Identification of Palaeochannels of Mihe River: An Approach Based

on the Integrated Geophysical Methods

Longfeng Guo

, Liangliang Li

, Lidong Fang

, Xu Wang

, Hengyu Jin

and Gang Wang

1,*

College of Water Conservancy & Civil Engineering,Shandong Agricultural University,Tai’an 271000;

Shandong Zhengyuan Construction Engineering Co.,Ltd,Jinan 250101;

China Metallurgical Geology Bureau,Geological Exploration Institute of Shandong Zhengyuan,Weifang 261000.

Email: gwang@sdau.edu.cn

Keywords: Mihe River, paleochannel, vertical electrical sounding, high-density electrical resistivity tomography,

resistivity

Abstract: Palaeochannel plays an important role in the economic construction. Geophysical methods are appropriate

tool to identify palaeochannel. On the basis of analyzed formation genetic type of the study area, this study

combining Vertical Electrical Sounding & High-density Electrical Resistivity Tomography were measured

paleochannel and the results were compared with drilling information. The resistivity results strongly

reflects the spatial distribution characteristics of stratum in the research area, where the palaeochannel is

represented by the high-resistivity values which is in the range of 18-40Ω•m indicating, the lithology mainly

as fine sand and silty-fine sand. It will establish the foundation for rational exploitation and comprehensive

utilization of palaeochannel resource in future in the area.

1 INTRODUCTION

Palaeochannels (Wu and Zhao, 1993) are old river

channel beds containing coarser unconsolidated

deposits which are formed either during the course

of dying of the river or deflection of the river

channel in a different direction. The sand layers,

with or without a clayey cover (of variable thickness)

at top, generally form potential aquifers owing to

better conductivity and availability of storage space

in the interstices of coarser granular material. The

palaeochannels also act as good groundwater

recharge avenues and pathways for groundwater

flow. Often the palaeochannels yield good quality

water in comparison to the adjoining areas. Such

palaeochannels can be used for sustainable

groundwater development and management for

different needs such drinking and irrigation.

Many researchers have earlier also studied

palaeochannels through using different techniques.

Especially the new techniques and methods have

greatly improved the accuracy and depth of research

in the last several decades (

Wu and Zhao, 1993; Wu et

al., 1991; Dave et al., 2002; Fu et al, 2008; Zhu et al. 2013;

Cao et al. 2016

). Geophysical methods provide some

of the uncomplicated and reliable groundwater

exploration techniques. As the research going in

various fields, the conventional resistivity method is

difficult to meet the work demand while High-

density Electrical Resistivity Tomography arising. It

is widely applied especially in recent years (

Mauro et

al. 2013; Wang et al. 2016; Dai and Xie 2015; Chen S et

al. 2017

The study area is located in Shouguang City,

China. It is in downstream of the Mihe River

alluvial fan (North of the Mount Tai and the Mount

Yi, South of Laizhou Bay, Figure 1). Because of the

Mihe River erosion forms vast and flat plain where

Quaternary is thick and continuous. It is composed

of clayey sand mainly (

Han,1996). Under the

influence of the Ice Age in geological time Mihe

River forms paleochannel (

Han et al. 1999a). Its

mainstream belt is mainly composed of gravel, sand

and silt, the top and bottom plates are clay or loam.

The Mihe River palaeochannel is the main

enrichment area of groundwater. In the 1970s the

saltwater intrusion occurred in the area. As the main

secondary landform, palaeochannel is the main

channel of saltwater intrusion and also the fastest

intrusion, the most complex changes and the worst

Guo, L., Li, L., Fang, L., Wang, X., Jin, H. and Wang, G.

Identiﬁcation of Palaeochannels of Mihe River - An Approach Based on the Integrated Geophysical Methods.

In Proceedings of the International Workshop on Environment and Geoscience (IWEG 2018), pages 353-359

ISBN: 978-989-758-342-1

353

affected landform units (

Han, 1996; Han et al. 1999b

Due to the over-exploitation of groundwater, three

large-scale regional groundwater funnel areas

（average groundwater level about -58m）centered

on Hualong, Shengcheng Street and Luocheng Street

have been formed. Therefore saltwater intrusion is

gradually intensified.

In order to meet the challenges arising out of the

crisis in water supply sector and saltwater intrusion

in the fresh water aquifers, it has been imperative to

map the palaeochannels of the rivers, which might

host fresh water. For this purpose this study will

combining Vertical Electrical Sounding & High-

density Electrical Resistivity Tomography identify

palaeochannel. The results will be compared to the

information obtained in the exploration drilling.

2 METHODOLOGY

Affected by saltwater intrusion, groundwater in the

area is divided into the northern saltwater zone, the

middle salty-freshwater transitional zone and the

southern freshwater zone (

Zhao et al. 2000

). The

survey lines is laid from west to east according to

the boundary of saline water & freshwater and the

direction of the road, as shown in Figure 1. Based on

the respective advantages of the two methods and

the preliminary determination of the distribution and

direction of palaeochannel, firstly we use One-

dimensional Electrical Sounding in the "macro" to

detect palaeochannel information; then High-density

Electrical Resistivity Tomography is conducted in

areas with palaeochannel response according to the

first step exploration results.

2.1 Vertical Electrical Sounding

Vertical Electrical Sounding (VES) is an electrical

exploration technique, which by changing the pole

pitch of AB and MN to control detection depth in

artificial electric field to obtain the vertical variation

rule of the rock resistivity characteristic at the

sounding point (Figure 2). The collection of VES

data uses DZD-6A multi-function direct current

electrical prospecting apparatus with symmetrical

quadrupole devices according to Table 1 data layout

polar distance. This is 94 sounding points in all

(Figure 1).

2.2 High-density Electrical Resistivity

Tomography

High-density Electrical Resistivity Tomography

(ERT) is an array of exploration method based on

conventional resistivity method. The measuring

principle is identical to VES, except that ERT

electrode arrangement completed once and more

collecting data along with high efficiency can more

intuitively reflect the characteristics of the

underground space electrical abnormal body [8].

Figure 1: Illustration map showing location of the study area and measured points & profiles.

IWEG 2018 - International Workshop on Environment and Geoscience

354

Figure 2: The schematic diagram of VES.

Start inversion program

Open data file

（electrode coordinates、actual

measured potential, et al）

Generate grid

Delete dad date

Start inversion

Establish initial model

Iterative time n =0

Known

information

Forward calculation

Calculate fitting error

Solve linearization problem

Modified model

Forward calculation

Calculate fitting error

Satisfied stop conditions

（RMS、

max

）

Stop

Iterative time n=1

To next iterative time n=n+1

Figure 3: The flow diagram of resistivity inversion.

The collection of ERT data uses 61-channel

FlashRES-UNIVERSAL ultra-high-density direct

current electrical exploration system. The system

uses advanced full-waveform ZZ data acquisition

device breaking through conventional data

acquisition method. In the electrode system

composed of 64 measuring electrodes any one of

AB supply electricity forms an electric field, MN

can measure 61 potential (difference) data at the

same time. Therefore when A and B composed of

odd and even numbered electrodes respectively, ZZ

device can obtain 32×32×61=62464 data each data

acquisition.

According to the results of VES, it is

preliminarily assessed that the palaeochannel

response exists at the locations 36#～42# and 44#～

47#. To further understand the spatial distribution of

palaeochannel, ERT measurements were performed

near this location. Figure 1 shows four profiles E1～

E4 in total using full-waveform ZZ data acquisition

device. Each profile is arranged with 64 electrodes,

the unit electrode distance is 6m, and effective

measuring line length is 378m.

The collected data of VES and ERT can

calculate apparent resistivity



according to the

following formula.







(1)

1111

AM BM AN NB







(2)

Where

U

is potential difference between M

and N (V); I is current between A and B (A); K is

device coefficient; AM、BM、AN、NB are distance

between electrodes respectively (m).

Due to heterogeneity and anisotropy of the actual

stratum,



is not enough to reflect the electrical

characteristics of the medium. Therefore, the true

resistivity of the medium is calculated by the least

square inversion method. The inversion process is

shown in Figure 3.

Identiﬁcation of Palaeochannels of Mihe River - An Approach Based on the Integrated Geophysical Methods

355

Table 1: Data acquisition list of VES (Unit: Meter).

Sequence

Number

AB/2 MN/2

Sequence

Number

AB/2 MN/2

1 2.5 1.5 8 25 4

2 4 1.5 9 32 4

3 6 1.5 10 40 4

4 9 1.5 11 50 4

5 12 1.5 12 60 6

6 16 1.5 13 74 6

7 20 1.5 14 90 6

3 ANALYSIS AND DISCUSSION

OF RESULTS

3.1 Analysis of VES Results

Sand and clay perform high and low resistivity

respectively in palaeochannel sediments. Figure 4

shows VES resistivity curves after inversion of

36#～42#、44#～46# sounding points.

Figure 4 shows the "K" sounding curves carried

out in the area. It indicates three different electrical

layers vertically. Numerically, the curve is

characterized by high center and low sides. The high

and low resistivity zones are caused by enrichment

of sand and clay respectively. This shows that the

sounding points have palaeochannel distribution

characteristics, and the corresponding lithology from

top to bottom is clay - sand - clay.

From resistivity values and distribution of curves,

it is evident that the maximum resistivity is 40Ω•m

(40#, 41#) and the minimum is 16Ω•m (37#). The

maximum is 2.5 times the minimum. The obvious

contrast of resistivity indicates that there are

different electrical strata at the sounding points.

According to the range of resistivity values of clay

& sand, all of the electrical sounding curves show an

increasing trend at 18-20Ω•m, which indicates the

existence of sand layer. With deepening of VES

dipole depth, the resistivity increases to the

maximum and then decreases gradually, which is

caused by disappearance of sand and appearance of

clay. Therefore the enrichment of sand

characteristics in palaeochannel can be inferred

based on characteristics of the curves. Table 2 shows

characteristics of palaeochannel sand layer deduced

from sounding curves.

Table 2 shows that the average buried depth of

the top of sand layer (as deduced from the 10

electrical sounding curves) is 9.9m; the maximum

depth of 16m (36#, 39#) and the minimum of 4m

(44#). The average of bottom depth is 45.2m, the

maximum depth is 80m at 40# sounding point and

the minimum is 16m at 46#. It is deduces that the

average sand thickness is 35.3m ,the maximum

thickness is 58m (39#) and the minimum is 10m

(46#).

3.2 Analysis of ERT Results

Figure 5 shows resistivity spatial distribution of the

ERT profiles E1 ～ E4 after inversion. The non-

uniform distribution of resistivity in profiles

indicates spatial heterogeneity in the stratigraphic

lithology. Four profiles on the structure show overall

three layers of "low - high - low " resistivity from

top to bottom. This is consistent with the features

reflected by VES.

Figure 4: The graphs of VES.

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356

Table 2: The inference statistical table of the palaeochannel characteristics of sand layer.

Method

Sand features

(m)

Serial number of sounding points

Average

36# 37# 38# 39# 40# 41# 42# 44# 45# 46#

VES

Top depth

16 15 12 16 9 9 6 4 6 6 9.9

Bottom depth 68 35 33 74 80 60 32 32 22 16 45.2

Thickness 52 20 21 58 71 51 26 28 16 10 35.3

ERT

Top depth 18 15 12 22 7 8 5 3.5 5 6 10.1

Bottom depth 70 35 32 78 78 59 30 29.5 22 14 44.8

Thickness 50 20 20 56 71 51 25 26 17 8 34.4

Re (%)

Top depth

11.8 0.0 0.0 21.6 25.4 11.8 18.3 0.5 18.3 0.0 2.0

Bottom depth 2.9 0.0 3.1 5.3 2.5 1.7 6.4 8.1 0.0 13.4 0.9

Thickness 3.9 0.0 4.9 3.5 0.0 0.0 3.9 7.4 6.0 22.5 2.6

The resistivity values obtained by ERT are

slightly less than VES, the maximum is 38Ω•m and

the minimum is 3Ω•m. The maximum value is the

same as VES. However the minimum value has a

certain difference between them. It may be due to

amount of data collected, difference of data values

and regionalization of inversion process between

them and others. Based on collected data, the criteria

for dividing clay and sand is 18Ω•m. The calculation

results of two methods are shown in Table 2. The

location of VES sounding points in ERT profiles is

shown in Figure 5.

Table 2 shows that the average buried depth at

the top of the sand layer estimated from the ERT

profiles of the corresponding location is 10.1m, the

relative error (



) with the VES inference is 2.0%,

the absolute error (e

) of other sounding points is not

more than 2m except for 39# (e

=4m) and the results

of 37#, 38# and 46# is same to VES inference. The

estimated bottom sand depth is 44.8m,



with the

VES inference is 0.9%, the absolute error of other

sounding points is not more than 2m except for 39#

=4m). The



of 36#, 37#, 38#, 40#, 41#and 45#

is less than 5% and the results of 37#and 45# is

same to the VES

inference. This implies that the

results of two methods are basically in agreement.

The top and bottom positions of 39# are 4m deeper

than the VES result. From continuity of spatial

distribution in the high resistivity region of ERT

profiles, the result of ERT is more reliable. The

average thickness of the sand layer estimated is

34.4m, with



of 2.6% from the VES results. The

largest



is located at 46#, followed by 44# and

45# are 7.4% and 6.0% respectively. Others are

within 5% and 37#, 40# & 41# are 0.

Figure 5: The resistivity profiles of ERT and drilling

information of K1 & K2.

Through the above comparative analysis, ERT

and VES can more conclusively deduce the

information of clay layer and sand layer in the

stratum except for 44#. Profile E1 show that the

sand layer in other areas are mainly enriched in the

stratum within a depth of about 15m except for the

Identiﬁcation of Palaeochannels of Mihe River - An Approach Based on the Integrated Geophysical Methods

357

sand layer exposed at 345m-357m. The thickness of

sand layer at 65m-125m is about 50m, and the

thickness of other areas is about 15m. The location

of Profile E2 is the main enrichment zone of the

sand layer with a depth of about 8m. The thickness

of the sand layer is about 25m at 0-50m and 350m-

390m. The middle area is fully enriched with sand in

exploration depth and extends to deeper. The typical

"U" type distribution is in accordance with

sedimentary structure of palaeochannel. The spatial

distribution of sand layer at profile E3 is more

uniform. Except for at 225m-260m and 360m-390m

near the surface, the average sand depth in other

areas is about 5m and the average thickness is about

24m. There is a discontinuity in the sand space at

profile E4, which occurs at 270m-290m. The sand is

mainly concentrated at 0-270m, the average depth is

about 5m and the average thickness is about 22m.

There is a sand lens body at 290m-370m with an

average depth of about 5m and a thickness of about

10m.

3.3 Comparative Analysis of

Exploration Drilling Results

In order to further verify characteristics of

palaeochannel, this study arranged K1 & K2 drilling

lithological data on E1 & E4 profiles respectively

(Figure 5). The comparison of two results shows that

high conformity between them and reliability of this

integrated geophysical method.

4 CONCLUSIONS

Based on the integrated geophysical methods of

VES & ERT and stratum differences in resistivity

parameters, prospective palaeochannels have been

identified. The results of geophysical methods were

in accordance with the drilling data. It will establish

the foundation for rational exploitation and

comprehensive utilization of palaeochannel resource

in future in the area.

The geophysical methods show the sand layer in

palaeochannels reflected by high resistivity which is

in the range of 18-40Ω•m in the average buried

depth of about 10m. The buried depth in the middle

is deep, the depth at both ends is shallow, and the

average sand thickness is about 35m. The typical

"U" type distribution is in accordance with the

sedimentary structure of palaeochannel. The clay in

palaeochannel reflected by low resistivity which is

in the range of 5-18Ω•m. It constitutes the top & the

bottom floor and protection structure of

palaeochannel.

ACKNOWLEDGEMENT

This study was supported by the Natural Science

Foundation of China (41202174).

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