on phase.

3.3 Mutual Approximate Calculation

Method between Apparent

Resistivity and Phase

Because the MT impedance is the minimum phase

response function in one-dimensional media, for the

minimum phase response function, the relationship

between amplitude and phase angle is provided from

the Hilbert transform formula (Fu, 1983; Fu, 198)

lg ( )

()

∞

−∞

=−

−

∫

(1)

The approximate formula can be obtained from:

()

44lg

ππ

θω

≈+

(2)

Concurrently, the following relationship can be

obtained:

ωπ

=−

(3)

Figure 4: Results for station No.C14 based on the mutual

approximate calculation method between apparent

resistivity and phase curves. As shown, the quality of the

low frequency signal is poor for strong electromagnetic

noise. (a) Measured curves and calculated theoretical

curves and (b) results for station No.C14 after correction.

Electrical Resistivity Structure of Changbai Volcanic Mountain - Results from Magnetotelluric Exploration Methods

347

The apparent resistivity and phase curves can be

calculated using Equations 1-3, and some noise

influence can be corrected with this method. Figure

4 shows the processing results for station No.C14

based on the apparent resistivity and phase mutual

calculation technology method. The curve with open

symbols indicates the results calculated using the

trend in apparent resistivity and phase data. As

shown in Figure 4(b), the MT data acquired is closer

to the theoretical data when after modifying the

apparent resistivity and phase data at a noise

frequency while referencing the theoretical

calculations value.

3.4 Other Processing Techniques

In addition to the described methods, there are

additional advanced processing technologies, such

as robust processing, rhoplus analysis, impedance

tensor decomposition and others (Egbert and Booker,

1986; Chave et al., 1987). We ultimately obtained

relatively reliable electromagnetic response data in

the strong electromagnetic noise area using these

different techniques.

4 DATA ANALYSIS AND

QUALITATIVE EXPLANATION

A qualitative estimate of underground electrical

structure in the Tianchi Volcanic area was obtained

using 2D skewness and the analysis of the real

induction vector. The profile was generally

characterized using the 2D structure. The tectonic

direction is primarily northwest. In addition, there is

a 3D low resistivity body localized at shallow and

deep depths.

4.1 Analysis of Two-Dimensional

Characteristics

The two-dimensional deviation S (skewness) of the

MT sounding impedance is a parameter reflecting

the 2D dimension characteristics (Chen and Wang,

1990). For one dimension and the ideal two-

dimensional structure, S=0. Generally speaking,

smaller S values results in a more two-dimensional

underground medium. It is generally believed that

when the S value is less than 0.3 ~ 0.4, the

underground structure can be considered a two-

dimensional feature. Finally, S values greater than

0.5 indicate a three-dimensional medium in the

corresponding depth underground. The MT result

data after editing was used to calculate skewness

along the profile (Figure 5). As shown, most of the

stations along the profile show 2D structure

characteristics in shallow space and strong three-

dimensional characteristics at depth. Affected by

strong electromagnetic noise, the low frequency

skewness may be influenced and unreliable.

Figure 5: Analysis of 2D skewness along the observation

profile. Most stations show 2D structure characteristics at

shallow depths and three-dimensional characteristics

deeper. The 2D skewness value is larger at low frequency

(less than 0.1 Hz), which is likely to be affected by noise

and unreliable.

4.2 Analysis of Two-Dimensional

Tectonic Direction

Due to the continuous subduction of the Western

Pacific plate toward mainland China, Northeast

China has been squeezed. The direction of

maximum principal compressive stress in the study

area is NEE - SWW, and the main fault shows strike

slip movement. The results from satellite

photographs and gravitational field interpretation

show that a group of NW westward faults are

distributed parallel at the Tianchi volcanic crater and

its vicinity (Li et al., 2006). Groom and Bailey

proposed the tensor decomposition theory to manage

the local surface current distortion near the surface,

which is called GB decomposition. The theory

decomposes the impedance tensor into a distortion

tensor and regional two-dimensional tensor. This

restores the undistorted region two-dimensional

impedance tensor data, providing the regional

tectonic direction angle, and solving the distortion

factors, such as shear and distortion in the local

distortion effect (McNeice and Jones, 2001). In this

study, the method for impedance tensor GB

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decomposition was used to analyze all station

tectonic trends. Figure 6 is a statistical analysis

diagram of tectonic trend showing that the tectonic

trend for most stations is primarily northwest.

Figure 6: Analysis of the 2D tectonic direction using the

impedance tensor GB decomposition method.

4.3 Induction Vector Analysis

The induction vector for MT sounding was obtained

through the transfer function of the magnetic field.

The induction vector is an important vector

constructed by tilting data. It plays an important role

in delineating low resistivity anomaly bodies and

analyzing structural trends and deep dimensional

characteristics. Its greatest advantage is that it can

provide lateral electrical information about the

corresponding depth below, which is mainly used to

reflect large scale structural features. In addition, the

magnetic field is almost unaffected by local

distortion, and there are no static MT curve shifts.

The induction vector is divided into the real

induction vector and virtual induction vector. The

real induction vector has been widely recognized

and applied because of its clear directivity and

stability. In this study, the direction of the induction

vector is directed to high resistance. If the

underground resistivity varies with depth alone

without lateral variation (one dimensional medium),

the magnetic induction vector is zero.

According to the distribution map of induction

vectors at different frequencies, the approximate

position and distribution range of the

inhomogeneous medium in the underground

medium can be determined. The relevant

relationship is that lower frequencies correspond to

deeper reactions. Due to serious electromagnetic

noise in the work area, the induction vector analysis

in this study can only analyze the characteristics of

the macroscopic electrical change. Figure 7 shows

the distribution of real induction vectors along the

observed profile, in the map upward direction

towards west.

Figure 7: Analysis of the real induction vector; here, up

indicates west. In this paper, the direction of the induction

vector is directed to high resistance. The whole profile

contains five abnormal zones with low resistivity.

From the regular direction of the real induction

vector, from shallow to deep, the whole profile

contains five abnormal zones with low resistivity.

The No.1 low resistivity zone is located at the

bottom of the Tianchi Volcano and below, and the

real induction vectors on both sides are all away

from the direction of the crater. This direction may

represent the Tianchi water body and magma

channel. The No.2 low resistivity zone is located in

the north direction of the Tianchi volcanic crater.

There is a local solid induction vector pointing

northwest, indicating that there is a local low

resistivity anomaly in the southeast direction. The

No.3 low resistivity zone is located between stations

C07 and C09, which is also located near the

Changbai Mountain gate. From the morphology of

the real induction vector, the anomalous area is

approximately vertical. The No.4 low resistance

zone is located between stations C04 and C05, and

is also an approximately vertical strip. As the

frequency decreases, the real induction vector begins

to change, and gradually loses obvious regularity.

The No.5 low resistivity zone is located in the deep

part of the earth's crust. The real vector direction

deviates from the position 20 km south of Tianchi

Volcano, suggesting that the range of the deep and

low resistance abnormal body is large, and the

anomaly area may correspond to a deep magma

capsule. With a further reduction in frequency, the

Electrical Resistivity Structure of Changbai Volcanic Mountain - Results from Magnetotelluric Exploration Methods

349

direction of the real induction vector becomes

disordered and its directivity may be influenced by

strong electromagnetic noise.

5 ANALYSIS OF 2D INVERSION

RESULTS

According to direction of the profile, we can obtain

corresponding accurate TE and TM polarization

mode data by rotating the impedance tensor -90

degrees. A large body of research has shown that a

more accurate inversion result is usually obtained

using the TM polarization mode for the two-

dimensional inversion. Especially in complex

geological conditions, the inversion results from the

TM polarization mode are more reliable than other

inversion results using other polarization mode data

(Dong et al., 2012; Cai and Chen, 2010; Rodi and

Mackie, 2001). The two-dimensional electrical

structure model for the Changbai Mountain igneous

area is obtained through nonlinear conjugate

gradient two-dimensional inversion and

regularization "L curve" factor analysis (Figure 8).

Figure 9 shows the apparent resistivity and phase of

the inversion result fitting. As shown, the calculated

values of the model are in good agreement with the

observed values, and the inversion results are more

reliable.

Figure 8: MT 2D inversion result for the volcanic area at

Changbai Mountain. From the diagram, there are clear low

resistivity anomalies below the Changbai Mountain

volcanoes.

Figure 9: Fitting the inversion result. The calculated

values of the forward model are in good agreement with

the observed values, and the inversion result is reliable.

Note that the left side shows the apparent resistivity map

and right side shows the phase map. The top panels are

observed data, and bottom panels are the calculated values

from the 2D inversion result model.

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From the two-dimensional inversion results in

Figure 8, there are clear low resistivity bodies and

electrical gradient zones at depth along the profile,

which correspond to low resistivity abnormal bodies

and fault structures at different depths. Because

Tianchi Volcano at Changbai Mountain is located on

the subduction zone of the Western Pacific Ocean,

and is affected by the compression of the Western

Pacific plate, the subduction produces tension in the

back arc area, and a series of northern oblique faults

developed near the shallow surface of the working

area. According to the characteristics of large

variation gradient and dense contour line, 11 fracture

structures are deduced along the profile. Most of the

shallow surface of the working area is covered by

volcanic debris and Quaternary sediments, which are

characterized by low resistance in the inversion

results. At depth (~20 km), the profile is

characterized by low resistance in the middle region

and high resistance at the south and north areas. The

high resistivity body corresponds to the relatively

high deep resistivity of the crystalline matrix rock

mass, and the middle low resistance corresponds to

the magma channel and the low resistivity anomaly

body. The magma channel located beneath Tianchi

Volcano is an erect structure and closed at about 10

km. At the bottom of the crater, the position

approximately 7 km north shows a clearly abnormal

body of low resistance, resistivity < 10Ω•m, which

connects with the magma channel. Based on these

characteristics, we propose that it may be a

developing magma chamber in the shallow surface.

The abnormal regions between C07 and C09 stations

and between C04 and C05 stations show two clear

nearly vertical low resistivity zones at a depth of

about 7 – 17 km, directly connecting with the below

low resistance body, suggesting that the low

resistivity zone is an active fault zone. With

increasing depth, the crust low resistivity anomaly

bodies are widely developed at depths of 13 – 30 km

approximately 20 km south of Tianchi Volcano,

suggesting that it may be active magma chamber.

There have been differing opinions about the

formation mechanism of volcanoes in northeast

China. A variety of viewpoints have been proposed

by prior researchers, and Tang Ji and others believe

that the modern volcanic activity in the northeast

region is closely related to the dewatering of the

subduction plate of the Western Pacific plate (Hu et

al., 2007; Tang et al., 2006). From the two-

dimensional inversion result model presented in this

study, we can see that the eruption of Changbai

Mountain volcano is closely related to the Western

Pacific plate.

6 CONCLUSIONS

Evaluating the possibility of an eruption of the

Changbai Mountain Tianchi Volcano has become an

important research topic in recent years. In this

study, the MT sounding method was applied to

detect deep electrical structures at Changbai

Mountain, and a nearly north-south observation

profile was surveyed across Tianchi Volcano. A

two-dimensional electrical structure model was

constructed, and the following conclusions obtained.

The electrical resistivity structure of Changbai

Mountain clearly shows the presence of a magma

channel beneath Tianchi Volcano; the magma

channel is closed at a depth of 10 km.

There is an obvious abnormal body of low

resistance, resistivity < 10Ω•m, which connects to

the magma channel. It lies about 7 km north from

the bottom of the crater, suggesting that it may be a

developing magma chamber in the shallow surface.

There are clearly two nearly vertical low

resistivity zones at a depth of 7 - 17 km located

between stations C07 and C09 and between stations

C04 and C05, directly connecting with the low

resistance body, suggesting that these low resistivity

zones are active fault zones.

With increasing depth, the crust low resistivity

anomaly bodies are widely developed at depths of

13 – 30 km approximately 20 km south of Tianchi

Volcano, suggesting that it may be an active magma

chamber.

A number of fault structures have developed in

the working area. According to the electrical

resistivity structure along the profile, 11 fracture

structures were deduced. Due to the extrusion

influence of the Western Pacific plate, a series of

extensional fracture structures inclined toward the

northwest have developed. The heat energy deep

underground is transmitted through the fault to

shallower depths, and a number of hot springs have

formed in the shallow space.

The exploration results obtained using the MT

sounding method show that Changbai Mountain has

a high potential eruption risk. Furthermore, the MT

sounding method performed well detecting the

spatial distribution and active state of the magma

capsule below the crater.

Electrical Resistivity Structure of Changbai Volcanic Mountain - Results from Magnetotelluric Exploration Methods

351

ACKNOWLEDGMENTS

We thank the reviewers for their valuable comments.

We also thank all the staff involved in the field data

acquisition of this project, they worked hard to

obtain valuable data. This study is supported by the

National Key R&D Program of China

(No.2016YFC0600201), the National Natural

Science Fund (No.41574068), Scientific Research

For Public Welfare Industry (No.201211095-2),

Geological Investigation Project

(No.121201108000160913, No.12120114005801),

and CAGS basic research project (No.AS2016J12,

No.JYYWF20180902).

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