Influence of Soil Moisture on Magnetic Susceptibility Measurement - 1-s2.0-S0926985105000856-Main

8/13/2019 Influence of Soil Moisture on Magnetic Susceptibility Measurement - 1-s2.0-S0926985105000856-Main

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The influence of soil moisture on magnetic

susceptibility measurements

G. Maiera,*, R. Scholgera, J. Schon b

a Department of Applied Geosciences and Geophysics, University of Leoben, Peter-Tunner-Str. 25-27, 8700 Leoben, Austriab

Joanneum Research, 8700 Leoben, Austria

Received 30 November 2004; accepted 14 October 2005

Abstract

An important methodological question for magnetic susceptibility measurements is if a variation of the soil conductivity, as a

result of a change in soil moisture, influences the measured susceptibility values. An answer to this question is essential because an

accurate magnetic susceptibility mapping requires a grid of comparable magnetic susceptibility values, which indicate the magnetic

iron-mineral contents of the soils. Therefore, in the framework of the MAGPROX project (EU-Project EVK2-CT-1999-00019), the

study aims at investigating the influence of soil moisture and the possible correlation between magnetic susceptibility and electric

conductivity. This approach was realised by model experiments in the laboratory and a field monitoring experiment, which was

performed in an analogical manner as the model. For the laboratory experiment, a plastic tub with a water in- and outflow system

and installed lines of electrodes was used. The measurements were carried out with layers of different magnetic material within the

experimental sand formation under varying water saturation conditions. For the field experiment, which was carried out from Julyto December 2003, two test sites were selected. The magnetic susceptibility was measured by means of the recently developed

vertical soil profile kappa meter SM400 and a commonly used Bartington MS2D probe. The electric resistivity was recorded using

a 4-point light system (laboratory) and a ground conductivity meter EM38 (field). The knowledge of the resistivity of the sand

formation enabled an estimation of porosity and water saturation in consideration of the Archie equations. The laboratory

experiment results showed a very slight variation of measured magnetic susceptibility under different degrees of moisture,

indicating mainly the influence from the diamagnetic contribution of the water volume. A measurement error in connection

with the measurement method, for example caused by an interfering effect of soil conductivity variations, was not found. The

authors conclude, that in practical use of the investigated instruments for topsoil magnetic susceptibility mapping in the field, the

influence of soil moisture and resulting soil conductivity can be neglected, especially compared to the influence of the contact

between measurement loop and soil. The study presented here verifies the magnetic susceptibility data reproducibility and

comparability, which provides the basis for magnetic susceptibility monitoring. Additionally, new application approaches of

magnetic susceptibility measurements were proposed, which show again the versatility and the potential of the method.D 2005 Elsevier B.V. All rights reserved.

Keywords: Magnetic susceptibility; Electric conductivity; Soil moisture; MAGPROX

1. Introduction

During the last few years magnetic susceptibility

measurements have become an established method to

detect polluted regions and their spatial demarcation.

0926-9851/$ - see front matterD

2005 Elsevier B.V. All rights reserved.doi:10.1016/j.jappgeo.2005.10.001

* Corresponding author. Tel.: +43 1 40440 23334; fax: +43 1 40440

623334.

E-mail address: [email protected] (G. Maier).

Journal of Applied Geophysics 59 (2006) 162175

www.elsevier.com/locate/jappgeo



Magnetic susceptibility mapping has been used for

investigations around power plants (Heller et al.,

1998; Kapicka et al., 2001), iron industry and mining

areas (Strzyszcz and Magiera, 1998; Lecoanet et al.,

2001; Strzyszcz and Magiera, 2001; Hanesch et al.,

2003), urban environments (Hanesch and Scholger,2002) and roads (Hoffmann et al., 1999). It has also

proved to be useful for studying the influence of atmo-

spheric processes on distribution and deposition of air

pollutants (Maier and Scholger, 2003, 2004) and for

discriminating different soil-contamination sources

(Lecoanet et al., 2003). A comprehensive overview of

magnetic monitoring methods in pollution studies is

given byPetrovskyand Ellwood (1999).

In most cases, a Bartington magnetic susceptibility

meter for field measurements was used. Such measure-

ment systems operate on the principle of alternatingcurrent induction (Bartington Operation Manual, 2002).

An alternating magnetic field (of low intensity) is pro-

duced by a sensor when it is connected to a source of

alternating current. The sensor consists of an oscillator

circuit for which a wound inductor is the principle

frequency-determining component. The magnetic sus-

ceptibility k is related to the relative permeability of a

mediuml rand this parameter is closely associated with

the characteristics of alternating current circuits contain-

ing inductive elements (Collinson, 1983). When the

inductor contains only air the value of permeability of

air l0 determines the frequency of oscillation. If theinductor is placed within the influence of the material

to be measured, the value oflrdetermines the frequency

of oscillation. Thus, the relative change in inductance

and frequency resulting from the difference between the

permeability of airl0and the relative permeabilitylrof

another medium (e.g., soil) is a measure of magnetic sus-

ceptibility. In short, magnetic susceptibility is the ratio of

induced magnetisation to the applied magnetic field.

However, in these kinds of measurements, an im-

portant methodological question is if a variation of soil

conductivity, as a result of a change in soil moisture,influences the measured magnetic susceptibility values.

The theoretical background of this possible interaction

is given by the third and forth Maxwell equation. In the

case of soils, soil moisture is the most significant factor

affecting electrical conductivity. Furthermore conduc-

tivity is also influenced by porosity, particle size and

salinity. Since the principle of susceptibility measure-

ment is based on applied alternating magnetic fields,

the influence of conductivity has to be considered.

The basic idea of this work is based on experiences

of the MAGPROX project partners (EU-Project EVK2-

CT-1999-00019) during magnetic susceptibility field

measurements on topsoils. Particularly in the low mag-

netic susceptibility range, dry and wet or waterlogged

soils yielded different values. This possible influence

has not been studied yet in empirical form. An answer

to this question is essential because accurate magnetic

susceptibility mapping requires a grid of comparablemagnetic susceptibility values, which indicate the mag-

netic iron-mineral contents of the soils. Therefore the

study aims at investigating the interfering influence of

soil moisture and the possible correlation between mag-

netic susceptibility and electric conductivity.

For the recently developed vertical soil profile kap-

pameter MAGPROXk SM400,Petrovskyet al. (2004)

studied the effect of conductivity on magnetic suscep-

tibility measurements theoretically. Based on exact an-

alytical formulas derived from the Maxwell equations,

the authors calculated the negative effect of electricalconductivity on relative change of inductance and mag-

netic susceptibility. According to Petrovskyet al., con-

ductivity affects the imaginary part of the complex

magnetic susceptibility that cannot be compensated in

single-coil systems and contributes to total magnetic

susceptibility as a negative component. The authors

concluded that in practical use of SM400 for typical

soil conductivities the effect of conductivity could be

neglected. Additionally, by reference to the Bartington

operation manual the Bartington probes are particularly

insensitive to sample conductivity. After the manual the

response of kappameters to conductors is high if theinstruments feature a high operating frequency. Due to

the fact that the operating frequency of the SM400 is

relatively high (8 kHz) compared to the Bartington

MS2D probe (operation frequency of 0.958 kHz), it

could be expected, that the Bartington MS2D probe is

less sensitive to electrical soil conductivity than

SM400. However, the theoretical specifications had to

be examined empirically in the form of experiments.

This experimental investigation should quantify the

importance of conductivity variations for magnetic sus-

ceptibility measurements and the influence of soil mois-ture on their accuracy and comparability.

The approach of this study was the investigation of

the relationship between magnetic susceptibility and

electric resistivity (the reciprocal of conductivity) as a

result of changing water content. This approach was

realised by model experiments in the laboratory and a

field monitoring experiment, which was performed in

an analogical manner as the model and should confirm

the laboratory results under natural soil conditions. For

the field experiment, which was carried out from July to

December 2003, two test sites were selected where

previous surface measurements (Bartington MS2D

G. Maier et al. / Journal of Applied Geophysics 59 (2006) 162175 163



probe) and soil profile measurements (SM400) showed

enhanced magnetic susceptibility values.

2. Laboratory experiments

2.1. Experiment 1

For the model experiments, a plastic tub with a water

in- and outflow system and installed lines of electrodes

was used (Fig. 1ac). In the lower part of the tub, a

layer of coarse grained gravel to fine grained gravel was

implemented, which guaranteed an undisturbed water

flow. Above the experimental sand formation (consist-

ing of homogeneous silica sand with a grain size of

0.52 mm) was implemented by means of underwatersedimentation. In this way, a grading of the silica sand

should be avoided as good as possible. Three plastic

tubes for magnetic susceptibility measurements with

Fig. 1. (a) Experiment plastic tub. (b) Position of the experimental sand formation and the drainage system. (c) Water in- and outflow system with

flowmeter. (d) Electrode array and their position on the chamber. (e) Magnetic susceptibility measurement with the Bartington MS2D probe at the

formation surface. (f) The markers and the self-weight of the probe guaranteed an identical measurement position and contact pressure of the coil foreach measurement. (g) Magnetic susceptibility measurement with the vertical soil profile kappameter SM400.

G. Maier et al. / Journal of Applied Geophysics 59 (2006) 162175164



the recently developed vertical soil profile kappameter

MAGPROXk SM400 were built in to control the

three-dimensional uniformity of the magnetic layers.

The tubes were installed before the implementation of

the sand formation to eliminate vertical drainage in the

sand formation during the experiment. Between thegravel layer and the experimental sand formation, a

fleece prevents the sand from being washed out during

the drainage. The measurements were carried out with

layers of different magnetic material within the exper-

imental sand formation under varying water saturation

conditions. Two magnetic layers were implemented, the

first in a depth of about 6 cm with a thickness of about

1 cm, the second as a 10 cm thick layer between depths

of about 40 and 50 cm. After determination of the

required mass of the highly magnetic concentrate

(with Bartington-MS2B5 Sensor), the concentrate(ironsilicon-oxide, mass susceptibility= 1.32d106

m3/kg, grain size = 63250 Am) was mixed with watery

sand in different steps of the dosage. After wetting of

the sand, the concentrate was scattered in and the

material was mingled using a stirring staff. This process

was repeated until the required volume for the layer was

reached. Before the implementation of the magnetic

layers, the mobility of the magnetic concentrate and

the risk of an unwanted migration were investigated in

form of a preliminary test. A magnetic layer of 1 cm

was installed in the middle of a plastic tube (50 cm

length) filled with water saturated sand. The suscepti-bility of the tube was recorded with the Bartington-

MS2C-Sensor (Core Logging Sensor), and after the

outflow of the water through a hole at the bottom of

the tube, susceptibility was measured again. The results

showed a similar susceptibility distribution before and

after the water outflow and thus no disturbing migration

of the magnetic concentrate. The magnetic layer stayed

stable in its position.

The electric resistivity was recorded using a LIPP-

MANN 4-point-light system in dipoledipole configu-

ration with current- and measurement electrodes atevery side of the chamber (Fig. 1b,d). The knowledge

of the resistivity of the sand formation enabled an

estimation of porosity and water saturation in consid-

eration of the Archie equations (Archie, 1942). Firstly,

magnetic susceptibility was measured by means of a

Bartington MS2D probe (in SI units at the more sensi-

tive range 0.1). Markers guaranteed that the horizontal

and radial measurement position was always the same

(Fig. 1e,f). The self-weight of the probe guaranteed an

identical contact pressure of the coil for each measure-

ment. Secondly, magnetic susceptibility was measured

with the soil profile kappameter SM400 (within in-

stalled plastic tubes) (Fig. 1g). The penetration depth

of this instrument is limited to some few cm, since the

measurement probe integrates more than 90% of the

signal at a distance of 12 mm. The temperature of the

experimental sand formation was measured using a

temperature sensor installed in the formation.For the first measurement the water level was low-

ered and raised in 5 steps (1 step per day), based on the

estimated total water volume in the experimental sand

formation which amounted to 73.3l (Fig. 2). After the

final drawdown step, the dehydration of the sand for-

mation was observed for days.

The results of the magnetic susceptibility measure-

ments with the MS2D-sensor showed a very slight

variation of magnetic susceptibility under different

degrees of water saturation (Fig. 3b). The water satu-

ration is presented by means of the term b

effectivewater saturationQ. This parameter considers the penetra-

tion depth of the MS2D probe and the thereby caused

different measurement influence of the water saturation

conditions of each measurement level by weighting the

values. bEffective water saturationQ means the sum of

the depth-weighted water saturation values of each

measurement level within the reach of the MS2D

probe. The weighting was carried out based on the

specific penetration depth of the MS2D probe described

byLecoanet et al. (1999). With decreasing water satu-

ration and increasing sand formation resistivity during

the drawdown the magnetic susceptibility measured atthe sand formation surface increased. With increase of

water saturation and decreasing resistivity during the

refilling of the tub the magnetic susceptibility de-

creased. The presented magnetic susceptibility values

represent the average of 10 measurements. Additional-

ly,Fig. 3b shows that the water saturation values, which

are estimated based on the resistivity did not return to

100%, although the sand formation was again totally

saturated at the end of the refilling process. The appar-

ently lower water saturation of only 80% is a result of

the settlement of the experimental sand formation dur-ing the drawdown of the water level (Fig. 2). Thereby

the porosity and consequently the amount of water (the

conductive electrolyte) after the refilling were reduced.

That resulted in higher resistivity values, compared

with the initial sand formation resistivity, and to appar-

ently lower water saturation values. The settlement

effect can also be observed in consideration of the

magnetic susceptibility data at the end of the refilling

process. Firstly, the lower distance between MS2D

measurement coil at the sand formation surface and

the magnetic layer after the settlement, led to a slight

increase of magnetic susceptibility, compared with the




Fig. 2. Water saturation values for each measurement level, initial porosity, end porosity and formation volume content during the measurementprocedure (drawdown of the water level).




initial magnetic susceptibility. Secondly, the real reduc-

tion of the amount of water within the defined volume,

which influences the measurement, added to the mag-

netic susceptibility increase.

The magnetic susceptibility values measured using

the soil profile kappameter SM400 showed a slight

variation under different water saturation conditions.

However, outliers did not allow a satisfying interpreta-

tion. Based on these data, a correlation between mag-

netic susceptibility and the variation of the water

saturation conditions was not observable. A modified

laboratory experiment (experiment 2, Section 2.2)

aimed at improving the configurative precision and

comparability of the measurements with SM400.

During the second measurement the water level was

lowered and raised more carefully, adapted to the

results of measurement 1. The results showed an im-

proved data density between 30% and 60% of water

saturation (Fig. 3c). Again the magnetic susceptibility

measured at the sand formation surface with the MS2D

Fig. 3. (a) Example of the vertical magnetic susceptibility progression of the experimental sand formation (experiment 1, measured with SM400);

the signals show the implemented magnetic layers. (b) Measurement 1: magnetic susceptibility kvs. effective water saturation Sw during the

drawdown of the water level and subsequent filling of the experimental sand formation. (c) Measurement 2: improved drawdown and filling

procedure, adapted to the results of experiment 1. (d) Combined results of measurements 1 and 2; cross plot of magnetic susceptibility Dkvs. watersaturation Sw (the error bars show the standard deviation of the values).




probe increased with decreasing water saturation and

increasing sand formation resistivity during the draw-

down and vice versa during the refilling.

A cross plot of magnetic susceptibility vs. water

saturation, including the combined results of measure-

ments 1 and 2, showed a very slight increase of mag-netic susceptibility in the range of 5.3 d106 SI with

decreasing water saturation (Fig. 3d). The largest mag-

netic susceptibility value also has the largest vertical

error bar. If we would neglect this value, which seems

to be an outlier, susceptibility would only show an

increase of about 4 d106 SI. An additional determi-

nation of the bulk magnetic susceptibility of the tap

water used during the laboratory experiments with the

GEOFYZIKA kappabridge KLY-2, showed a value of

9.0 d106 SI. A sandwater mixture sample with a

water content equal to the content in the totally satu-rated experimental sand formation amounted to a mag-

netic susceptibility contribution of 3.9 d106 SI.

Petrovskyet al. (2004)predicted for the soil profile

kappameter SM400 a magnetic susceptibility change of

8.7 d106 SI for a conductivity change of 20 S/m.

The average conductivity of the totally saturated exper-

imental sand formation (initial state, cp.Fig. 2) amounts

to 16 mS/m. In consideration of these facts, the inves-

tigation results of the Bartington MS2D probe indicate

predominately the true decreasing and increasing influ-

ence of the diamagnetic contribution of water. An

interfering effect of conductivity variations was notfound.

The correlation between magnetic susceptibility and

water saturation shows a nearly linear behaviour for

high to medium water saturation conditions (100% to

35%). Under low water saturation conditions (35% to

7%) the curve shape changes and the correlation shows

a curvature with a non-linear increase of magnetic

susceptibility (Fig. 3d). These results indicate the pres-

ence of a second conductivity component, the interface

component, in addition to the electrolytic conductivity

component. Under low water saturation conditions ordehydration this component becomes more important.

As a result of the decreasing water saturation and

following dehydration the continuous water contact

within the pore channels collapses and the electrolytic

conductivity does no longer exist. Only the interface

conductivity component, which is independent of a

continuous water contact in the sand formation, persists

and the direct correlation between conductivity and

water saturation (which is the basis of the Archie-rela-

tions) does not exist anymore (Archie, 1942). Conse-

quently the water saturation values, which were

determined using the 2nd Archie-equation, have been

overestimated. This results in an apparently non-linear

increase of magnetic susceptibility.

2.2. Experiment 2

The objective of experiment 2 was an improvementof the configurative precision and comparability of

measurements with SM400 by the usage of an adapted

measurement configuration. For this purpose, a plastic

measurement cell was constructed. For the conductivity

measurements four lines of 9 electrodes were installed

on opposing sides of the cell.

The water level was lowered and raised in 5 steps (1

step per day), based on the estimated total water volume

in the experimental sand formation that amounted to 2.88

l. By means of the 4-point-light-system, the resistance

and the water saturation conditions were investigated inthe same way as in the previous experiment, with a

dipoledipole configuration with current and measure-

ment electrodes on each side of the chamber to measure

the transversal resistance of the sand formation.

The vertical soil profile kappa meter SM400 was

implemented in the experimental sand formation (homo-

geneous silica sand) and the temperature of the sand

formation was held constant to ensure identical condi-

tions during the experiment (Fig. 4a).Fig. 4bshows the

magnetic susceptibility progression in the measurement

tubes detected with the vertical soil profile kappameter

SM 400. The amplitude in a depth of about 31 mm showsthe implemented magnetic layer, the hatched section

shows schematically the dimension of the experimental

sand formation. The coherence between magnetic sus-

ceptibility and different water saturation was observed

with regard to the magnetic susceptibility peak value of

the amplitude of the magnetic layer and the bbase lineQ in

a depth of 170 mm. The behaviour of these signals was

observed during the drawdown and the refilling process.

Fig. 4c shows the behaviour of the magnetic suscep-

tibility peak value of the magnetic layer marked in Fig.

4b during the drawdown and refilling process. Fig. 4dshows the behaviour of the bbase lineQ (cp. Fig. 4b).

The results of experiment 2 showed also a slight vari-

ation of magnetic susceptibility under different water

saturation conditions, in a comparable range as in the

previous experiment. The adapted setup improved the

configurative precision of the experiment with the soil

profile kappameter SM400 and provided sufficient data

quality. Magnetic susceptibility values showed an in-

crease during the drawdown and dehydration process

and a reversible decrease during the refilling process.

For the SM400 the observed magnetic susceptibility

change was marginally higher than for the Bartington




MS2D probe. Whether this difference was caused by

the higher operation frequency of the SM400 or was

only the result of a configurative measurement error

could not be found.

The similar behaviour of the magnetic susceptibility

peak values of the magnetic layer (Fig. 4c) and the

depth section which was evidently unaffected by the

magnetic concentrate (Fig. 4d) was an essential infor-

mation. Although the magnetic susceptibility levels of

the two sections were quite different at the initial state

of the experiment, the magnetic susceptibility values

showed variations in the same range.

Consequently, even for the investigation with the soil

profile kappameter SM400 the correlation between mag-

netic susceptibility values and different water saturations

indicates mainly the true decreasing and increasing in-

fluence of the diamagnetic contribution of water.

3. Field monitoring experiment

The test sites were investigated with the Bartington

MS2D probe (for magnetic susceptibility) and with the

ground conductivity meter EM38 (for conductivity and

as a result of the conductivity values, for soil moisture).

Fig. 4. (a) Measurement cell and experimental sand formation with implemented vertical soil profile meter SM400. (b) Vertical magnetic

susceptibility progression of the formation; the amplitude shows the implemented magnetic layer. (c) Magnetic susceptibility k vs. effective

water saturation Sw; behaviour of the magnetic susceptibility peak values of the magnetic layer during the drawdown and refilling process.

(d) Behaviour of a depth section (170 mm) which is evidently unaffected by the magnetic concentrate.




In field it could not be guaranteed, that the measure-

ment position and the contact between the measurement

coil and soil are always exactly the same (because of

the vegetated surface). Consequently, magnetic suscep-

tibility measurements with MS2D probe were done at

the rapid 1.0 range. A comparison showed that the useof the 0.1 range would not improve the measurement

precision under these conditions. The conductivity

measurements were performed in vertical and horizon-

tal dipole mode, which provide different penetration

depths and sensitivities.

The measurement procedure and the magnetic sus-

ceptibility distribution at surface are demonstrated in

Fig. 5ad and Fig. 5g. The sites had a size of 120 by

120 cm with regard to the length of EM38. Magnetic

susceptibility was measured in a grid of 10 by 10 cm,

which equals 36 measurement positions. Conductivity

was measured at 6 positions of both directions (12

measurement values). Magnetic susceptibility values

and conductivity values were averaged to provide a

mean value of the test sites at each measurement inter-

val. The sites were measured two times per week from07/27/2003 to 12/01/2003. From July to December, air

temperature and soil temperature at surface and in the

depths of 5 and 10 cm were measured with a digital

thermometer and the temperature in 50 cm depth was

measured with a HOBO H8 Temperature Logger. The

knowledge of the temperature variation during the ex-

periment allowed a temperature correction of the electric

conductivity data. In addition to that, the real soil mois-

ture was determined with the gravimetric method. 4 soil

sample cores were taken from the direct surrounding of

Fig. 5. Measurement methodology (example of a test site). (a) Magnetic susceptibility measurement at surface with MS2D probe. (b) Magnetic

susceptibility measurement positions. (c) Conductivity measurement with EM38 in vertical position. (d) EM38 in horizontal position; conductivitymeasurement positions. (e) HUMAX-Soil sampling tool. (f) Soil sample core. (g) Example of the measurement procedure.



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the investigation areas once a week. For sampling, the

HUMAX-Soil sampling tool was used (Fig. 5e). The

moisture was determined for the upper 10 cm of the

sample core (Fig. 5f). With regard to the specific pen-

etration depth of the Bartington MS2D-probe, this depth

section is particularly interesting for a comparison ofmoisture and magnetic susceptibility variations.

Test site 1 was located at a dumping ground, where

slag from mining processes was deposited. The soil

profile showed a uniform soil development from 0 to

20 cm depth above the slag deposition. The soil mate-

rial consisted of silty sand with mainly slag components

as coarse fraction. Test site 2 was also located in a

region influenced by mining. The profile showed a

uniform horizon between 0 and 30 cm. The soil mate-

rial of this horizon consisted of loam and sandy clay,

respectively. From 30 to 50 cm depth this soil materialis mixed with clay schists, shale and slag components.

A grain size distribution showed significantly higher

clay contents of the samples taken from test site 2.

The ideal meteorological conditions in 2003 sup-

ported the significance of the experiment. The summer

months were extraordinarily hot and dry and provided

anomalously low moisture values. Heavy rainfalls dur-

ing autumn and the frequent changing of snowfall and

melting from November to December resulted in a

strong wetting of the investigated soils. It can be as-

sumed, that the moisture contrast of this period topped

the average annual contrast in this climatological re-gion. The experiment was finished on the 1st of De-

cember. Freezing of the soil and a permanent snow

cover made further measurements impossible.

As expected, the soil conductivity values showed a

strong dependence on the degree of soil moisture. The

corrected soil conductivity values are demonstrated in

Fig. 6ad. The results showed higher values for test site

2 than for test site 1. This is probably caused by the

higher clay content in the soil of test site 2. The higher

clay content and its water-retaining property is also

reflected in a slightly higher maximum soil moistureof test site 2. Conductivity values measured in horizon-

tal dipole mode showed for both measurement areas a

more dynamic behaviour under varying soil moisture in

shallow depths than the values measured in vertical

dipole mode. This is due to the higher sensitivity of

the H-mode in these shallow depth sections. Conse-

quently, conductivity values measured in vertical mode

showed a slightly delayed behaviour indicating the

infiltration of the rainwater. When the electrolyte mi-

grated down in depth sections where the V-mode fea-

tures the highest sensitivity it results in a delayed

increase of conductivity.

Fig. 6e,f shows crossplots of temperature-corrected

electrical conductivity (horizontal and vertical mode)

versus real soil moisture. The crossplots demonstrate

that for both sites the coefficients of determination (r2)

of the horizontal mode are higher than the coefficients

of the vertical mode. It is based on the fact, that the realsoil moisture was determined only for the particularly

interesting upper 10 cm of the soil and due to the above

mentioned higher sensitivity of the H-mode for this

depth section.

The good correlation between conductivity and soil

moisture allowed a depth-related estimation of soil

moisture in depths of 2.5 and 7.5 cm. According to

Durlesser (1999), the sensitivities of horizontal and

vertical dipole mode were assessed (McNeill, 1980).

Finally, soil moisture in depths of 2.5 and 7.5 cm

could be calculated. The resulting moisture values aredemonstrated inFig. 6g,h, together with the gravimet-

rically determined soil moisture and the magnetic

susceptibility.

Although the soil moisture varied from July to De-

cember in the range of 38%, the magnetic susceptibility

values showed the expected independent and nearly

constant behaviour for both investigation areas. Since

the magnetic susceptibility measurements with MS2D

probe were done at the rapid 1.0 measurement range,

the data were displayed in steps of 105 SI, which is

the usual scale for field measurements. The slight in-

fluence of the diamagnetic contribution of water mea-sured under laboratory conditions was inferior and not

visible in the field, because of the more important

influences of vegetation, surface roughness, measure-

ment position and the contact between the measurement

probe and the soil. The stronger magnetic susceptibility

variation during the first 4 weeks with stable dry soil

conditions and the following constancy indicates the

growing measurement routine. However, as expected

an increase or decrease of magnetic susceptibility as a

result of different water saturation conditions could not

be observed.

4. Discussion and conclusions

The experimental investigations presented here

quantified the importance of conductivity variations

as a result of changing soil water contents for magnetic

susceptibility measurements. The influence of soil

moisture on the accuracy and comparability of magnet-

ic susceptibility values was successfully evaluated. The

properties of two magnetic susceptibility sensors, the

Bartington MS2D probe and the recently developed

vertical soil profile kappa meter SM400, are presented.




(a) The different experiments and calculations

showed that the magnetic susceptibility values of soils

and sediments are dominated mainly by the volumetric

composition of the space within the reach of the mea-

surement coils of the investigated instruments. The

results proved that for typical soil conductivities theeffect of conductivity on magnetic susceptibility can be

neglected. This is in agreement with the theoretical

predictions ofPetrovsky et al. (2004).

Magnetic susceptibility measurement systems with a

measurement resolution in the range of 106 SI, but

ideally in the range of 108 to 107 SI, could provide

an additional, specific information for the estimation of

the soil water content, independent of the chemism of

water, pore structure, water contact within the pore

channels and temperature (e.g., applicable in frozen

soils or aquifers). Additionally magnetic susceptibilitymeasurement could open a new perspective for fluid

monitoring in reservoirs (Ivakhnenko and Potter, 2004).

(b) The magnetic susceptibility changes in depen-

dence of soil moisture are limited to a maximum var-

iation of9.0 d106 SI for a change in soil water

Fig. 6. (a) Conductivity EC measured in horizontal position vs. soil moisture Hw; temperature corrected (25 8C), test site 1. (b) Test site 2.

(c) Conductivity EC measured in vertical position vs. soil moisture Hw; temperature-corrected (25 8C), test site 1. (d) Test site 2. (e) Crossplots of

temperature-corrected electrical conductivity DEC (horizontal and vertical mode) vs. real soil moisture Hw, test site 1. (f) Test site 2. (g) Magnetic

susceptibilitykvs. soil moisture Hw; the soil moisture curves show the gravimetrically determined moisture of the upper 10 cm and the depthspecific moisture calculated based on electrical conductivity information, test site 1. (h) Test site 2.




content from 0% to 100%. This observation was con-

firmed in the form of different experiments and verifiedby calculations.

For example, the laboratory measurement results of

the Bartington MS2D probe showed a magnetic suscep-

tibility change of about +5.3 d106 SI during a water

saturation change of 92% and corresponding changing

water content from 40% to 3%. The magnetic suscepti-

bility change correlated with the water content and the

resulting electrical conductivity. Furthermore the labo-

ratory measurements with SM400 showed similar mag-

netic susceptibility changes for the behaviour of a

magnetic layer and a depth section which was evidently

unaffected by the magnetic concentrate. This indicates

that the magnetic susceptibility variation was predomi-

nately affected by the water volume content.Consequently, this study quantified the influence of

water on magnetic susceptibility measurements as very

low and showed that the influence is based mainly on

the diamagnetic contribution of the water volume. A

measurement error in connection with the measurement

method, for example caused by an interfering effect of

soil conductivity variations, was not found. To con-

clude, a physically founded significant dependence of

the magnetic susceptibility values of polluted soils

(which are commonly in the range of several

100 d106 SI) on soil moisture caused by weather or

season does not exist.

Fig. 6. (continued).




This information is of great importance for the

method of magnetic susceptibility mapping and

answers open questions about data reproducibility and

comparability. The answers confirmed the reliability of

the screening standardprocedure developedwithin the

MAGPROX project (Schibler et al., 2002). The repro-ducibility of repeat measurements was verified, which

provides the basis for the magnetic susceptibility mon-

itoring of a test site or an investigation area.

(c) On the other hand, there is no doubt that weather

(heavy rainfalls) and season (snow, freeze processes)

can change the magnetic properties of the soil itself.

The determination of the vertical distribution of mag-

netic susceptibility in soils, for example with the

SM400, may provides the exact depth sections of trans-

port channels, boundary layers and redox zones and

allows the observation of material movements in asufficient resolution.

(d) For the Bartington MS2D probe, the above de-

scribed slight magnetic susceptibility variation under

different water saturation conditions (in the range of

106 SI) was only measurable at the more sensitive

measurement range of the instrument during the labo-

ratory experiment (under ideal conditions). In this case

the magnetic susceptibility data were displayed in steps

of 106 SI.

During the field experiment the measurements were

done at the rapid range and the data were displayed in

steps of 105 SI, which is the usual scale for fieldmeasurements. Under field conditions the magnetic sus-

ceptibility showed a stronger variation (in the range of

105 SI) and did not correlate with the soil water content.

Consequently, the slight influence of the diamagnetic

contribution of water was concealed by other influences

like measurement position, vegetation, surface rough-

ness and generally, slight differences in the contact be-

tween the measurement probe and the soil.

Based on these facts and the practical experiences

during the field measurements, it could be assumed that

these influences are generally much more importantthan the influence of water. Wet soils show in many

cases well developed vegetation. Often, they are abun-

dantly covered with grass or moss. The specific pene-

tration depth of the Bartington MS2D probe was

mentioned above. The change in sensitivity with dis-

tance is of great importance for measuring vegetated or

rough surfaces. A layer of diamagnetic material or of

low density overlying a surface will have a significant

effect on the measured value.

For example, a natural cover (plants or litter) of 5

mm thickness would have the effect of reducing the

loop reading to 75% of the value which would be

expected if the loopwas in contact with the underlying

soil (Dearing, 1999).

The authors conclude, that in practical use of the

investigated instruments for topsoil magnetic suscepti-

bility mapping in the field, the influence of soil mois-

ture and resulting soil conductivity can be neglected,especially compared to the influence of the contact

between measurement loop and soil. Whenever a

high reproducibility and comparability is required,

this point should be kept in mind and an identical

horizontal and radial measurement position and an

identical contact pressure should be guaranteed or

aimed.

Acknowledgement

This study was carried out in the framework of theMAGPROX project (EU-Project EVK2-CT-1999-00019).

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