Considerations On Very Low Frequency Electromagnetic Techniques For Contactless Measurements In Hidrogeology

Considerations on very low frequency electromagnetic techniques for contactless measurements in hidrogeology

Tudor Burlan-Rotar 1, Constantin D. Stănescu, Liliana Cainiceanu 2

1,2,3 Polytechnic University, Bucharest, Romania

E-mail: [anonimizat], [anonimizat], [anonimizat]

Abstract. Studies of groundwater consist in data acquisition, their processing and interpretation. In areas of interest hidrogeological is assumed that there is a network of wells drilled. This network provides a first in the hidrogeological information. Electromagnetic (EM) mapping through the use of such areas, using data obtained from existing network of wells drilled, calibration and confirmation. Measurements using the EM can highlight the existence of several layers with different characteristics: clay, limestone, sand, etc. Studies of groundwater interpretation are used for developing a regional hydrogeologic model. The application of electromagnetic techniques for measuring soil resistivity or conductivity has been known for a long time. Conductivity is preferable in inductive techniques, as instrumentation readings are generally directly proportional to conductivity and inversely proportional to resistivity. The operating principle of this method is: a Tx coil transmitter, supplied with alternating current at an audio frequency, is placed on the ground. An Rx coil receiver is located at a short distance, s, away from the Tx coil. The magnetic field varies in time and the Tx coil induces very small currents in the ground. These currents generate a secondary magnetic field, Hs, which is sensed by the Rx receiver coil, together, with primary magnetic field Hp. The ratio of the secondary field, Hs, to the primary magnetic field, Hp, (Hs/Hp) is directly proportional to terrain conductivity. Measuring this ratio, it is possible to construct a device which measures the terrain conductivity by contactless, direct-reading electromagnetic technique (linear meter). This technique for measuring conductivity by electromagnetic induction, using Very Low Frequency (VLF), is a non-intrusive, non-destructive sampling method. The measurements can be done quickly and are not expensive. The Electromagnetic induction technology was originally developed for the mining industry, and has been used in mineral, oil, and gas exploration, and archaeology. In these applications, differences in conductivity of subsurface layers of rock or soil may indicate stratified layers or voids that could be of interest.

Introduction

Studies of groundwater consist in data acquisition, their processing and interpretation.

In areas of interest hidrogeological is assumed that there is a network of wells drilled. This network provides a first in the hidrogeological information. Electromagnetic (EM) mapping through the use of such areas, using data obtained from existing network of wells drilled, calibration and confirmation. Measurements using the EM can highlight the existence of several layers with different characteristics: clay, limestone, sand, etc. Studies of groundwater interpretation are used for developing a regional hidrogeologic model.

Electromagnetic method for measuring soil resistivity

The application of electromagnetic techniques (EM) for measuring soil resistivity or conductivity is known for a long time. Conductivity is preferable in inductive techniques, as instrumentation readings are generally directly proportional to conductivity and inversely proportional to resistivity.

Figure 1 presents the principle of electromagnetic method for measuring soil conductivity.

Figure 1. Principle of electromagnetic soil conductivity measurement

The operating principle of this method is: a Tx transmitter coil supplied with alternating current at a frequency audio is placed on the ground. A Rx receiver coil is located at a distance s from Tx coil.

The magnetic field varies in time and the Tx coil induces very small currents in the ground. These currents generate a secondary magnetic field, Hs, which is sensed by the Rx receiver coil, together, with primary magnetic field Hp.

The current induced in the coil receiver Rx is directly proportional to the conductivity of the soil:

(1)

where :

= secondary magnetic field at Rx coil; = permeability of vacuum;

= primary magnetic field at Rx coil; = soil conductivity;

= 2f (pulsation); s = distance between coils;

f = frequency; =

Since the ratio of the secondary magnetic field and the primary magnetic field is directly proportional to the soil conductivity, can write apparent conductivity indicated by the instrument as defined by the equation:

(2)

The unit for conductivity is Siemens per meter or, more conveniently, milli Siemens per meter (mS / m).

Characteristics of the device according to the type of polarization

Table 1 shows the penetration depth depending on the type of polarization and the distance between the coils.

Table 1. The penetration depth

Consider the following initial conditions:

For a homogeneous or stratified horizontal ground current flow is entirely horizontal. In addition, the current flow at any point in the ground is independent of current flow at any point and the magnetic coupling between the current loops are negligible. Accordingly the depth of penetration is limited only by the distance between the coils.

The response of the device as a function of depth (in a homogeneous halfspace):

Whether on a homogeneous halfspace surface which are located the Tx and Rx coils at distance s. Consider a thin layer dz at a depth z.

The thin layer dz at a depth z is presented in figure 2.

Dragi colegi,

Revenim cu precizari privind buna desfasurare a procesului de recenzare a lucrarilor pentru ACME 2016

1. Responsabilii de sectiune isi vor stabili lista de recenzori functie de numarul lucrarile primite.
2. Responsabilii de sectiune vor trimite fisele de recenzie, impreuna cu lucrarile alocate, fiecarui recenzor. Rugamintea este ca recenziile (cel putin doi recenzori trebuie sa fie implicati in evaluarea fiecarie lucrari)) sa fie realizate in maxim 10 zile.

3. Fisele de recenzie se vor intoarce tot la responsabilii de sectiune, care vor trebui sa ia legatura cu autorii pentru corecturile cerute in urma evaluarii.

4. Dupa reprimirea lucrarii corectate, responsabilii de sectiune vor hotari daca lucrarea este acceptata sau respinsa si vor comunica autorilor decizia.

Figure 2. The thin layer dz at the depth z

The depth plotted as fractions of s – distance between coils, is represented on :

(3)

It can be built, so for the vertical polarization, the function , which describes the relative contribution of the secondary magnetic field due to a thin layer at a depth z.

In figure 3 is presented the function for the vertical polarization.

It is observed that the layer located at a depth of about 0.4s gives maximum contribution of secondary magnetic field, but that layer to a depth of 1.5s, yet contribute significantly.

Figure 3. Operation of the device in vertical polarization mode (VD)

It is interesting to note that in the neighborhood of the surface layer has a very small s contribution to the secondary magnetic field and, therefore, this configuration is insensitive to changes in conductivity near the surface.

In figure 4 is presented the function for the case when the transmitter and receiver operate in the operating mode to horizontal coplanar dipoles.

Figure 4. Operation of the device in horizontal polarization mode (HD)

For comparison of the different ways to respond to layers at different depths, now are shown in the same coordinate system, both functions: vertical polarization (VD) and horizontal polarization (HD). In figure 5 are presented both functions: and .

Figure 5. Representation of both functions: and

(to highlight how different the response of different layers)

It is noted that at depths slightly smaller than the distance between the coils, the signal measured by the device is about twice higher for vertical polarization to horizontal polarization case.

The horizontal dipole orientation, the instrument is more sensitive to soil layers in the vicinity. The vertical dipole orientation device is more sensitive to the deeper layers.

Thus, by performing measurements in both modes, it is possible to measure the increase or decrease in conductivity with depth.

Block diagram of the device based on the method of electromagnetic (EM)

Figure 6 presents a block diagram of the device and the types of polarization used: vertical dipole (VD) and horizontal dipole (HD).

Figure 6. Block diagram of the device and polarization types used:

Vertical dipole (VD) and horizontal dipole (HD)

The device is composed of two parts. The emission consists of a transmitting coil that receives signal Tx emission module (a square wave generator of fixed frequency of 10-20 kHz). The reception desk is made of Rx coil and receiver (amplifier with one or more floors, followed by a detector). Receiver modulator output is connected to a measuring instrument (mA) through a potentiometric circuit. Level zero is set in the potentiometer.

The polarization is selected by positioning the two coils, Tx coil and Rx coil. It uses horizontal polarization dipole when the coil axis is parallel to the soil surface and vertical dipole when the coil axis is perpendicular to the soil surface.

Hidrogeology measurement

The operating principle of this method is: a Tx coil transmitter, supplied with alternating current at an audio frequency, is placed on the ground. An Rx coil receiver is located at a short distance, s, away from the Tx coil. The magnetic field varies in time and the Tx coil induces very small currents in the ground. These currents generate a secondary magnetic field, Hs, which is sensed by the Rx receiver coil, together, with primary magnetic field Hp. The ratio of the secondary field, Hs, to the primary magnetic field, Hp, (Hs/Hp) is directly proportional to terrain conductivity. Measuring this ratio, it is possible to construct a device which measures the terrain conductivity by contactless, direct-reading electromagnetic technique (linear meter).

This technique for measuring conductivity by electromagnetic induction, using Very Low Frequency (VLF), is a non-intrusive, non-destructive sampling method. The measurements can be done quickly and are not expensive.

Wells are drilled at a minimum distance of 3 – 5 Km within 15 – 20 Km. The distances of 30 – 40 Km are too high to provide a basic level of primary information. Measurements using the EM can highlight the existence of several layers with different characteristics: clay, limestone, sand, etc. Type of sediment permeability and thus the condition of existing water quantity. It should be taken into acount and the season in which they occur measurements: rainy or dry season.

The transmitter( Tx) and receiver(Rx) must be kept dry to function properly.

This is accomplished by floating them in plastic storage boxes. The receiver coil must be perched on legs made from four-foot long wooden dowels

The transmitter output current of 3A into a 100 m x 100 m loop gives good response and resolution to depths of 150 m. So it is an ideal instrument for resistivity sounding over a large area. When the transmitter output current of 2,5 A to an 8 – turn, 5 m x 5 m moving transmitter loop with base frequency of 75 Hz, the configuration is optimal for horizontal loops (vertical dipole) surveys for mineral exploration to shallow depths, and for groundwater exploration. The transmitter has a 12V battery.

The transmitter current is a modified symmetrical square wave. In figure 7 are presented the transmitter current and the signal in the receiver loop.

Figure 7. the transmitter current and the signal in the receiver loop

The receiver coil is located at the middle of the transmitter loop. In figure 8 is presented the central loop sounding configuration.

Figure 8. Central loop sounding configuration.

red: Tx transmitter and transmitter loop, blue: Rx receiver and receiver loop

Better spatial resolution is obtained with a moving transmitter configuration with a short intercoil spacing, but is limited to ashallower depth of exploration. Greatest depth is obtained with large fixed loop.

The loops are air-cored coils for both: transmitter and receiver.

The system is adequate to be employed for general geological applications, such as mapping groundwater.

Conclusions

In areas in vicinity of the sea is likely to be revealed some degree of intrusion of saltwater. Areas with saltwater and freshwater identify the difference in resistivity. The interpreted EM measurements show a distinct range of layer resistivities, which correspond to freshwater and saltwater saturated materials.

Based on this and the results of geophysical borehole measurements, the EM results can be used to map. The data points which do not fit in smoothly with neighboring points are rejected. On the map are indicated depths separate.

References

Diaz, L. and J. Herrero, “Salinity Estimates in Irrigated Soils Using Electromagnetic Induction”, Soil Sci. 154(2): 151-157, 1992;

Kachanoski RG Gregorich EG & Van Wesenbeeck IJ, “Estimating Spatial Variations of Soil Water Content Using Noncontacting Electromagnetic Inductive Methods”, 1988., Canadian Journal of Soil Science 68, 715-722;

Rhoades, J. D., P. A. Raats, and R. J. Prather. “Effects of Liquid-Phase Electrical Conductivity, Water Content, and Surface Conductivity on Bulk Soil Electrical Conductivity” (1976), Soil Sci. Soc. Am. J. 40:651-655;

Sheets K.R. and J.M.H. Hendrickx, “Noninvasive Soil Water Content Measurement Using Electromagnetic Induction”, Water Resources Res. 31(10): 2401-2409, 1995;

Williams, B.G., Fiddler, F-T., “The Use of Electromagnetic Induction for Locating Subsurface Saline Material. In Relation of Groundwater Quantity and Quality”, 1983, (Proceedings of the Hamburg Symposium, August 1983). IAHS Publishing No 146 189-196;

Williams BG & Hoey D, “The Use of Electromagnetic Induction to Detect the Spatial Variability of the Salt and Clay Contents of Soil”, 1987, Australian Journal of Soil Research 25, 21-28;