configuration and specification of equipments used in dc resistivity survey

5/27/2018 Configuration and Specification of Equipments Used in Dc resistivity survey

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CONFIGURATION ANDSPECIFICATION OF EQUIPMENTSUSED IN DC RESISTIVITY SURVEY

Aditya kumar anand

M.Sc (final ) Geology 4th semester

Department of geology

University of Delhi


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DC RESISTIVITY SURVEY D.C. resistivity (electrical resistivity) techniques measure earth

resistivity by driving a direct current (D.C.) signal into the

ground and measuring the resulting potentials (voltages)

created in the earth.

In geophysical and geotechnical literature, the terms "electrical

resistivity" and "D.C. resistivity" are used synonymously .

The terms "resistivity" or "electrical" are often used to refer to

the same methods or techniques, although "electrical" is

sometimes used to encompass a broader range of techniques

including the electromagnetic methods.


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Resistance Voltage Current Resistivity surveying investigates variations of electrical

resistance by causing an electrical current to flow through the

subsurface using wires (electrodes) connected to the ground.

Resistivity = 1 / Conductivity

To get current to flow you must provide a push

The push is called a potential difference or voltage(V)

The flow is called the current

Symbol: I (I = amperes / amps)


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RESISTANCE AND RESISTIVITY The amount of potential difference required to push a given current is

directly proportional to the Resistance

OHMS LAW: V = IR , R = V/I

Resistance, R Resistivity, (rho)

They are related but are fundamentally different things

Resistance depends on:

The material properties i.e. the resistivity, (so is a material property)

The shape of the material that has current flowing through it.

R= Resistance a = cross sectional area l = length

ThereforeResistance is higher when current is forced through a

Small area

Long length


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Apparent ResistivityIn a VES survey the ratio V/I is measured with increasing electrode spacing.

The ratio changes for two reasons

Layers of differing resistivity are encountered.

The electrodes are now farther apart

Current diverges at one electrode and converges at the other.

R is directly proportional to length and inversely proportional to cross

sectional area.

At depth 2d:The length of the path is doubled.The cross sectional length is doubled in both dimensions,

so area is 4x.

The measured resistance (V/I) will be as much.


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To account for the effects of changes in electrode spacing theapparent resistivity is found as

Here is a geometrical factor equal to a/l for a rod

The geometrical factor varies depending on array

configuration / type.

Apparent Resistivity


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CONFIGURATIONAND SPECIFICATIONOF EQUIPMENTS


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CAPACITIVELY COUPLED RESISTIVITY

The capacitively-coupled resistivity(CCR) experimentswere conducted using the multi-channel OhmMapper.

In the CCR method there is no need to plant electrodes

into the ground.

With CCR very rapid near surface surveys are possiblecompared to conventional D.C. resistivity survey .

In Disaster mitigation or for environmental survey it is

important to understand the geologic structure of the

near surface to a depth to around 10 m.

In such surveys rapid and cost-effective survey methods

are needed.


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In a CCR survey because it is not necessary to use

ground stakes to measure the resistivity of the

ground and for this reason very rapid

measurement is possible compared to the D.C.

galvanic-resistivity technique.

The CCR survey has the advantages that dataacquisition is possible in highly resistive areas.

Values of apparent resistivity greater than 10,000

ohm-m such as in permafrost may experiencesevere contact resistance problem with using a

conventional galvanic resistivity meter.


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METHOD The concept of the capacitively-coupled resistivity measurement is

shown in the Figure.

When voltage is applied to the conductor inside the CCRtransmitter an electric charge appears between the conductor andthe ground which are separated from one another by theinsulation.

The conductor and the ground act as two plates of a capacitorseparated by a strong dielectric resistor(the insulation).

This capacitance between the conductor and the ground acts as apath for an A.C. current to flow into the ground from the conductor.

According to the same principle it is possible with a CCR receiver todetect the A.C. voltages in the ground generated by the transmitter.

In this manner the resistivity of the ground can be acquired.


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OHM MAPPER Figure 2 shows the appearance of the five-receiver OhmMapper

TR5 and Figure 3 shows the schematic diagram of the OhmMapper.

The receivers are connected to each other by shared "dipole

cables" and the transmitter is connected to the receiver array by a

nonconductive rope.

The transmitter/receiver array is towed by a person or a vehicle.


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Two dipole cablesare connected to the transmitter and also twodipole cables are connected to each of receivers in the multi-receiver array.

The electrode configuration is equal to a dipole-dipolearray.

The depth of investigation can be controlled by changing the lengthof the dipole cables and the spacing between the transmitter andthe receivers.

The transmitter- receiver separation should not exceedone skin depth.

Skin depth is defined as the following:

= 503 SQRT (/f)

where = skin depth, = resistivity of the ground , f =transmisssion

frequency eg: f=8 kHz, =20ohm-m, =25m


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SPECIFICATION AND APPLICATIONS OF OHM MAPPER

Applications for Capacitively-Coupled Resistivity OHM MAPPER .

Monitoring dykes and levees for damage and leaks.

Shallow minerals exploration.

Shallow ground-water exploration.

Monitor environmental sites for leakage plumes.


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CASE STUDY OF CONVENTIONAL GALVANIC RESISTIVITYSURVEY AND OHM MAPPER SURVEY ON SAME GROUND The OhmMapper and a traditional resisitivity survey were conducted on

the same survey line for comparison.

The site is located in Tsukuba city in Ibaraki Pref., Japan. The comparisonline was 250 m long on cohesive soil. The surface of the survey line wasthe grass.

The OhmMapper measurement used 5 m dipole cables and the separationbetween the transmitter dipole and the receiver dipoles was from 5 m atminimum to 35 m at maximum.

The electrode array of the D.C. resistivity survey was pole-pole array. Theminimum electrode spacing was 1 m and the maximum was 15 m.

the results of the OhmMapper and the D.C. resistivity from thecomparison line. There is a resistive layer of more than about 140 ohm-m,and below this layer, is a less resistive layer of less than 60 ohm-m

Although there are differences at the surface, the OhmMapper resultroughly agrees with the D.C resistivity result. The differences in the verynear surface may be caused by the difference in the type of electrodearray.


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CORIM The corim system measures the electrical resistivity of the ground

at a few metres depth using capacitively coupled carpets pulled onthe surface behind a towing veichle .

The logging speed of few kilometres permit to carry out as many asreadings per day and makes the corim system an efficient tool forshallow structure investigation .

Computer controls the whole system . Images obtained for eachprofile are apparent resistivity pseudo section

These apparent resistivity condition point out the conductive and resistiveareas related to the presence of fractures , voids to lateral variation inlithology and clay content

The main applications are

Dike diagnosis

Soil detection

Cavity detection

Archaeology


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The CORIM system using alternating current which penetrates in to the

ground by capacitively coupling and the potential difference are

measured in the same way .

Electrodes are simply laid on the ground so the whole system can beeasily pulled along .

This enables a much higher acquistion speed than in standard dc

prospecting

The potential depends upon the resistivity of the ground . Farther the

receiving carpet from the transmitting carpet deeper will be theinvestigation.


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RESISTIVITY SURVEYING

Aim: Imaging the underground geological structures throughsurface electrical measurements

Principle: Transmitting a current I through two electrodes andmeasuring a voltage V with two other electrodes

Apparent resistivity: = K*V/I, K depending on the chosen

electrode array and the electrode separation Electrical sounding: Determining the depths and thickness of

layers through the variations of the electrical resistivity withdepth .

Electrical profiling: Delineating anomalous areas through the

lateral variations of the resistivity Applications: environmental studies, groundwater

investigation, civil engineering, archaeology...


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CLASSICAL RESISTIVITY

AND MULTI-ELECTRODE RESISTIVITY


2 (A, B) electrodes : current transmission

2 (M, N ) electrodes : potential measurement

MULTI-ELECTRODE RESISTIVITY

"n" electrodes (n = 48, 72, 96, )

successively "current" or "potential"

aim : to save time in the acquisition


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Principle of multi-electrode resistivity imaging

The evolution of electronic components and of computer processing have

permitted to develop field resistivity equipment (SYSCAL Switch and SYSCAL Pro

Switch units) which includes a large number of electrodes located along a line at

the same time.

This technique, called Resistivity Imaging or Electrical Resistivity Tomography (ERT),

finds applications in the environment, groundwater, civil engineering and

archaeology fields.

The multi-electrode resistivity technique consists in using a multi-core cable with

as many conductors (24, 48, 72, 96, ) as electrodes plugged into the ground at a

fixed spacing every 5m for instance (Figure 1)

The various combinations of transmitting (A,B) and receiving (M,N) pairs of

electrodes construct the mixed sounding / profiling section, with a maximuminvestigation depth which mainly depends on the total length of the cable.


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MULTI-ELECTRODE RESISTIVITY IMAGING


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SYSCALa. Syscal junior

b. Syscal junior switch 48

c. Syscal junior switch 72

d. Syscal kid

e. Syscal kid switch 24

f. Syscal pro

g. Syscal pro switch

h. Syscal pro deep marine

i. Syscal R1 plus

j. Syscal R1 plus switch 48

k. Syscal R1 plus switch 72

l. Syscal R2


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The SYSCAL resistivity meter is placed in the central part of the

sounding.

The metallic electrodes have to be plugged into the ground as

deeply as possible to decrease the ground resistance for both the

transmitting electrodes A, B, and the receiving electrodes M, N. A resistance of a few k ohms is convenient (10 to 20 k ohm max).

When possible, water can be poured on the electrodes or two

electrodes can be set in parallel at each point to decrease this

value.

The wires going from the SYSCAL to the A, B electrodes (up to

several hundreds volts) have to be placed as far as possible from

the wires going to the M, N electrodes (down to a few mV) to

prevent insulation troubles.


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SYSCAL KID Syscal kid is a very compact unit specially designed shallow electrical

survey. Easy to use , field proof and light weight .

Syscal kid is ideal for archaeological , geological and civil engineering applications .


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SYSCAL JUNIOR

Resistivity meter for environmental applications .

Computation of resistivity for most electrode arrays:Schlumberger, Wenner , Gradient, Dipole-Dipole, Pole-Dipole,

Pole-Pole.

Compact, easy to use

Measurement of

electrical resistivity

2 simultaneous

reception channels

Outputs:

400 V100W1.25A


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RESISTIVITY MEASUREMENT IN SYSCAL JUNIOR


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SYSCAL JUNIOR SWITCH 48 The SYSCAL JUNIOR Switch-48 is an all-in-one multi node

resistivity imaging system.

It features an internal switching board for 48 electrodes andan internal 200W power source.

The system is designed to automatically perform pre-defined

sets of resistivity measurements with roll-along capability. Four multi-core cables with 12 electrodes takeout each are

connected on the back of the resistivity meter. These heavy-duty cables are available with standard 5 or 10 m electrodespacings.

It is ideal for environmental and civil engineering applicationssuch as pollution monitoring and mapping, salinity control,depth-to-rock determination and weathered bedrockmapping. It can also be used for shallow groundwaterexploration (depth and thickness of aquifers)


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FIELD LAY OUT OF SYSCAL JUNIORSWITCH 48


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FIELD LAY OUT OF SYSCAL JUNIORSWITCH 72


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SYSCAL Pro Switch MAIN FEATURES

The SYSCAL Pro Switch is a versatile electrical resistivity meterwhich combines a transmitter, a receiver and a switching unit

in one single casing. It is supplied by a 12V battery.

The measurements are carried out automatically (output

voltage, stacking number, quality factor) after selection oflimit values by the operator, and are stored in the internal

memory.

The SYSCAL Pro Switch uses multi-core cables for controlling a

set of electrodes connected in a line or in several lines.

The ten channels of the system permit to carry out up to 10

readings at the same time for a high efficiency.


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SYSCAL PRO DEEP MARINE

SYSCAL Pro "deep marine":

This version has been specifically designed for marine survey in highconductive medium (like salt water) thanks to the high output currentcapability.

Marine survey with GPS:

A GPS/Sounder can be directly connected to the unit by a serial link for acontinuous recording of the location of the 10 channels and of the waterbottom all along the profile.

In that mode, using the 10 reception channels a set of 10 resistivities ismeasured and stored approximately every 2 seconds.

Graphite electrodes:

Specific cables with graphite electrodes can be supplied to fit to thatenvironment; this allows to get low resistance values and to avoidcorrosion due to water contact.


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SYSCAL PRO DEEP MARINE


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SYSCAL R1 PLUS Resistivity meter for medium depth exploration.

Compact, easy to use .

Measurement of electrical resistivity .

2 simultaneous reception channels.

Outputs : 600 V - 200 W - 2.5 A .

MAIN FEATURES

Power source, transmitter and receiver in a single unit

Display of noise level before measurement

Measurement and display of ground resistance, current, voltage,self potential and standard deviation .

Computation of the apparent resistivity for the various electrodearrays: Schlumberger & Wenner (sounding or profiling), Dipole-Dipole, Gradient.


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Syscal R2 plus

The SYSCAL R2 unit is a high-power system designed for DC

electrical surveys applied to groundwater exploration,

environmental studies, civil engineering, structural geology

investigation and mineral exploration.

Easy to use:

The SYSCAL R2 computes and displays the apparent resistivity

automatically for the most common electrode arrays

(Schlumberger and Wenner sounding and profilinggradient

dipole-dipole )


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THANK YOU

configuration and specification of equipments used in dc resistivity survey

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