Webinar-ERI-complete.pdf

Geophysics Webinar Series
Webinar-1: Electrical Resistivity Imaging
AF Academy Dr. Sanjay Rana

Geophysical Webinar Series
Schedule:
• Electrical Resistivity Imaging/ Tomography: Best Practices
June 19, 2021 15:00 IST / 09:30 UTC
• Geophysical Methods for Dam Investigations & Health Checks
June 26, 2021 15:00 IST / 09:30 UTC
• Multi-Channel Analysis of Surface Waves: Best Practices
July 03, 2021 15:00 IST / 09:30 UTC
• Ground Penetrating Radar: Best Practices
July 10, 2021 15:00 IST / 09:30 UTC
• Subsurface Utility Engineering- Detection & Mapping of Underground Utilities
July 17, 2021 15:00 IST / 09:30 UTC
• Seismic Refraction & Seismic Refraction Tomography: Best Practices
July 24, 2021 15:00 IST / 09:30 UTC

Session Guidelines:
• Questions towards the end of session
in chat box
• Preferred to receive questions by
email, enabling development of
comprehensive Q&A
• Questions related to this session or
previous sessions only
• Please allow 24 hours for reply to your
emails
Geophysical Webinar Series

AFA provides training & knowledge sharing
platform to decision & policy makers, working
professionals, operating level personnel and
aspiring students willing to specialise in technical
sector through mediums of trainings & courses.
Visit https://www.afacademy.org/ for details of
all offered online courses
About AF Academy

Online Courses (https://www.afacademy.org/ )
Latest Course Added
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Crosshole Seismic Tomography

• Professional Geophysicist, with 31 years of work experience.
Chairman AF Academy & Managing Director, PARSAN, An
engineering geophysics company
• Gold Medalist- 1990, University of Roorkee (Now IIT-Roorkee)
• Pioneered use of Near Surface Geophysics (Private Sector) in India
in 1995.
• Completed geophysical investigations of >2000 projects.
• Member of various working committees for development of Code
of Practices and Standards, including IRC.
• Principal author of
• Guidelines on Geophysical Investigation of Dams
• Guidelines for Geophysical Investigation of Tunnels (TAI)
• Guideline for Geophysical Investigations for Bridges (IRC)
• Indian Code for Subsurface Utility Mapping.
About Speaker (Dr. Sanjay Rana)

Geophysical
Investigations……

Geophysics as a Science
GEOLOGY
PHYSICS
GEOPHYSICS
MATHS-
COMPUTER
SCIENCE

Why Use Geophysics…….
• Low Cost
• Rapid Coverage
• Continuous information
• Optimization of dill holes
• Minimization of ‘Surprises’
• Early stage application…Better
planning, smooth execution.

Choice of method- An Important Step
• The user has considerable scope for choice.
However, some factors to be considered are:
• What type and shape of feature is being imaged?
• What physical properties will show the best
contrast?
• Are there any strong but irrelevant contrasts that
will mask the results?
• To what depth must the survey penetrate?
• What spatial resolution is needed?
• What are the time or cost constraints?
• Are there any special restrictions e.g. on access
or damage?

• Geophysical methods are subdivided into active or passive
methods, depending on whether or not the instrument
puts energy into the ground.
• Active methods:
• Seismic
• Electrical resistivity
• Ground penetrating radar
• Passive methods
• Gravity
• Magnetics
Active & Passive Methods

• Geophysical methods are further subdivided into
contacting or non-contacting methods, depending on
whether or not the source or receiver is actually in contact
with the ground.
• Contacting methods:
• Seismic
• Electrical resistivity
• Non-contacting methods
• Gravity
• Magnetics
• Ground penetrating radar
• EM conductance methods
Contacting and non-contacting methods

Advantages & Limitations
• Advantages of geophysics
• Rapid and cheap survey tool
• Easily integrated with other forms of ground survey
• Non-destructive (archaeology, Dams, urban areas
generally)
• Modern processing methods give a visual image of
the subsurface
• Limitations of geophysics
• Can be ambiguous without controls
• Poor discrimination in some cases
• Can suffer from noise or artefacts

Different Method for Different Property
• Some methods that are commonly used are:
• Seismic methods = Elastic wave velocity
• Ground Penetrating Radar (GPR) = EM pulse velocity
• DC resistivity methods = Electrical DC resistance
• EM conductivity methods = Electrical AC conductivity
• Magnetic methods = Magnetic field strength
• Gravity methods = Gravity field strength
• The value of the surface measurement is
determined by the contrast in the relevant
property (hence material type) and by the three
dimensional structure.

Setup: Establish the geoscience objectives, consider conventional practice,
and identify how geophysics might contribute.
Contrasts: Characterize materials that can be expected and establish the likely
physical property contrasts.
Survey Design: Determine a suitable geophysical survey, and design an effective and
efficient field survey. Identify possible sources of error, noise and mis-
interpretation.
Data Collection: Carry out the field survey, taking all necessary actions to ensure
complete, high quality, and cost effective data sets.
Processing: Plot the data, and apply appropriate processing and analysis.
Interpretation: Interpret results in terms of physical property distribution, and then in
terms of the original geoscience objectives.
Synthesis: Combine interpretations with prior knowledge about the problem, and
with other relevant information. Decide if your results are adequate for
the particular problem. Decide further investigations, if needed.
Seven-step framework of Geophysics

Geophysics- Huge ROI...
• Detailed investigation of site…Saving
huge costs towards changed plans,
project delays when surprises crop
up….
• No drilling, No digging…Vast
information at fraction of cost of
traditional methods.
• Early stage application…Better
planning, smooth execution.

Electrical Resistivity Imaging

Resistance & Resistivity…
-m(ohm- meters)
(ohm)

Soils and rocks are composed mostly of silicate minerals, which are essentially insulators. Exceptions
include magnetite, specular hematite, carbon, graphite, pyrite, and pyrrhotite. Therefore conduction
is largely electrolytic, so conductivity depends mainly upon:
• porosity;
• hydraulic permeability, which describes how pores are interconnected;
• moisture content;
• concentration of dissolved electrolytes;
• temperature and phase of pore fluid;
• amount and composition of colloids (clay content).
Resistivity/ Conductivity of Soils/ Rocks…

Current Path % of Total Current
1 17
2 32
3 43
4 49
5 51
6 57
Two electrode current sources…
Almost 50% of the
current placed into the
ground flows through
rock at depths shallower
than or equal to the
current electrode
spacing.

Current Flow in Subsurface…
Galvanic currents will flow towards regions of high conductivity and
away from regions of high resistivity.

Equipment…
1. Resistivity Meter
2. Energy Source
3. Electrodes
4. Connecting Cables

To measure resistance in the ground,
four electrodes are required
– 2 current electrodes
(inserting current)
– 2 potential electrodes
(measuring voltage)
R = V / I
300 Ω
C1 = A P1 = M P2 = N C2 = B
Measurement Procedure…

Resistivity Survey Types…
Soundings: a fixed geometry of electrodes is expanded symmetrically about a central point of the array. The data
provide information about how the electrical structure varies with depth. The data curve is often called a “sounding”
and a single sounding can be inverted to produce a 1D conductivity model.
Profiling: a fixed array is moved along a line. The data provide information about lateral variations to a depth that is
determined by the length of the array.
2D Electrical Resistivity Imaging: uses an array of electrodes connected by multicore cable to provide a linear depth
profile, or pseudo-section, of the variation in resistivity both along the survey line and with depth. Switching of the
current and potential electrode pairs is done automatically using a laptop computer and relay box.

+
+
+
+
+
+
M N
A
A
A
A
A
A B B B B B B
Basic Theory…VES

+
+
+
+
+
+
A
A
A
A
A B B B
VES Survey
Resistivity 1
A M N B
+
+
+
+
+
+
+
APPARENT RESISTIVTY
SYNTHETIC
MODEL 1
SYNTHETIC
MODEL 2
SYNTHETIC
MODEL 3
SYNTHETIC
MODEL 4
REAL WORLD
“RESIDUAL”
% Value
Lower = Better
Basic Theory…VES

A M N B
+
A M N B
+
A M N B
+
A M N B
+
A M N B
+
A M N B
+
A M N B
+
A M N B
+
A M N B
+
A M N B
+
A M N B
+
A M N B
+
A M N B
+
A M N B
+
+
A M N B
+
a=1 + +
+ +
+ +
+ + +
+ +
+ +
+
A M N B
+
A M N B
+
A M N B
+
A M N B
+
A M N B
+
A M N B
+
A M N B
+
A M N B
+
A M N B
+
A M N B
+
A M N B
+
A M N B
+
a=2 + ++ +++ +++++ +
A M N B
+
A M N B
+
A M N B
+
A M N B
+
A M N B
+
A M N B
+
A M N B
+
A M N B
+
A M N B
+
a=3 + +++ ++++ +
Basic Theory…Electrical Resistivity Imaging

Electrical Resistivity Imaging- Field Work

2D Resistivity Section of Subsurface...

Data Acquisition Parameters...
• Length of Spread (controls depth of
investigation)
• Electrode Spacing (controls
resolution)
• Survey Configuration (Wenner,
Schlumberger, Dipole-Dipole etc.)

Data Acquisition Parameters- Length...
• Ideally 5-6 times of depth information required
• Information tapered towards end
• Roll along required to cover constant depth

Data Acquisition Parameters- Electrode Spacing...
• To be chosen based on spread length required (depth of investigation)
• Smaller electrode spacing provides higher resolution
• Resolution also depends on chosen array type

Data Acquisition Parameters- Survey Configurations...

Data Acquisition- Wenner Array...
• The Wenner array is relatively sensitive to vertical changes in the subsurface resistivity below the
center of the array. However, it is less sensitive to horizontal changes in the subsurface resistivity.
• In general, the Wenner is good in resolving vertical changes (i.e. horizontal structures), but
relatively poor in detecting horizontal changes (i.e. narrow vertical structures).
• Among the common arrays, the Wenner array has the strongest signal strength.
• For Wenner array, the depth of investigation is approximately 0.5 times the “a” spacing used.

Data Acquisition- Wenner-Schlumberger array...
• This array is moderately sensitive to both horizontal and vertical structures.
• In areas where both types of geological structures are expected, this array might be a good
compromise between the Wenner and the dipole-dipole array.
• The median depth of investigation for this array is about 10% larger than that for the Wenner array
for the same distance between the outer (C1 and C2) electrodes.
• The signal strength for this array is smaller than that for the Wenner array, but it is higher than the
dipole-dipole array.

Data Acquisition- Dipole-Dipole array...
• The dipole-dipole array is very sensitive to horizontal changes in resistivity, but relatively
insensitive to vertical changes in the resistivity.
• It is good in mapping vertical structures, such as dykes and cavities, but relatively poor in mapping
horizontal structures such as sills or sedimentary layers.
• One possible disadvantage of this array is the very small signal strength for large values of the “n”
factor. Difficult to collect data in poor ground contract cases.

Data Acquisition- Making the Choice of Array...
• If your survey is in a noisy area and you need good vertical resolution and you have limited survey
time, use the Wenner array.
• If good horizontal resolution and data coverage is important, and your resistivity meter is
sufficiently sensitive and there is good ground contact, use the dipole-dipole array.
• If you are not sure, or you need both reasonably good horizontal and vertical resolution, use the
Wenner-Schlumberger array with overlapping data levels.
Practical Note: It’s always recommended to record data at least with Wenner & Wenner-
Schlumberger Arrays for most projects, if time permits. Dipole-Dipole, in most cases, suffers from
poor data due to poor grounding of electrodes, and should be used in favorable areas in addition to
above.

Data Acquisition- Equipment Settings...

Data Acquisition- Wenner Video...

Data Acquisition- Wenner-Schlumberger Video...

Data Acquisition- Dipole-Dipole Video...

Electrical Resistivity Section from dam crest of a masonry dam Showing Zone of Saturation (Green & Blue)
Electrical Resistivity Imaging Results...

Electrical Resistivity Imaging Results...

Electrical Resistivity Imaging Results- 3D Volume...

Best Practices- Survey Planning...
• Define your target- Resistivity Contrast is a MUST
• Size of your target- Theoretical Resolution is ½ the electrode spacing laterally and ¼ the electrode
spacing vertically. Resolution decreases with depth.
• Depth of investigation
• 1D, 2D, 3D or 4D

Best Practices- Survey Planning...
• Field Environment
- Natural Ground
- Paved Surface
- Forest with loose top soil
- Access
• Time Constraints (multichannel options)
• Signal to noise ratio (nested array v/s distant array)
• Not sure of survey plan? Model it (RES2DMOD)

Best Practices- Electrode Placement...
Credit- GuidelineGeo

Best Practices- Power Supply...
Credit- GuidelineGeo
Effects: Unstable Current, Reduced Power, Transmitter Shutdown
Solutions: Suitable Battery, Good charging regime, In-field Charging Solution, discard old batteries, sensible
transmitter settings

Best Practices…
• Keep connectors clean and dry. Keep a maintenance kit.
• Keep electrode contact improvement kit: Water
containers, salt, gel, clay, bentonite, hammers, shovels,
vegetation cutters, etc.
• Leave connectors to dry after working in wet area
• NEVER use the system during an electrical storm.
• Try reciprocal measurements
• Try crosslines
• Observe readings while in field

Best Practices-
Never Interpret Pseudosection….
One useful practical application of the
pseudosection plot is for picking out bad
apparent resistivity data points, which
have unusually high or low values.
Observe difference in pseudosection
generated from different arrays.

3 Month’s Advanced Certification Course:
• Survey Planning
• Modelling (Live on RES2DMOD)
• Survey Parameters
• Data Acquisition
• Data Processing (Live on RES2DINV)
• Data Interpretation

3 Month’s Advanced Certification Course: Approach
• LMS based course delivery. Live sessions (day and
time of sessions will be suitably chosen so as not
to conflict with your working hours).
• Training Videos & Training Modules (Reading
material) on LMS
• Membership of a member’s only Facebook group
for interaction within the group members on
various aspects of ERI.
• Email/ online support
• Certificate from AF Academy on completion of
course

Discounted Course Pricing (Valid till 8 PM IST today)
• Investment (LMS Based): INR 3,000/ USD 60 + GST (50% discounted)
• Investment (Live Only): INR 1,000/ USD 20 + GST
• Additional Bonus: Any one of the following course for FREE
• Subsurface Utility Engineering- Self Study
• Ground Penetrating Radar- Short Course
• Certificate: INR 300/ USD 6 + GST
For any payment gateway linked assistance, please contact:
Mr. Rashik Kathuria, Email: support@hummz.com
Regular Fee without discount
INR 6,000/ USD 120

June 19, 2021; 20:00 hrs IST
(for LMS based Course with videos)
https://hmz.io/AFAERT
1. Open the course page using the short URL
2. Click on enrol button
3. Add your details
4. Enter ERT21 in the "Discount Code” box The amount will update to give 50% discount.
5. Add the free course selection in the "Add a note to your order” box (SUE or GPR)
6. Make payment

June 19, 2021; 20:00 hrs IST
https://hmz.io/AFAERT
1. The discount after 8:00pm IST will be 20% for 24 hours - that is Jun 20th
8:00pm.
2. For participation certificate, payment link is https://hmz.io/ERTCERT

This course can be attended during the live sessions only.
Session recordings will not be available for viewing. Session
presentations and notes shall be available to download.
For Live Course without videos on LMS)
https://hmz.io/ERTLIVE

Contact Details:
• For any LMS, Facebook linked assistance,
course enrolment etc., please contact: Mr.
Rashik Kathuria, Email:
support@hummz.com
• For any requirement of geophysical
investigation, please contact Cdr Ashutosh
Kaushik, CEO, PARSAN Overseas Pvt Limited,
Email: akaushik@parsan.biz
• For any technical clarification, please contact
Dr. Sanjay Rana, Chairman AF Academy &
Managing Director, PARSAN, Email:
sanjay@parsan.biz

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