Grav-nghjghgh jhj jhjjj jhjh jhjhjMag.pptx

Introduction to
Geophysical Exploration
Methods -Gravity
DR. RAJIB KUMAR SINHARAY
PhD, MScTech (Applied Geophysics)
Associate Professor
Department of Petroleum Engineering
Maharashtra Institute of Technology (MIT)-World Peace University (MIT-WPU), Pune

o Associate Professor, MIT-World Peace University (WPU), Pune (2018-till date)
o Senior Geophysicist, Reliance Industries Limited, Mumbai (2009-2018)
o EM Geophysicist, Regional Technology Centre (RTC), Schlumberger, Mumbai (2007-2009)
o Junior Data Processor and Interpretor (DPI), WesternGeco Electromagnetics, Mumbai (2007)
o Member of XXII Indian Scientific Expedition to Antarctica (ISEA), Antarctica, 2003
o Research Assistant, Central Water and Power Research Station, Pune (2001-2006)
o Junior and Senior Research Fellow, Indian School of Mines (IIT(ISM)), Dhanbad (1998-2001)
o Master in Science and Technology in Applied Geophysics, IIT, Dhanbad (1994-1997)
o Bachelor of Science in Physics, University of Calcutta, Kolkata (1990-1993)
DR. RAJIB K. SINHARAY
Mobile/WhatsApp: +91-9004071757
email: rajib.Sinharay@mitpune.edu.in.com/ rsrism@gmail.com

Safety in Field Work
The highest priority in any field work!
1. Be Alert!
2. Read instrument manual thoroughly
3. Follow dos and don’ts strictly
4. Must wear safety gears
5. Carry First-Aid Box
6. Know locations and area before hand
7. Save data always in different storages simultaniously

Exploration Methods
■ Geology
■ Geochemical
■ Geophysical
■ Well Logging

Design and Decision: Integration of All!

Geophysical Methods
■ Gravity (Density of Rocks)
■ Magnetic (Magnetic Property)
■ Self Potential (Natural Electric Potential)
■ Resistivity (Resistance of Rock)
■ Seismic (Sound Wave)
■ Electromagnetic (Electromagnetic Wave)

Integration of Geophysical Data!
Seismic
Gravity Magnetic
CSEM
MMT

Gravity Survey
■ Theoretical Background
– Newton’s gravitational acceleration
– Earth’s structure and gravity reductions
– Gravimeter for gravity measurements
– Gravity data processing
■ Survey Planning
■ Data Acquisition
– Land Gravity
– Marine gravity
– Satellite gravity
■ Gravity Interpretation
■ Case studies

Equator: gE = 9,78,033 mgal; Radious= RE = 6378 km
Pole: g P = 9,83,219 mgal; Radious= RP = 6357 km
gP = g E +3370 mgal
Gravity Value and Earth’s Shape

Centrifugal Forces
gP = g E +3370 mgal

Mass Distribution and Gravity
g E = gP - 6467 mgal,

Latitude Corrections
gP = gE + 6467+ 3370 - 4800 mgal = gE +5037 mgal
These variation of g with latitude θ:
g (θ) = 9.78031846 (1+ 0.0053024 sin ² θ – 0.0000058 sin² 2θ)
This equation is called the Geodetic Reference System for 1967.

Calculate
What is gravity value @ θ = 53° 30’ 25” N in GRS 1987 ?

Calculate
What is gravity value @ θ = 53° 30’ 25” N ?
g = 9.78031846 ( 1+0.003417902-0.000005395) m s −2
= 9.81369388 m s −2

Geoid
The geoid is a surface of constant potential energy that coincides with mean sea level over
the oceans. This definition is not very rigorous. First, mean sea level is not quite a surface of
constant potential due to dynamic processes within the ocean. Second, the actual
equipotential surface under continents is warped by the gravitational attraction of the
overlying mass. But geodesists define the geoid asthough that mass were always
underneath the geoid instead of above it. The main function of the geoid in geodesy is to
serve as a reference surface for leveling. The elevation measured by leveling is relative to
the geoid.

ESA's GOCE mission has delivered the most accurate model of the 'geoid' ever produced.
Red corresponds to points with higher gravity, and blue to points with lower gravity.

FAC
Δg = 3.086 Δh .10 −6
m s −2
= 0.3086 Δh mgal
Difference of Gravity ?

BC
BC = 2πGρ Δh BC = 0.00004193 ρ Δh mgal
For ρ = 2670 kg m−3 BC = -0.1119 Δh mgal

Corrections
■ Drift corrections
■ Tidal corrections

Processing or Reductions
■ Latitude Corrections
■ Free Air Corrections
■ Bouguer Corrections
■ Terrain Corrections

Regional Residual Gravity Separation

Gravity Anomaly of a Sphere
 Volcanic intrusions
 Salt domes
 Near surface cavities
 Basically all near-spherical
shape of geological feature

Interpretation: Half Wavelength Method

Gravity Anomaly on a Fault
G= gravity constant,
t= thickness of the slab
△ρ= density contrast Kg/m3

Gravity Survey Design
■ Station Spacing
■ Profile Length
■ Profile Spacing
■ Base Stations

Different Gravimeter
■ Absolute gravimeter
– Pendulum
– Free Fall
■ Relative gravimeter
– Portable pendulum
– Torsion balance
– Stable type
– Unstable type
– La-Coste Romberg
– Worden Gravimeter

La-Coste Romberg Gravimeter
Telford, Applied Geophysics,
Oxford University Press

Gravity Measurements
Worden
Gravimeter

Case Study
Gravity anomalies of the active mud diapirs off southwest Taiwan
W. Doo, Shu-Kun Hsu, Shu-Kun Hsu, Yi-Ching Yeh, Yi-Ching Yeh…….
December 2015
Geophysical Journal International 203(3):2089-2098
DOI: 10.1093/gji/ggv430

Gravity Response of Sedimentary
Basin

Terrain corrected Bouguer anomaly
map of the central India at 5 mGal
interval (GMSI, 2006)

Filtering Methods
■ Measured gravity values are plotted at different measurement points
■ Spatial wavelengths are calculated
■ High Cut, Low Cut and Band Pass filters are used to separate different spatial
frequency responses
■ Depth are estimated depending on frequencies of the signals

Applications for Hydrocarbon
Exploration
■ Mapping of sedimentary basins
■ Study of salt domes
■ Mapping of basement of sedimentary basin
■ Mapping of intrusive bodies i.e. dyke, seals, vents
■ Mapping of basalts and carbonate formations

(1642-1727).
Gauss Law: Magnetic Field
0
. 



B
0



 

A
B A
d
B


Units of Measurements
■ The magnetic field strength, H, is defined as the force per unit pole strength.
■ MKS unit: Force is N and magnetic monopoles in Amp - m, the units associated with
magnetic field strength are N / (Amp - m).
■ N / (Amp - m)=T (Tesla). Earth’s magnetic field=0.5 T
■ Magnetic anomaly are measured in nano Tesla (nT) which is 10-9
T
■ 1 Gamma=1 nT= 10-9
T

Magnetic
Susceptibility
of
Rocks

Magnetism of Rocks
■ Induced
– Present geomagnetic
field
■ Remanant
– Depositional
– Thermal
– Chemical

Magnetometers
■ Measures the magnetic field intensity (in nT or Gamma) along with inclination and
declinations
■ Schmidt type magnetometer
■ Torsion magnetometer
■ Flux gate magnetometer
■ Nuclear or the proton magnetometer
■ Rubidium vapour magnetometer

Maxwell-Faraday’s law
Electromagnetic induction was discovered by Michel Faraday in 1831 and
Joseph Henry independently at the same time.
t
B






t
B
E
X








Fluxgate Magnetometer
■ The fluxgate magnetometer is based on the
magnetic saturation circuit.
■ Two parallel bars of a ferromagnetic material
are placed closely together.
■ Each bar is rapped with a primary coil in reverse
direction
■ Difference of magnetic field creates potential
difference which is measured by secondary coil
■ Accuracy: 0.5 to 1.0 nT

Proton Precession Magnetometer
■ The most commonly used magnetometer and measures
the total amplitude
■ The sensor is a cylindrical container filled with a liquid
rich in hydrogen atoms (water/ oil/ alcohol etc.)
surrounded by a coil
■ Measures current produced by spinning hydrogen
nuclei, they begin to precess around the direction of the
Earth’s total field
■ Frequency of precession is proportional to the strength
of the total field
■ Accuracy: 0.1 nT

Magnetic Field Survey &
Interpretation

Survey Modes
■ Airborne - Both fluxgate and proton precession magnetometers can be
mounted within or towed behind aircraft, including helicopters. Most
difficult problems associated with aeromagnetic surveys is fixing the
position of measuring point. Realtime, differential GPS systems are used to
solve this problem.
■ Shipborne - Magnetic surveys can also be completed over water by towing a
magnetometer behind a ship.
■ Ground Based - Magnetic surveys are also commonly conducted on foot or
with a vehicle. Ground-based surveys may be necessary when the target of
interest requires more closely-spaced readings.

Magnetic Anomaly
■ Geomagnetic forces are
perpendicular to the surface only at
poles
■ Geomagnetic field is horizontal at
equator
■ Geomagnetic field inclination varies
between equator and pole from 0 to
90o
depending on latitude
■ Magnetization and polarity of rocks
depends on latitude of occurrences

Rate of change of declination at Greenwich (GRW), Abinger (ABN), Hartland (HAD), Eskdalemuir (ESK) and
Lerwick (LER) observatories

http://www.geomag.bgs.ac.uk/education/earthmag.html

Variation
of
Earth’s
Magnetic
Field

Diurnal Variation of Inclination
http://www.geomag.bgs.ac.uk/education/earthmag.html

Source of Geomagnetic Field
■ 99% from the Earth
– 94% dipole field
– 5% non-dipole field
■ 1% current in ionosphere
– diurnal variations
– magnetic storms

Diurnal Correction
■ Reference magnetometer at Base Station (BS)
■ Continuously record magnetic field variation
with time (diurnal) at BS
■ Compensate the variation depending on the
time of measurement
■ Range of variation may be up to 20-80 nT
BS

Regional-Residual Separation
■ Estimates of the regional field :
– IGRF (large area)
– Graphical method (local investigations)
– polynomial curves generated using least squares
– inversion at a large scale to define a regional field.
BResidual= BObserved−BRegional

Total Magnetic Intensity (TMI)

Magnetic Anomaly & Declination
Magnetic anomaly from a vertically orientation 2 m pole for differing inclinations of the Earth’s field: a) 90o
, b) 45o
and c) 0o
.

Anomaly of Magnetized Sphere
Pole
Equator

Reduced To Pole (RTP)
 Preferred when measurements are at higher latitude
 Removes the dependence of magnetic data on the magnetic inclination
 Converting data to what they would have looked like at Pole where field is
vertical
 Reduction to the pole removes anomaly asymmetry caused by inclination
 Locates anomalies above the causative bodies
 Assumes that the remanent magnetism is small compared to the induced
magnetism.
 Difficult to do at low magnetic inclinations

Quantitative Interpretation of RTP
■ Dipole: Object is a compact body
Z X
∼ 1/2
■ Monopole: Object is an extended
body
Z ½ * X
∼ 1/2
Half-width, X1/2, as the width of the anomaly at half its
maximum

Reduced To Equator (RTE)
■ Preferred when measurements are at low latitude
■ Reduction-to-Equator is a mathematical transformation of the total magnetic
intensity field at its observed inclination (I) and declination (D) to that of the
magnetic equator. I=0o
.
■ Difficult to use if measurements are in higher latitude

Magnetic Gradient: Vertical
The vertical magnetic gradient offers a better means of
detecting near surface magnetic sources than total
magnetic field measurements, making the gradient method
ideal for mineral prospecting.

Analytical Signal
 2D the analytical signal:
and
 3D the analytical signal:

(a) Map of the vertical gradient of the total magnetic field and (b) the amplitude of the analytic
signal.

Electrical Resistivity Surveys

Resistivity and Apparent Resistivity
■ Electrical resistance of a material with unit cross-
sectional area and unit length is called the resistivity
(ῼ-m) or True Resistivity.
■ Resistance of a specific lithology can change depending
on physical dimension but resistivity remains constant.
■ The resistance of a formation comprising more than
one lithological units is called Apparent Resistivity. It
depends on the number of different lithology units,
true resistivity of each units and their thickness.

Electrical Conductivity
■ Degree of ease for flowing the electrical charges
■ Conductivity (σ) is a tensor, (S/m)
J is current density (A/m2
) and E is electric field (V/m). The
linearity hold for most of the materials
E
σ
J



E
x,y,x)
J
E
J





Current and Potential Electrodes
■ Current Electrodes (A&B): Used to sent current
in the ground and connected to DC source.
Current (I) through the circuit is measured for
a certain separation AB. If AB increases
current decreases for a fixed voltage.
■ Potential Electrode (M&N): Connected to
voltmeter to measure voltage difference (∆V)
for a certain distance MN.

Potential Difference at Half Space
R= ρ L/A
ρ = (Δ V/I) (A/L)
V = Iρ / 2πR
VP = (Iρ/2π) (1/R)

Potential Difference
V = ρI / 2π (1/R1 – 1/R2)

Different Array Configurations

Depth of Penetration and Plotting
Point
■ Plotting point is the mid point of MN
■ Increase of AB helps deeper penetration or
larger depth of penetration (DOP)
■ Higher ∆V indicates good signal to noise ratio
or more accuracy in measurements
Plotting Point

Vertical Resistivity Sounding (VES)
■ Schlumberger Array is the common survey
method used for VES,
■ Resistivity of the formations along the
depth can be obtained at the plotting
points by increasing AB separations.
■ Apparent resistivity are plotted against
AB/2 on Log-Log graphs to obtained VES
curves.
Plotting Point

Two Layers Schlumberger VES
Curve

Three Layer Schlumberger VES
Curves

Resistivity
Interpretation
2
Layer
Cases

Ultrasonic? No Sonic (Seismic)
Method!
1. Uses Sound Wave with higher frequency (2.5-3.5 MHz) than audible sound (20Hz-2.0 MHz)
2. Create image of soft tissues
3. Object is few mm to few cm below the skin (surface)

Seismic Method
■ Uses seismic waves of low frequencies (~ 12-120 Hz)
■ Image soft sedimentary rocks up to ~ 5 Km below surface
■ Seismic waves reflect back from sedimentary bed interfaces
Geophone

Offshore Seismic
Survey
Sparkers Boomer
Hydrophones

Seismic Signals
1. Low frequencies penetrates deeper
2. High frequencies have higher resolution
3. Longer off-sets have deeper penetration
4. Reflected energy is proportional to seismic impedance (density x velocity) contrast
5. Difficult to image below hard and compact rocks, i.e. Basalt

Seismic Wave Velocity in Different
Rocks

Seismic Sections and Interpretations

Difficult Questions!
1. What if there a hard rock above sedimentary
rock, i.e. basalt or carbonates ?
2. How to map petroleum system below a salt
dome?
3. Am I sure that this beautiful structure is not
volcanic?
4. Am I sure that there is Petroleum in my
prospect NOW ?
5. Is the Gas saturation is commercial?
Golden Pot
But
Is there
water now?

Electromagnetic Methods
■ Magnetotelluric (MT)
– Natural Source
– Very Low Frequency (0.1-0.001 Hz)
– Deep penetrating up 100 Km
– May be both Land and Marine
■ Controlled Source Electromagnetic (CSEM)
– Artificial source
– Frequency 0,1 -10 Hz
– Depth of penetration
– Marine method

Working Principle of MT
Natural EM field from Ionosphere
Different Frequency
Senses Different
Depth
f

 503

MMT Frequency
Range 1-0.0001 Hz

Survey Layout of MMT
Basalt (Resistive)
Sub-Basalt Sediment (Conductive)
Basement (Resistive)
Induced Current
Sediment (Conductive)
Reference
MT Station

H
E



How E & H Behaves ?
“Time Series” Data for Marine Magnetotelluric (MMT). H(s) are Magnetic and E(s) are Electric
fields. On land we measure Hz also.
Ex
Ey
Hx
Hy
Time
Amplitude

Concept of MT: Cagniard’s Relation
)
(
)
(
)
( 

 y
x H
Z
E



E (Secondary) H
(Primary)
Earth
Air

Apparent Resistivity and Phase
Apparent
Resistivity
(Ohm-m)
Phase (Degree)
Period (S)
2
0.2
ij
ij
Z
a f
 
1
Im
tan
Re
ij
ij
ij
Z
Z
 
 
 

 
 
Apparent resistivity (ρ) and
Phase (φ) can be calculated
from Z at each frequency (f):

Salt Bodies : Gulf of Mexico, WGEM
Sandburg
et
al.
(2008)

■ Basement mapping
■ Salt body mapping and subsalt basin studies
■ Volcanic basin studies
■ Crustal studies
Application of MT/MMT

Navigation Dome
Emergency Receiver Catcher

Working Principles of CSEM
Field components are recorded continuously
against time (Time-series data).
Field amplitude and phase are plotted
against Tx-Rx offset distances for a particular
frequency
Amplitude value increases due to presence of
sub-surface resistive bodies (i.e.,
hydrocarbon filled reservoirs)
Tx
Rx
Amplitude
(V/Am
)
2
Tx-Rx Offset (km)
Time
Hydrocarbon
saturated
reservoir
(Resistive)
Increased
amplitude
response
Seabed

CSEM Put Colours on Seismic
2.5 D CSEM Results Adding Value in
Conventional Seismic Section

CSEM for Hydrocarbon Exploration
 Capable to detect thing HC saturated resistive reservoir layer
 Prospect Evaluation and de-risking
 Ranking and Prioritization of prospects
 Mapping the extension of reservoir
 Reservoir Monitoring (Experimental)

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