Evaluation of the OhmMapper Instrument for

Measurement of Soil Moisture

1,2Jeffrey P. Walker and 1Paul R. Houser

1. Hydrological Sciences Branch, Laboratory for Hydrospheric ProcessesNASA’s Goddard Space Flight Center, Greenbelt, MD 20771, USA

2. Goddard Earth Sciences and Technology Center

Submitted to:Soil Science Society of America Journal

7th November, 2000

Corresponding Author:Jeffrey P. Walker

Rm B309, Building 33NASA/GSFC Mail Code 974Greenbelt MD, 21144 USA

cejpw@land.gsfc.nasa.gov

ACKNOWLEDGEMENTS

The United States Department of Agriculture is acknowledged for allowing access to the

OPE3 field site at the Agriculture Research Center in Beltsvill e, MD. Assistance was given

in the field by Kristi Arsenault, Brian Cosgrove, Dan Hymer and Melinda Pouss. The loan

of an OhmMapper by Geometrics Inc. and analysis of OhmMapper data by Brandy

Rutledge is acknowledged.

Evaluation of the OhmMapper Instrument forMeasurement of Soil Moisture

Abstract

An instrument for making rapid measurements of the soil moisture content in the

top few meters of soil i s an essential tool for many applications, including understanding of

soil water dynamics, evaluation of agriculture water stress, validation of soil moisture

modeling, and assimilation of remotely sensed near-surface soil moisture data. Studies have

shown that electrical resistance measurements may be used to measure soil moisture

content. In this paper, electrical resistivity (resistance multiplied by a geometric factor)

measurements of the soil using the OhmMapper instrument of Geometrics Inc. are

compared with time domain reflectometry point measurements of soil moisture to a depth

of 70 cm. It was found that the OhmMapper could measure soil moisture content with a

correlation coefficient of up to 0.6. It is li kely that this correlation coeff icient would be

greater if much deeper measurements of soil moisture were compared with the OhmMapper

measurements.

1

1. INTRODUCTION

Knowledge of the spatial distribution in soil moisture content for the top meter or

two of the earth’s surface is important for many environmental studies, including

meteorology, hydrology and agronomy. However, as a result of the heterogeneity of soil

properties, topography, land cover, evapotranspiration, and precipitation, soil moisture is

highly variable both spatially and temporally (Engman, 1991; Wood et al., 1993). Hence,

numerical soil moisture models are prone to error (Wood et al., 1993) and spatial coverage

by point measurements is limited by logistics (Giacomelli et al., 1995). Moreover, soil

moisture measurement depth from remote sensing observations is limited to the top few

centimeters (Schmugge, 1985). As a result, research is being directed toward a combination

of these three approaches for estimating the spatial distribution and temporal variation of

soil moisture content; assimilation of remote sensing observations into the numerical soil

moisture model (eg. Houser et al., 1998) and calibration/evaluation of the soil moisture

model from point measurements (eg. Walker et al., 2000). However, validation of such a

system over even moderately sized areas is diff icult, due to a lack of knowledge about

spatial variabili ty in the soil moisture profile. This necessitates a dependence on sparse

point measurements of soil moisture and measurements of surrogate variables, such as

surface temperature and evapotranspiration, for validation of the soil moisture estimates. In

order to overcome such limitations, there is a demonstrated need for an instrument that

allows rapid measurement of the spatial variabili ty in soil moisture for the top meter or two

of the earth’s surface.

2

Studies have shown that soil moisture content can be determined from

measurements of the soil electrical resistance (eg. Seyfried, 1993; Amer et al., 1994; Hymer

et al., 2000) using point measurement techniques. A new instrument on the market for

mapping the spatial variabili ty of electrical resistivity (resistance multiplied by a geometric

factor) over the top several meters of the earth’s surface is the OhmMapper by Geometrics

Inc. In this paper we evaluate the OhmMapper for its potential to map the soil moisture

profile over large areas in a timely fashion.

2. DATA AND METHODOLOGY

2.1 Study Area

Evaluation of the OhmMapper was undertaken at the OPE3 (Optimizing Production

inputs for Economic and Environmental Enhancement) field site located at the United

States Department of Agriculture, Agriculture Research Center, in Beltsvill e, MD (Gish et

al., 2001). The OPE3 field site has a sandy loam soil with terrain slopes from 1% to 3% and

is used for growing corn during the spring and summer. This field site consists of four

gently sloping 4ha watersheds that feed a wooded riparian wetland and first-order stream. A

single 250m transect running from top to bottom of one of the watersheds was monitored

using both widely accepted point measurement techniques (capacitance and time domain

reflectometry) and the OhmMapper. Point measurements were made at 25m spacing to a

depth of 70cm. Evaluation was made over a 1-month period immediately following

planting in the spring growing season of 2000. Soil moisture content and OhmMapper

measurements were made on a total of 8 days.

3

2.2 Point Measurements

The point measurements of soil moisture content were made with the commercially

available Theta Probe from Delta-T Devices Ltd. (Delta-T Devices Ltd., 1996) and the

Trase Connector Time Domain Reflectometry (TDR) soil moisture instrument from Soil

Moisture Equipment Corp. (Soil Moisture Equipment Corp., 1989). (The mention of trade

and company names is for the benefit of the reader and does not imply an endorsement of

the product.) By inserting the probes/waveguides vertically from the soil surface, the Theta

Probe provided a soil moisture measurement over the top 6cm of soil , while the TDR

instrument provided soil moisture measurements over the top 15, 30, 45, 60 and 70cm of

soil . Comparison of Theta Probe and 15cm TDR measurements with thermogravimetric

measurements showed that the manufacturers calibration was not satisfactory for this

evaluation (Figure 1). Hence a correction was applied to the point measurements of soil

moisture content.

2.3 OhmMapper

The OhmMapper instrument is a capacitively-coupled resistivity system that

measures the electrical properties of the ground without the cumbersome galvanic

electrodes used in traditional resistivity surveys. A simple coaxial-cable array with

transmitter and receiver sections is pulled along the ground either by a single person or a

small all -terrain vehicle (Figure 2). In conventional resistance surveys, four electrodes are

inserted into the soil and a current injected into the ground by connecting a DC power

source to two of the electrodes. The voltage is then measured at the remaining two

electrodes, and the resistance calculated using Ohm’s Law. The resistance measured varies

as a function of the distance and geometry between the probes, so it is normalized with the

4

addition of a geometric factor that converts the measurement to resistivity (Geometrics Inc.,

1999). This is the basis for the electrical resistance sensors commercially available for

measurement of soil moisture content.

Like some configurations of traditional galvanic resistivity, the OhmMapper uses a

dipole-dipole array (ie. injection of current following measurement of voltage rather than a

nested measurement) to measure resistivity, except that contact is made with the ground

capacitively. The dipole-dipole array is very sensitive to horizontal changes in resistivity

but relatively insensitive to vertical changes, meaning that it is good in mapping vertical

structures but relatively poor in mapping horizontal structures (Loke, 1991). By increasing

the dipole cable length and/or tow-link cable length, and hence the total length of the

dipole-dipole array, the depth of the resistivity measurement is increased. For dipoles

separated by a dipole length (see Figure 2) this measurement depth is approximately 14%

of the total length of the dipole-dipole array (Loke, 1999). The measurement depth

increases to approximately 22% of the total length of the dipole-dipole array for dipoles

separated by six dipole lengths (Loke, 1999). Hence, by making multiple passes with

different configurations of the OhmMapper it is possible to measure the variation of

resistivity with depth. In addition, data collection is continuous in time so the resistivity is

finely sampled along any given towpath. Nominal dipole cable lengths of 1, 2.5 and 5m,

and nominal tow-link cable lengths of 0.25, 0.5, 1.0 2.0 and 4.0m were used in this

evaluation.

5

2.4 Evaluation

Hymer et al. (2000) used a power relationship between volumetric soil moisture

content (θ) and resistance (R) of the form

baR=θ , (1)

where a and b are fitted parameters. A relationship of this form is compatible with Archie’s

Law (Archie, 1942), where a is the inverse of soil salinity and b is a soil texture parameter.

Similar non-linear relationships have been used by Seyfried (1993) and Amer et al. (1994).

In this paper, we take the OhmMapper apparent resistivity measurement to be

equivalent to the resistance value in (1). We then solve for the parameters a and b by fitting

(1) to OhmMapper measurements of apparent resistivity and TDR/Theta Probe

measurements of soil moisture at each 25m interval along the 250m transect. Although the

variation of resistivity with depth in the soil profile could be evaluated from an inversion of

the multiple OhmMapper measurements with different dipole cable and tow-link cable

lengths using a commercially available inversion software package, we have concentrated

our efforts on evaluating the single pass resistivity data. Our reasons for this are: (i) the

TDR measurements used for evaluation give a depth integrated measurement of soil

moisture content; and (ii ) the single pass data must be shown to have information about soil

moisture before attempting the measurement of soil moisture variation with depth.

6

3. RESULTS AND DISCUSSION

The results from fitting the function given in (1) with the measured soil moisture

content and resistivity data are summarized in Table 1 for the correlation coeff icient. These

results show a typically low correlation coeff icient, having a value of less than 0.6, for all

dipole cable, tow-link and soil moisture depth measurement combinations. However,

generally speaking, for a given dipole cable length the soil moisture depth yielding the

greatest correlation coeff icient increases with tow-link length. The correlation coefficients

cease to follow this trend only when soil moisture measurements are too shallow. This

trend was expected, as a greater separation between the midpoints of receiver and

transmitter dipoles is claimed to yield a deeper measurement (Geometrics Inc., 1999).

However, using the general rule that resistivity measurement depth is half of this

separation, it was expected that the greatest correlation coeff icient for the 1m dipole cable

length and 0.25m tow-link length would be with the 70cm soil moisture measurement

depth. Given that the correlation coeff icients are so low for the 1m dipole cable length, the

higher correlation coeff icient for the 15cm soil moisture measurement depth may be a

purely spurious correlation.

The scatterplots with fitted function are given in Figures 3 to 5 for each dipole cable

and tow-link length for the soil moisture measurement depth having the greatest correlation

coeff icient. However, it may be seen that there is still as much as 15% v/v variation in soil

moisture content for a given resistivity value, with most scatterplots bearing no

resemblance to the fitted function, or any other relationship, being simply a cloud of points.

The nominal 2.5m dipole cable length with 1m tow-link cable length is the only

OhmMapper configuration that yielded a scatterplot that bore a significant resemblance to

7

the fitted function, as indicated by its higher correlation coeff icient. In fact, this

configuration yielded a consistently higher correlation coeff icient for all soil moisture

measurement depths when compared with the other configurations. It is suggested that the

lower correlation coeff icients for shorter dipole cables and tow-link lengths are a result of

the noisy resistivity measurement yielded by the OhmMapper when the dipole cables were

short and/or the dipole cable separation was small relative to the dipole cable length. The

lower correlation coeff icients for longer dipole cable and/or tow-link lengths are likely a

result of the OhmMapper resistivity measurement being for a depth much greater than the

deepest TDR measurement of soil moisture. The higher correlation coeff icients for all soil

moisture measurement depths with the OhmMapper resistivity measurements for 2.5m

dipole cables and 1m tow-link cable length are likely a result of high correlation in soil

moisture content for the top 70cm of soil .

The soil moisture variation along the 250m transect is plotted in Figure 6 for the

60cm TDR measurements, where it is compared with the OhmMapper measurements for

2.5m dipole cables and 1m tow-link lengths using the fitted function from Figure 4c. Most

notable in this figure is that TDR point measurements of soil moisture content have a

variation of approximately 7% v/v from dry to wet, while the OhmMapper measurements

have only approximately 3% v/v variation from dry to wet. However, the day on which

TDR measurements indicate the wettest soil moisture coincides with the day on which

OhmMapper measurements indicate the wettest soil moisture. Likewise, the TDR and

OhmMapper measurements both indicate the driest soil moisture on the same day.

Moreover, the OhmMapper measurements exhibit approximately the correct trend of soil

moisture content along the transect.

8

It is well known that the near-surface soil moisture content responds rapidly to both

infilt ration and exfilt ration events while the soil moisture content for a deep layer of soil

has a much slower response time. Moreover, changes in the near-surface soil moisture

content have only a minimal impact on the average soil moisture content of a thick soil

layer (ie. 3m). Hence, an explanation for the OhmMapper having the correct response to

soil moisture content but the wrong magnitude may be that the OhmMapper is actually

measuring the resistivity, and hence soil moisture, of a much deeper layer than the TDR,

being approximately 3m as compared to 60cm. Furthermore, OhmMapper measurements of

resistivity are an average measurement made over not only some depth (eg 3m), but over

the entire length of the instrument (ie. 11m for 2.5m dipole cables and 1m tow-link),

meaning that interpretation of comparisons with individual point measurement is diff icult.

4. CONCLUSIONS

This study has shown that there is a correlation coeff icient of approximately 0.6

between TDR measurements of soil moisture content and the OhmMapper with 2.5m

dipole cables and a 1m tow-link. All other OhmMapper configurations resulted in smaller

correlation coefficients for the soil moisture depths measured. Results have indicated that

this configuration is optimal for the purpose of measuring soil moisture in the top few

meters, with the correlation coeff icient likely to be greater if TDR measurements of soil

moisture content could be made for a greater depth. Moreover, unless the noise in

OhmMapper measurements of resistivity can be reduced for shorter dipole cables and/or

shorter dipole cable separations, it is unlikely that the OhmMapper will be applicable for

measuring the soil moisture content of shallower soil depths or be used to measure the

variation of soil moisture content with depth in the soil profile.

9

REFERENCES

Amer, S.A., T.O. Keefer, M.A. Weltz, D.C. Goodrich, and L.B. Bach, 1994. Soil moisture

sensors for continuous monitoring. Water Resources Bulletin, 30:69-83.

Archie, G.E., 1942. The electrical resistivity log as an aid in determining some reservoir

characteristics. Transactions of the American Institute of Mineral Engineering,

146:54-62.

Delta-T Devices Ltd., 1996. Theta Probe Soil Moisture Sensor Type ML1 User Manual.

Delta-T Devices, Inc., Cambridge, England, 18pp.

Engman, E.T., 1991. Application of microwave remote sensing of soil moisture for water

resources and agriculture. Remote Sensing of the Environment, 35:213-226.

Geometrics Inc., 1999. OhmMapper TR1 Operation Manual. Geometrics Inc., San Jose,

USA, 113pp.

Giacomelli , A., U. Bacchiega, P.A. Troch, and M. Mancini, 1995. Evaluation of surface

soil moisture distribution by means of SAR remote sensing techniques and

conceptual hydrological modelli ng. Journal of Hydrology, 166:445-459.

Gish, T.J., W.P. Dulaney, C.S.T. Daughtry, and K.-J.S. Kung, 2001. Influence of

preferential flow on surface runoff f luxes. ASAE Preferential Flow Symposium.

10

Houser, P.R., W.J. Shuttleworth, J.S. Famiglietti, H.V. Gupta, K.H. Syed, and D.C.

Goodrich, 1998. Integration of soil moisture remote sensing and hydrologic

modeling using data assimilation. Water Resources Research, 34: 3405-3420.

Hymer, D.C., M.S. Moran, and T.O. Keefer, 2000. Soil water evaluation using a hydrologic

model and calibrated sensor network. Soil Science Society of America Journal,

64:319-326.

Loke, M.H., 1999. Electrical imaging surveys for environmental and engineering studies: A

practical guide to 2D and 3D surveys. Geometrics Inc., San Jose, USA, 57pp.

Schmugge, T., 1985. Chapter 5: Remote Sensing of Soil Moisture, In: M.G. Anderson, and

T.P. Burt, (Eds.), Hydrological Forecasting, John Wiley and Sons, New York,

101-124.

Seyfried, M.S., 1993. Field calibration and monitoring of soil -water content with fiberglass

electrical resistance sensors. Soil Science Society of America Journal, 57:1432-

1436.

Soil Moisture Equipment Corp., 1989. Trase System 1 Operating Instructions. Soil

Moisture Equipment Corp., Santa Barbara, USA, 54pp.

Walker, J.P., G.R. Willgoose, and J.D. Kalma, 2000. Three-dimensional soil moisture

profile retrieval by assimilation of near-surface measurements: Simpli fied Kalman

filter covariance forecasting and field application. Water Resources Research, In

Preparation.

11

Wood, E.F., D.S. Lin, M. Mancini, D.J. Thongs, P.A. Troch, T.J. Jackson, J.S. Famiglietti,

and E.T. Engman, 1993. Intercomparisons between passive and active microwave

remote sensing and hydrological modeling for soil moisture. Advances in Space

Research, 13:167-176.

12

Table 1: Comparison of correlation coeff icient between OhmMapper resistivity and soil

moisture measurements for nominal dipole cable, tow-link and TDR probe lengths given.

Maximum R-value for dipole cable and tow-link length configuration are given in bold.

R-Value for Soil Moisture Measurement DepthDipole CableLength

Tow-LinkLength 6cm 15cm 30cm 45cm 60cm 70cm0.25m 0.064 0.221 0.168 0.168 0.125 0.0850.50m 0.104 0.183 0.272 0.266 0.186 0.1651.00m 0.017 0.143 0.174 0.176 0.240 0.128

1.0m

2.00m 0.251 0.336 0.310 0.333 0.318 0.348

0.25m 0.218 0.361 0.322 0.332 0.252 0.2990.50m 0.069 0.181 0.179 0.246 0.287 0.2381.00m 0.512 0.557 0.490 0.546 0.585 0.516

2.5m

2.00m 0.010 0.213 0.168 0.194 0.186 0.185

0.50m 0.218 0.428 0.467 0.401 0.375 0.3221.00m 0.008 0.100 0.123 0.130 0.062 0.1512.00m 0.067 0.226 0.229 0.326 0.318 0.360

5.0m

4.00m 0.236 0.106 0.145 0.196 0.228 0.269

13

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Figure 1: Calibration of a) Theta Probe and b) 15cm TDR against thermogravimetric soil

moisture measurements.

14

Figure 2: Schematic of the OhmMapper instrument.

Dipole Cable Dipole Cable Dipole Cable Dipole Cable

Receiver TransmitterNon-conductivetow-link cable

Fiber opticisolator cable

Dipole Length

Resistivity Measurement

15

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Figure 3: Fitted function and scatter plot of apparent resistivity against the soil moisture

measurement giving the greatest correlation for the OhmMapper configuration of dipole

cable tow-link cable lengths. Nominal 1m dipole cable length with: a) nominal 0.25m tow-

link length against 15cm TDR measurements; b) nominal 0.5m tow-link length against

30cm TDR measurements; c) nominal 1m tow-link length against 60cm TDR

measurements; and d) nominal 2m tow-link length against 70cm TDR measurements.

16

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Figure 4: Fitted function and scatter plot of apparent resistivity against the soil moisture

measurement giving the greatest correlation for the OhmMapper configuration of dipole

cable tow-link cable lengths. Nominal 2.5m dipole cable length with: a) nominal 0.25m

tow-link length against 15cm TDR measurements; b) nominal 0.5m tow-link length against

60cm TDR measurements; c) nominal 1m tow-link length against 60cm TDR

measurements; and d) nominal 2m tow-link length against 45cm TDR measurements.

17

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Figure 5: Fitted function and scatter plot of apparent resistivity against the soil moisture

measurement giving the greatest correlation for the OhmMapper configuration of dipole

cable tow-link cable lengths. Nominal 5m dipole cable length with: a) nominal 0.5m tow-

link length against 30cm TDR measurements; b) nominal 1m tow-link length against 70cm

TDR measurements; c) nominal 2m tow-link length against 70cm TDR measurements; and

d) nominal 4m tow-link length against 70cm TDR measurements.

18

O P

Q R

Q P

S R

R P R O R R O P R Q R R Q P R

T UVWX UV YZ [\

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Figure 6: Comparison of 60cm TDR soil moisture measurements (symbols) with the

OhmMapper soil moisture estimate (continuous line and symbol); solid circle 7 June, solid

triangle 9 June, solid square 12 June, open circle 15 June, open triangle 19 June and open

square 23 June.