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Soil Resistivity Measurements

Introduction

Soil resistivity measurements constitute the basis of any grounding study and are therefore of capital importance.

Soil resistivity measurements are made by injecting current into the earth between two outer electrodes and measuring the resulting voltage between two potential probes placed along a straight line between the current-injection electrodes. When the adjacent current and potential electrodes are close together, the measured soil resistivity is indicative of local surface soil characteristics. When the electrodes are far apart, the measured soil resistivity is indicative of average deep soil characteristics throughout a much larger area.

In principle, soil resistivity measurements should be made to spacings (between adjacent current and potential electrodes) which are at least on the same order as the maximum extent of the grounding system (or systems) under study, although it is preferable to extend the measurement traverses to several times the maximum grounding system dimension, where possible. Often, it will be found that the maximum electrode spacing is governed by other considerations, such as the maximum extent of the available land which is clear of interfering bare buried conductors.

The attached data sheets provide electrode spacings that can be used, starting at short electrode spacings, for information about the surface soil layers, and ending at the largest electrode spacings. As can be seen, the electrode spacings increase exponentially in order to cover the entire range of required measurement depths as efficiently as possible.

Special Precautions

Background Noise. Due to nearby sources of 50 or 60 Hz current and its harmonics, electrical noise at these frequencies are expected in the measurements, particularly for the larger electrode spacings. Conventional measurement methods can confound this noise with the measurement signal, resulting in apparent soil resistivity readings that can be an order of magnitude or more in excess of the true values. This suggests the need for equipment that uses a signal frequency other than 50 or 60 Hz and its harmonics and can efficiently discriminate between the signal filter and the noise. A soil resistivity tester such as the SYSCAL Junior or R1 Plus (the latter is strongly recommended when very large pin spacings or high resistivity surface material are expected), manufactured by Iris Instruments (Orleans, France), or the SuperSting R1 IP single channel memory earth resistivity and IP meter manufactured by Advanced Geosciences, Inc. of Austin, Texas, USA, achieves this function: both equipment are able to accurately measure its low frequency signal, even when the background 50 or 60 Hz noise is several thousand times larger in magnitude. In the following, we refer to the SYSCAl resistivity meter. However, the SuperSting meter has equivalent capabilities as well. Some high end resistivity meters manufactured by other organizations may have similar capabilities or better. Please compare products before selecting the equipment that best fits your needs and budget.

Interlead Coupling. Another problem that can be encountered at large electrode spacings, particularly when apparent resistivities are low to moderate, is magnetic field coupling between the current injection leads and the voltage measurement leads. This coupling induces noise at the same frequency as the signal into the measured voltage and amplifies the measured resistivity. While some equipment can detect the resulting phase shift in the measured voltage and make a partial correction, other equipment cannot. The SYSCAL soil resistivity tester circumvents this problem through use of a very low frequency signal (from 500 millisecond to 2000 millisecond square wave pulses, with 2000 milliseconds being the preferred setting), which generates negligible magnetic field coupling.

Influence of Bare Buried Metallic Structures. Bare metallic structures (including concrete-encased metal) of significant length buried in the vicinity of the measurement traverse can distort measured earth resistivities. When a measurement traverse runs parallel to a long structure of this type, significant error begins when the clearance between the traverse and the structure is on the same order as the electrode spacing. The error increases as the electrode spacing increases compared with the clearance. A similar effect is observed when the electrodes are placed near relatively small grounding systems which are interconnected by means of overhead wires. As a rule of thumb, to avoid significant error, there should be no bare metallic structures of significant size buried within a radius r of any of the measurement electrodes, where r is the spacing between adjacent current and potential electrodes. When the measurement traverse runs perpendicular to a buried metallic structure without crossing it, the clearance requirement need not be as severe. Computer modelling of the buried structure and measurement electrodes can provide an estimate of the measurement error to be expected for different soil structure types.

Weak Signal. A weak measurement signal can result from a low-power source, a low-voltage source, or a high contact resistance of one or both of the current injection electrodes. The problem is most often experienced when driving electrodes in high resistivity surface soils or when the electrode spacing becomes large (the signal strength is inversely proportional to the electrode spacing, for the Wenner 4-pin method, and inversely proportional to the square of the electrode spacing, for the Schlumberger 4-pin method, all other things being equal). Use of a powerful, high-voltage source is an obvious first step to eliminating this problem. Even with a good source, however, contact resistance can easily become a problem in high resistivity soils at the larger electrode spacings. The solution in this case is to drive the current-injection electrodes as deep as possible and wet the soil around these electrodes with saltwater: this should be done only for the larger electrode spacings. If need be, multiple rods can be driven into the ground and connected together to constitute a larger, lower impedance electrode. On solid rock or in rock with a shallow soil layer over it, the electrodes can be laid horizontally on the rock and covered with conductive material, such as saltwater-moistened earth. If the rock is highly localized, then the electrode position can be altered (and noted) to avoid the rock; interpretation software such as the RESAP module of the CDEGS software package will account for this.

The SYSCAL soil resistivity tester constitutes a high-voltage, high-power source, compared to many other available models: its output voltage varies from 50 V to 400 V or higher and its output power can reach 50 –250 W (depending on the model).

A weak signal can be detected by examining the magnitudes of the measured signal voltage and injection current; also by verifying the consistency of the readings. The SYSCAL can provide reasonably accurate readings for injection currents as low as 1 mA (a minimum of 5 mA is preferable) and signal voltages as low as 1 mV, in the presence of 50 or 60 Hz noise which is 4389 times larger in magnitude. On the other hand, very low frequency background noise may require a stronger signal for good accuracy. Such a need can be detected by the standard deviation value, q, reported by the instrument as it takes a series of readings with a square wave of alternating polarity: when q is 0 at the end of the series of readings, the measurement is reliable; otherwise, a stronger signal should be sought. A series of 10 cycles or so should be selected for each measurement. Also, q should be watched as the measurements are made: if the string “***” appears during the readings, the measurement should be rejected. This is usually an indication that one of the current injection leads has become disconnected. A further precaution is to read the resistivity at two different injection currents, if possible: if consistent, then the reading is good. The attached data table reminds the instrument operator to record these important values.

Erratic Readings . Erratic readings can occur due to poor connections or high contact resistance, background noise at a frequency similar to that used by the measurement equipment, nearby buried metallic structures, equipment failure, operator error, and other factors. Measured resistivities should be plotted on log-log graph paper in the field to permit detection of irregular measurements, so that corrective action can be taken immediately. Resistivity should be plotted versus electrode spacing: a smooth curve is expected. Sharp changes suggest a need for checking the equipment set-up, repeating measurements, and taking additional measurements at shorter and larger electrode spacings close to the problematic one.

Excessive Voltage Magnitude. Certain versions of the SYSCAL require an input voltage (including both signal and noise) of less than 5 V to provide a reading. A voltage exceeding 5 V can occur when background noise is excessive: in this case the input voltage must be reduced with a voltage divider circuit (e.g., the voltage from the two inner potential electrodes is applied to a 100 k W resistor in series with a 1 M W resistor and the voltage across the 100 k W resistor is measured by the M-N terminals of the SYSCAL, resulting in a 90.9% voltage reduction). An excessive voltage can also occur at short electrode spacings due to excessive signal strength. In this case, the voltage can be reduced by reducing the source voltage setting or by decreasing the current electrode depth. The attached data sheet provides typical electrode depths.

Note that the depth of the current injection pins should never exceed 33% of the spacing between adjacent current and potential electrodes; the inner potential measurement pins should be driven to even shallower depths, as shown in the attached data sheet. This improves measurement accuracy at short electrode spacings.

Measurement Details

The current injection leads are connected to the instrument terminals labelled “A” and “B”, the potential leads are connected to the terminals labelled “M” and “N”. The electrode spacing is keyed in and the measurement process is launched. The instrument records and averages as many readings as the user sets the instrument to take (e.g., 6 or so).

Measurements are to be made along the traverses determined in conjunction with SES (provided in a separate document). It is important that the maximum spacing between the two current injection pins along the longest traverse be at least equal to three times the maximum extent of the grounding system being designed, as a bare minimum, if this can be achieved without interference from nearby buried metallic structures.

Schlumberger Method

The measurements are to be made based on the Schlumberger 4-pin method, taking the precautions described in this document. The P1 and P2 potential pins should be installed at the center of the traverse, initially 1.0 m apart. The C1 and C2 current pins are to be driven into the ground at progressively increasing distances from their respective potential pins, starting 0.10 m from the nearest potential pin and increasing up to the maximum pin spacing specified by SES for each measurement traverse. The “maximum pin spacing” indicated for any given traverse is the maximum distance between each potential pin and its adjacent current pin. The separation distance between the inner potential pins remains at 1.0 m for the first few measurements, then increases as necessary to obtain a sufficiently strong measurement signal (i.e., at least 1-10 mV, if possible, and a q value of 0). Note that before increasing the spacing between the potential pins, all practical attempts should be made to improve the contact resistance of the outer current injection pins: drive them deeper, use clusters of rods at the larger spacings, wet the ground close to the pins with saltwater (without wetting the ground close to the potential probes!). Ensure that the injection current is 5 mA or more and the measured signal voltage 1 mV or more, if at all possible. Each time the potential pin spacing is increased, repeat the preceding measurement: i.e., place the current pins to the distance separating them from the potential pins during the preceding measurement, for validation.

Wenner Method (Fixed C1 & C2 Pin): One-Sided

The measurements are to be made based on a modified Wenner 4-pin method, taking the precautions described earlier in this document. The test method chosen here gives greater weight to transferred potentials from the C1 electrode to locations tested in the future mine area and also obviates the need to move the C2 test electrode.

The C1 electrode should be installed at the center of the future 260 kV substation site under test (there are two such substation sites). The C2 electrode should be installed 4 km away (the locations are specified below). The C1 and C2 electrodes must be installed such as to have low ground resistance: we wish to obtain approximately 500 mA or more of current flow from the soil resistivity meter when it is connected in series with the two electrodes. Start by driving 3 ground rods 0.7 – 1.0 m into the ground, in a triangular formation, spaced about 1.7 m apart, with the 3 ground rods interconnected. Drive the rods deeper and add rods if need be; also pour salt water around each ground rod, if need be, to achieve a sufficiently low ground resistance. Connect a lead from the “A” terminal of the SYSCAL test meter to the C1 electrode array and another lead from the “B” terminal of the meter to the C2 electrode array. Use the Rtest function to verify that the total resistance of the C1-C2 circuit is on the order of 800 ohms or less.

This electrode set-up should be used for all readings in which the P1 electrode is 30 m or more from the center of the C1 electrode array. When the P1 electrode is less than 30 m from the C1 electrode, the latter should be reduced to a single ground rod driven 0.7 m or so into the ground. For even shorter spacings, the depth to which the C1 electrode is driven should never exceed 30% or so of the distance between C1 and P1. There is no need to modify your C2 electrode set-up: it can remain the same for all electrode spacings.

The P1 and P2 electrodes are installed between the C1 and C2 electrodes, such that all electrodes are in a straight line. Connect the P1 electrode to the “M” terminal of the SYSCAL and the P2 electrode to the “N” terminal. For the first measurement of each traverse, place P1 and P2 such that the distances between adjacent electrodes (i.e., C1-P1, P1-P2, P2-C2) are all equal. This is the standard Wenner arrangement. After this first test, however, only the two potential electrodes are to be moved and always towards the C1 electrode. The C1-P1 and P1-P2 spacings are always equal: indeed, if it were not for the C2 electrode that remains immobile, the test would be a true Wenner test. From each pin spacing to the next, the C1-P1 and P1-P2 spacings are reduced by 1/3: in other words, multiply each pin spacing by 2/3 to obtain the next smaller pin spacing. The minimum required pin spacing is 0.3 m.

The P1 and P2 rods (just one rod or spike each) should be driven only a few inches into the ground, such as to achieve reasonable contact resistance: if the electrode presents resistance when pulled out of the earth, then it is certainly deep enough. For small electrode spacings, the depth to which the P1 and P2 electrodes are driven should not exceed 10% of the electrode spacing.

To estimate apparent resistivity, use double the spacing between C1 and P1, instead of simply the spacing between C1 and P1 as you would for the regular Wenner method: i.e., apparent resistivity is approximately equal to 4 π a R, where a is the C1-P1 spacing and R is the apparent resistance. If you are using the SYSCAL to compute apparent resistivities for you, always enter double the spacing that you are testing. This will be valid for most pin spacings and is certainly satisfactory for the purpose of checking the data for erratic behaviour.

All the data indicated on the data sheets are to be recorded for each traverse, from the minimum pin spacing indicated on the form, up to the maximum pin spacing associated with the traverse (as indicated on the list of measurement traverses required for this project).

At each pin spacing, measurements are to be made at two significantly different injection current levels, if possible, which can be achieved by varying the applied source voltage: a factor of two difference (on that order) between the two injection currents is to be obtained. The measured resistivity should be the same for both current levels: if not, start troubleshooting!

On the attached form:

  • “Source Voltage” is the voltage applied to the current injection pins (12 V, 50 V, 100 V, 200 V, 400 V or 600 V) and is set by a knob on some instruments; on others, this is automatically set by the instrument.
  • “Q:***?” indicates whether 3 asterisks appeared as a value for Q, while the meter is injecting current into the C1 and C2 pins. Either “yes” or “no” should be entered in this column.
  • “Q%” is the standard deviation value, Q, reported by the meter.
  • “Vsignal” is the voltage measured by the soil resistivity meter between the potential pins (P1 and P2).
  • “Iinject” is the current injected by the soil resistivity meter into the current pins (C1 and C2).
  • “Apparent Resistivity” is the apparent resistivity of the soil, as computed by the meter or by hand.

The measured apparent resistivities are to be plotted versus pin spacing on log-log graph paper, as the measurements are taken. A fairly smooth curve should result. Wire connections and rod-to-soil contact should be verified if abrupt changes are observed; the presence of long buried metallic structures can also be responsible for such variations. As indicated above, if the low current test values do not match the high current test values, then a problem exists and the source of the problem should be investigated. Similarly, if Q% is greater than 0, connection and rod-to-soil contact should be checked.

In addition, for each measurement traverse:

  • Attach a sketch showing the traverse location & starting point in relation to existing nearby structures, including approximate distances from them; also show traverse on site plan.
  • Please report any signs of pipes, pipelines, conduits, long sections of reinforced concrete, fences, or any other long, metal-bearing structures anywhere near the traverse.

If an instrument is used which does not filter out voltages induced in the potential measurement leads by the current injection leads (this should be assumed if there is no indication to the contrary), then the following precautions should be taken:

  1. Separate the current injection test leads from the potential measurement leads by a fixed distance (e.g., 10 feet).
  2. For every pin spacing, take the measurements using another set potential leads that are significantly further away from the current leads and separated from them by a fixed distance (e.g., 100 ft).
  3. SES can compare these two sets of data to estimate induced voltages and correct the data for induced voltages.

Interpretation

The apparent soil resistivities measured at each site can be plotted along with the curve corresponding to the equivalent soil computed by the RESAP module of the CDEGS software package. Each graph also shows the equivalent soil structure corresponding to this data and indicates the location on the pipeline to which it corresponds. Good agreement between the two orthogonal traverses and the computed curve fits usually can be achieved, so that a multi-layer soil model is obtained and used for the ac interference analysis.


1. The Wenner and Schlumberger 4-pin methods differ only in the spacing of the two inner potential electrodes. Details on the recommended method will follow.

2. Note that only the area in the vicinity of the current injection electrodes should be wetted in this way. This will not influence the measurements significantly, provided that the wetted area is small compared to the inter-electrode spacing.

 

 
 

 

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