self potential method_sudeshna

8/12/2019 Self Potential Method_Sudeshna

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SELF POTENTI LEARTHS NATURAL ELECTRICAL CONDUCTIVITY

SUDESHNA BISOYI

4/5/2013Department of Geology

LAURENTIAN UNIVERSITY


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TABLE OF CONTENTS

FIGURE 1: Current flow and natural self potential filed developed around a Sulphide ore body

FIGURE 2: The membrane or shale potential

FIGURE 3: Schematic of flow induced negative streaming potential (Erchul and Slifer, 1989)

FIGURE 4: Measurement of electrical current spontaneously generated by a Sulphide ore body

TOPIC Page No

Title page

1 Introduction 2

2 Self Potential and how it is produced 3

3 Physical properties and measured parameters 4

4 Field deployment 5

5 Measurement Tools of SP Method 6

6 Data acquisition 9

7 Data interpretation 11

8 Applications of SP method 11

9 Summary 13

10 References 15


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SELF POTENTIAL / SPONTANEOUS POTENTIAL METHOD

ABSTRACT

All geophysica l techniques are based on the detection of contrasts in different physical properties

of materials. If contrasts do not exist, geophysical methods will not work. Most Self Potential

surveys use a qualitative evaluation of the profile amplitudes or grid contours to evaluate self-

and streaming-potential anomalies. Contrasts in the magnitude of the naturally existing(ambient

or passive) electric current within the earth can be detected by self-potential (SP) surveys.

Spontaneous electrical potentials occur in nature where e.g. electrolytes with different

concentrat ions are in galvanic contact with each other. The electric potential due to capillary

liquid flow through the ground is called streaming potential. A change in streaming potential can

be due to changes in porosity, saturation, salinity, pH or permeab ility. SP method is very

effective in sulphide ore bodies. Sulfide ore bodies oxidize when exposed to oxygen (e.g., when

the water table drops and exposes previously saturated rocks to air). The resulting chemical

reaction (sulfide to sulfate) makes the ore body act much like a large battery. Mapping SP has

outlined some ore deposits. Self Potential is measured in milli volts (mV). Massive Sulphide Ore

bodies may produce SP anomalies of several hundred mVs. The main uses of SP logs are in the

detection of permeable beds, the determination of Rw, the indication of the shaliness of a

formation and the stratigraphic correlation of beds and metalliferous bodies. However Self

Potential method, like any geophysical method is only shows the anomalies which should be

supplemented in the field by field observations, mapping of rock types and borehole drilling

data.

Introduction:

Geophysics is the study of the subsurface structure of the earth by using quantitative physical

methods (Smith, 2013). Exploration geophysics is a branch of Geophysics which measures the

physical properties such as seismic, gravitational, magnetic, electrical and electromagnetic of

rocks, and in particular, to detect the measurable physical differences between rocks that contain

ore deposits or hydrocarbons and those without. It can be used to directly detect the target style


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of mineralization, via measuring its physical properties directly. For example one may measure

the density contrasts between iron ore and silicate wall rocks, or may measure the electrical

conductivity contrast between conductive sulfide minerals and barren silicate minerals. In the

20th century, geophysical methods were developed for remote exploration of the solid Earth and

the ocean, and geophysics played an essential role in the development of the theory of plate

tectonics.

Geophysical methods can be divided into two general types: Active, which measure the

subsurface response to electromagnetic, electrical, and seismic energy; and passive, which

measure the earths ambient magnetic, electrical and gravitational fields. Geophysical methods

can be further subdivided into either surface or borehole methods. Surface methods are usually

non- intrusive and are used to collect subsurface data. Whereas, borehole methods require that

wells or borings be drilled so that in situ conditions of the subsurface can be measured (Ariail,

1997). All geophysical techniques are based on the detection of contrasts in different physical

properties of materials. If contrasts do not exist, geophysical methods will not work. Electrical

methods measure the contrasts in electrical resistivity. Seismic methods depend on contrast

compression or shear wave velocities of different materials. Gravity method uses reflection and

refraction contrasts in the densities of different materials. Contrasts in magnetic susceptibilities

of materials permit magnetic surveying to be used in some investigations. Self potential method

is one of the Electrical geophysical methods which measure the contrasts in the magnitude of the

naturally existing electric current within the earth (Hoover et al., 1992). Self potential method

provides quickest field procedure in finding sulphide mineralization. This paper discusses

various aspects of the Self Potential method such as the physical properties, physical principle,

instrumentation, Data acquisition, Data interpretation, and applications.

What is Self Potential and how it is produced on the ground?

pontaneous potential (SP), also called self potential, is a naturally occurring electric

potential difference in the Earth. Spontaneous potentials can be produced by mineralization

differences, electro-chemical action, geothermal activity, and bioelectric generation of

vegetation. Four different electrical potentials are recognized. Electro-kinetic or streaming,

potential is due to the flow of a fluid with certain electrical propert ies passing through a pipe or

porous medium with different electrical properties (figure 1). Liquid-junction, or diffusion,

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potential is caused by the displacement of ionic solutions of dissimilar

concentrations. Mineralization or electrolytic contact potential is produced at the surface of a

conductor with another medium. The origin of SP across rock formations can be attributed to two

processes involving the movement of ions

Figure 1: Current flow and natural self potential field developed around a sulfide ore body( From Dobrin, 1976 ).


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Physical Properties involving Self Potential Methods:

he Self Potential method is heavily based on two properties of the Earth materials;

Chargeability and Polarizability. Chargeability is the ability of the Earth (Soil and Rock) to

store charge and Polarizability is the flow of charge due to separation of charges. It occurs whencurrents are due to movement of ions and electrons. Self Potential or Spontaneous Potential

method is a passive geophysical method which measures a field- the naturally occurring potential

or current in the field. It measures electrical conductivity in the ground. It is called passive

because no external force was induced on the earth for measuring the contrast.

Measured Parameter:

SP method measures Electrical Potentials. Spontaneous potentials (SP) are usually caused by

charge separation in clay or other minerals, due to presence of semi-permeable interfaceimpeding the diffusion of ions through the pore space of rocks, or by natural flow of a

conducting fluid through the rocks.

Measuring Units:

Self Potential is measured in milli volts (mV). Massive Sulphide Ore bodies may produce SP

anomalies of several hundred mVs.

Suitable conditions for deploying SP method:

SP is a passive and naturally occurring feeble currents produced in ground due to ionic

exchanges or movement of electrons. Ore acts as a passive conductor, focusing currents

associated with Oxidation-Reduction reactions at the water table. Therefore ore bodies must cut

across or straddle the water table to produce SP for measurement. SP is strong in porous rock, oil

and clay contents, metallic deposits or objects and in non-magnetic deposits. The suitable

deposits are VMS, MVT Pb-Zn, Porphyry Copper and Magmatic Ni-Cu-PGE deposits.

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The membrane potential exists at the junction between the non-invaded zone and the shale (or

other impermeable rock) sandwiching the permeable bed. These beds are usually shale, and the

argument that follows applies mainly to shales, but is also valid to a less extent for other low

permeability rocks. Shales have the property that they can preferentially retard the passage of

anions. This is called anionic perm selectivity or electronegative perm selectivity and is a

property of membranes. It is due to an electr ical double layer that exists at the rock-fluid

interface, and that has the ability to exclude anions from the smaller pores in the rock

(sometimes called anion exclusion). The strength of this effect depends upon the shale

mineralogy, the fluid concentration and the fluid pH. Most other rocks exhibit the same

behaviour but to a lower degree for geologically feasible fluid concentrat ions and pH but

cationic perm-selectivity is possible, if rare. Most subsurface shales are such efficient anionic

perm selecting membranes that they repel almost all anions (say, chloride ions). This results in

the shale being more positive than the non-invaded zone, and hence there is an electrical

membrane potential, which causes current to flow from the invaded zone into the shale (and

hence borehole).

SP Measurement Tools:

P measurement requires very simple instruments which can be carried to the field for

recording measurements. The instruments are:

1. A pair of non-polarizing electrodes.

2. A high impedance Voltmeter

3. Some Cables

The Electrodes:

Electrodes in contact with the ground surface should be the non polarizing type, also called

porous pots (Figure 2). Porous pots are metal electrodes suspended in a supersaturated solutionof their own salts (such as a copper electrode suspended in copper sulfate) within a porous

container. These pots produce very low electrolytic contact potential, such that the background

voltage is as small as possible. Tinker and Razor manufacture models of porcelain no polarizing

electrodes that are reliable and sealed to avoid evaporation of the salt solution. Sealed pots can

keep their supersaturated solutions for more than a week, even in arid locales. Refilling the pot

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with solution must occur before a day's work due to the possible contact potential change while

performing a measurement set. A useful procedure is to mix remaining fluids from pots in a

single container, add new solution to the mixture in the pot, and use the mixed solution to fill the

pots. Then all pots contain the same solution mix.

Millivolts Meter:

An inexpensive, high-input-impedance voltmeter is used to read the potential in the millivolts

range. Actual field voltage will be in error when the source potential is within an order of

magnitude of the input impedance of the meter. The meter uses a bias current to measure the

desired potential. The input impedance should exceed 50 M. Higher input impedances are

desirable due to the impedance reduction o f air's moisture. The resolution o f the meter should be

0.1 or 1.0 mV. Several useful options on meters are available. Digital voltmeters are more easily

read. Water-resistant or sealed meters are extremely beneficial in field use. Notch filters about

60 Hz will reduce stray alternating current (AC) potentials in industrial areas or near power

lines.

Survey Wire (Cables):

The wire used in SP surveys must be strong, hardy, and of low resistance. Wire needs to have

sufficient tensile strength to be able to withstand long-term pulls of survey work for multiple

sites. For some field use, heavy twine or light rope may need to be twisted and knotted to long

lengths of wire to add strength. Survey wire must have abrasion-resistant insulator

wrapping. Pulling the wire over roadway surfaces can expose bare wire. Usually random bare

wire positions will not fully ground to the soil, and the effects will be variable as differing

lengths of wire are unreeled and occupy differing positions for the survey. This error will only

modify the signal by a few to tens of millivolts (mV). Twisted two-conductor, 18-gauge, and

multistrand (not solid conductor) copper wire has been found to be strong and abras ion resistant.

A small 1.5 V batte ry is also included commonly to ensure that the overall signal is measured on

the correct scale. The simplicity of the log means that it is extremely cheap, and therefore gives

tremendous value for money.


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Data Acquisition:

elf-potential field surveys are conducted by measuring electrical potential differences

between pairs of electrodes that contact the surface of the earth (or water, in water-coveredareas) at a number of survey stations in the area of interest. These stations may be along profiles

or spaced so as to obtain a real coverage. One station is selected as a base station and all

potentials are referenced to that point. The base station should be located at a point removed

from expected anomalous activity. Potential (voltage) measurements are made by contacting the

earth with non-polarizing electrodes. These electrodes, often called "porous pots," are designed

so as not to create any spurious chemical potential upon contact with the ground. Measurements

are made by connecting a high impedance voltmeter between two electrodes, usually the base

station and a roving electrode.

Only relative changes in potential are measured because the absolute value of the SP is

meaningless. Changes of the order of 50 mV are typical. For the log to be good, a good earth is

necessary, in which often a metal spike is driven 1 meter into the ground. Spontaneous potential

can be measured by placing one probe of a voltmeter at the Earth's surface (called surface

electrode) and the other probe in the borehole (called downhole electrode), where the SP is to be

measured. In fact, logging tools employ exactly this method. Since this measurement is relatively

simple, usually SP downhole electrode is built into other logging tools.

Since spontaneous potential is a measure of electrochemical potential and the ionic activity of a

solution is inversely proportional to its resistivity.

The SP tool is one of the simplest tools and is generally run as standard when logging a hole,

along with the gamma ray. SP data can be used to find:

Where the permeable formations are

The boundaries of these formations

Correlation of formations when compared with data from other analogue wells

Values for the formation-water resistivity

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Figure 3: Schematic of flow-induced negative streaming potentials

(Erchul and Slifer, 1989)

Figure 4: Measurement of electrical current spontaneously generated by a Sulphide ore body

(Fournier, 1989; Birch, 1993)


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Data Interpretation:

ost SP investigations use a qualitative evaluation of the profile amplitudes or grid

contours to evaluate self- and streaming-potential anomalies. Flow sources producepotentials in the d irection of flow. Fluid inflow produces negative relative potentials, as would

greater distance from the flow tube; outflow of the fluid results in positive

potentials. Quantitative interpretat ions for a dam embankment with possible under seepage

would be determined from the profiles across the crest. Negative anomalies may be indicative of

flow from the reservoir at some depth. The width of the half-amplitude provides a depth

estimate. Outflow at the toe of an embankment or at shallow depths beneath the toe would

produce positive, narrow anomalies. Mineral or cultural utilities produce varying surface

potentials depending on the source. Semi quantitative, forward solutions may be estimated by

equations or programs (Corwin, 1989; Wilt and Butler, 1990) for sphere, line, and plate potential

configurations. These solutions of potential configurations aid in evaluation of the corrected

field readings, but are solutions of the data set taken.

Applications of Self Potential Method:

SP can be measured on the ground, near surface or in borehole intersections. It is highly effective

in Magmatic Ni-Cu- PGE deposits and moderately effective in Volcanogenic Massive Sulphides

(VMS), Mississippi Valley Type PB-Zn deposits, Porphyry Copper-Gold deposits. SP method is

also used in oil and gas exploration but only in low noise land environment; it may not be

effective in Seas or Oceans because SP is very feeble and immeasurable there.

Mineral Recognition

Though not as good as some other logs, the SP log does react unusually to a few minerals

and formations, and is therefore sometimes useful in mineral recognition. The most common

occurrences are as follows, but are not reliable:

Coals Large negative kick (or none at all)

Pyrite Very large negative kick

Rhyolite Large negative kick

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Black shale Positive kick.

SP methods can be very useful for karst groundwater regimes in quick surveys of a site or in

long-term surveys during a rainy season. Sinkholes can be pathways of surface water flow. The

subsurface flow in karst can be erratic. There can be a qualitat ive evaluation of the flow volume

in different subsurface routes if the ground surface may be assumed parallel to the surface

through the irregular flow paths.

Applications in Boreholes:

The most useful SP component is the electrochemical potential, since it can cause a significant

deflection opposite permeable beds. The magnitude of the deflection depends mainly on the

salinity contrast between borehole and formation fluid, and the clay content of the permeable

bed. The SP log is therefore useful in detecting permeable beds and to estimate formation water

salinity and formation clay content. Due to the nature of the electric current, SP can only be

recorded in conductive mud.

Other applications on the surface

Electrodes can be placed on the ground surface to map relative changes in the SP value

(in millivolts, or mV), typically with the goal of identifying the path of groundwater flow in the

subsurface, or seepage from an earthen dam. A voltmeter measures the voltage between a fixed

liquid-junction electrode and a mobile one (rover), which is moved along a dam face or over an

area of investigation to collect multiple readings. Anomalies observed may indicate groundwater

movement or seepage.

Uses of the Spontaneous Potential Log. The main uses of this log are:

The detection of permeable beds.

The determination of Rw.

The indication of the shaliness of a formation. Correlation.
http://en.wikipedia.org/wiki/Electric_currenthttp://en.wikipedia.org/wiki/Electric_currenthttp://en.wikipedia.org/wiki/Electric_current


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The detection of Permeable Beds

The SP log is an extremely useful quick- look indicator of bed permeability. It is not

quantitative, and opinions differ to the extent to which one can associate the s ize of the

deflection with the degree of permeability. Given the large number of other parameters that

might affect the SP log, I prefer to say that one should not associate very large permeability

necessarily with large deflections and vice versa. However, the SP log is quite sensitive, and

even a small deflection in the SP log indicates that the bed has reasonable permeability. It

should be noted that some permeable beds might give no deflection, such as those where there

is no difference in salinity between the formation fluids and the mud fi ltra te .

Correlation and Facies

The SP log is sometimes a useful additional log to use in correlation, but is rarely used alone. If

used, the wells should be close together and drilled with the same mud, and the salinities in the

format ions should be constant between wells.

The SP log can be used to follow facies changes. However, it has been largely replaced by the

GR log, which has a higher resolution and is more reliable.

Summary

The self-potential method consists in the passive measurement of the distribution of the electrical

potential at the ground surface of the Earth and in boreholes. The purpose of this method is to

map the electrical potential to reveal one or several polarization mechanisms at play in the

ground. In some cases, the self-potential signals are monitored with a network of non-polarisable

electrodes, which provides both a better signal-to-noise ratio and the possibility to discriminate

between various sources. The two main contributions to the self-potential signals are (1) the

streaming potential or hydroelectric coupling (Fournier, 1989; Birch, 1993, 1998; Aubert andYn Atangana, 1996; Revil and Leroy, 2001) and (2) electro-chemical processes (membrane or

diffusion potentials) associated with gradients of the chemical potentials of ionic species in the

pore water (e.g., Sen, 1991; Naudet et al., 2003, 2004; Revil and Leroy, 2005).


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SP measurement requires very simple instruments which can be carried to the field by a single

person for recording measurements. The instruments are a pair of non-polarizing electrodes, a

high impedance Voltmeter and some Cables. SP method is good for low potential type land

environment, so not very effective in Oceans (Oil/Gas exploration).A shortcoming of the SP

method is the frequency and variety of spurious responses obtained. A more popular application

of the electrical method is where controlled electrical energy is applied to the earth and the

resulting electrical behavior of the ground is observed at closely spaced stations at regular

intervals over the surface. The SP method often produces misleading results and use of the

method has declined recently. Self potential method like any other geophysical method is not the

ultimate source of locating mineral bodies. This method is an associated tool to be used with

field mapping, sampling and bore hole drilling.

References:

1. http://www.agoenvironmental.com/Burr_Article.pdf

2. http://www.epa.gov/esd/cmb/GeophysicsWebsite/pages/reference/methods/Surface_Geop

hysical_Methods/Electrical_Methods/Self-Potential_(SP)_Method.htm

3. http://www.1800geophysics.com/GeoMethods/M10.html

4. http://www.eos.ubc.ca/~msheffer/MRS_MASc.pdf

5. Wightman, W. E; Jalinoo; Sirles and Hanna, K (2003). "Application of

Geophysical Methods to Highway Related Problems."Federal highway

administration of Central Federal Lands Highway Division, Lakewood, CO.

Publicat ion No. FHWA-IF-04-021, September 2003.

6. http://www.cflhd.gov/resources/agm
http://www.agoenvironmental.com/Burr_Article.pdfhttp://www.epa.gov/esd/cmb/GeophysicsWebsite/pages/reference/methods/Surface_Geophysical_Methods/Electrical_Methods/Self-Potential_(SP)_Method.htmhttp://www.epa.gov/esd/cmb/GeophysicsWebsite/pages/reference/methods/Surface_Geophysical_Methods/Electrical_Methods/Self-Potential_(SP)_Method.htmhttp://www.1800geophysics.com/GeoMethods/M10.htmlhttp://www.eos.ubc.ca/~msheffer/MRS_MASc.pdfhttp://www.cflhd.gov/resources/agm/http://www.cflhd.gov/resources/agm/http://www.eos.ubc.ca/~msheffer/MRS_MASc.pdfhttp://www.1800geophysics.com/GeoMethods/M10.htmlhttp://www.epa.gov/esd/cmb/GeophysicsWebsite/pages/reference/methods/Surface_Geophysical_Methods/Electrical_Methods/Self-Potential_(SP)_Method.htmhttp://www.epa.gov/esd/cmb/GeophysicsWebsite/pages/reference/methods/Surface_Geophysical_Methods/Electrical_Methods/Self-Potential_(SP)_Method.htmhttp://www.agoenvironmental.com/Burr_Article.pdf

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