CHAPTER 3

• APPLIED GEOPHYSICS

Introduction

• The petroleum geoscientist uses a wide range

of tools to help explore for and produce

petroleum.

• The petroleum geoscientist needs to describe

the distribution, at basin to pore scale, of rock,

fluid, and void in the Earth's subsurface.

• To do this, the geoscientist uses a large array of

data types and methods.

Data used in petroleum exploration

& production

Data type and source Use

Satellite images

Seismic data, including:

2D

3D

4D

4C (shear wave signal)

Wireline log data

Cuttings and cores from well

Outcrop data

Seepage of Petroleum

Largely in frontier exploration

Frontier exploration and exploitation

Exploitation, appraisal, development

& production

Production

Exploration to production

Exploration to production

Exploration to production

Frontier exploration

Frontier exploration & exploitation

Data used in petroleum exploration

& production

A Landsat satellite image of the Zagros

Mountains, Iran. The center of the image shows a

near-circular anticline above a salt dome.

Gravity Surveying

• Gravity data can be used to help define the

regional tectonic regime, prioritize areas for

seismic work, and identify the causes of

seismic structure (e.g., reefs, salt, and

basement uplift).

• Gravimetric data can be obtained at much

lower cost than seismic data.

• However, the resolution of gravimetric data is

lower than that of seismic data.

Gravity Surveying

• Gravitational prospecting uses Newton's Law,

which links the force of mutual attraction

between particles in terms of their masses and

separation.

• The law states that two particles of mass m1

and m2 respectively, and of small dimension

compared with the distance r that separates

their centers of mass, will be attracted to one

another by a force F as follows:

F = G(m1.m2)/r2

Gravity Surveying

• The acceleration (a) of a mass m2 due to the

attraction of mass m1 at distance r can be

calculated by dividing the attractive force by the

mass m2, thus: a = F/m2 = G m1/r

2

• If m1 is considered to be the mass of the earth

and r its radius, then a is the gravitational

acceleration on the earth’s surface.

• g = GM/R2

• G = universal gravitational constant

Gravity Surveying

• The value of a varies from place to place. This

variation is due to the effect of latitude, altitude,

and topography, as well as geology.

• These variations must be removed before the

last residual one can be detected.

• The acceleration due to gravity is measured in

Gals. The commonly used unit is the milliGal

(where 1000 milliGaIs=I GaI).

Gravity Surveying

• Variations in g:

Gravity Surveying

• Salt dome, ρ g

• Dense rock (anticline), ρ g

Gravity Measurement & Equipment

• Relative gravity:

• Mass on spring measurements. Two types:

• 1. Stable Gravimeter. Ex: Askania, Gulf dan Norgaard

• Change in g --> change in spring length.

• Δg = -k ΔL/m

Gravity Measurement & Equipment

• 2. Unstable gravimeter or astatic.

• Suitable choice of mass, spring constant and geometry

makes the system unstable and very sensitive to

changes in g.

• Ex: LaCoste-Romberg and Worden gravity meter.

Density Variations of Earth Materials

• Consider the variation in gravitational

acceleration that would be observed

over a simple model.

• Assume due to the presence of a

small ore body.

• Let the ore body have a spherical

shape.

• The gravity anomaly produced by a

buried sphere is symmetric about the

center of the sphere.

Gravity Surveying

• When corrections have been made for the

readings at each station, they may be plotted

on a map and contoured in milligals.

• Gravity maps are more useful for showing the

broad architecture of a sedimentary basin.

However, in some cases gravity maps may

indicate drillable prospects by locating salt

domes and reefs.

• Gravity surveys can be carried out on land, at

sea, and by air.

Gravity Surveying

Example: Mapping basin depth

Gravity Surveying

Example: Mapping basin depth

Gravity Surveying

Example: Mapping basin depth

Magnetic surveying

• The Earth's magnetic field may be divided into

three components, the external field, the main

field, and variations in the main field.

• At any point above the earth, the measured

geomagnetic field will be the sum of these

components.

• The main field is generated by the Earth’s

metallic core.

• Variations in the main field are commonly much

smaller than the main field signal.

Magnetic surveying

• They are produced by local magnetic

anomalies in the near-surface crust.

• The intensity of magnetization of a magnetic

mineral will normally be related to the regional

field strength:

• J=kH

• Where J is the intensity of magnetization

• k is the magnetic susceptibility

• H is the intensity of the magnetic field

Magnetic Surveying

• Rock magnetism has two components, induced and

remanent.

• The induced component is proportional to the Earth's

magnetic field and the proportionality constant is called

the "magnetic susceptibility.“

• Magnetic susceptibility measures the degree to which an

element or mineral can be magnetized.

• The magnetic susceptibility is very variable, ranging from

<10-4 emu/cm3 for sedimentary rocks to between 10-3 and

10-2 emu/cm3 for iron-rich basic igneous rocks.

Magnetic Surveying

• In exploration geophysics, anomalies are

measured in gamma units—equivalent to the

nanotesla (nT) in SI units—where 1 gamma

=0.00001 Oe.

• Magnetic data may be collected on land and

with shipborne or airborne magnetometers.

• By this method an aeromagnetic map, which

contours anomalies in the earth’s magnetic field

in gamma units, may be constructed.

Magnetic Surveying

• Like gravity maps, magnetic maps are more

useful for showing the broad-basin architecture,

but can seldom be used to locate drillable

petroleum prospects.

Magnetic Surveying

• Magnetic field anomaly. Bouger anomaly.

Magnetic Anomaly

• Let's now qualitatively

construct what the

magnetic anomaly of a

metallic sphere located

beneath the north pole

would look like.

Magnetic Anomaly

• Finally, let's examine the

shape of the anomalous

magnetic field for a

metallic sphere buried

somewhere in the

northern hemisphere

Magnetic Anomaly

• Suppose we have a buried dyke

with a susceptibility of 0.001

surrounded by sedimentary

rocks with no magnetic

susceptibility. The dyke in this

example is 3 meters wide, is

buried 5 meters deep, and

trends to the northeast. Thus,

we could determine the location

of the dyke and possibly its

dimensions by measuring the

spatial variation in the strength

of the magnetic field.

Seismic Surveying

• Reflection surveying:

Seismic Surveying

Before Seismic Existed

Seismic Surveying

• The most important of the three main types of geological prospecting.

• They are the only widely used data that give a complete

picture of the whole area of study, be it basin, play

fairway, prospect, trap or reservoir.

• Seismic imaging of the Earth's shallow structure uses

energy waves created at a sound source and collected

some distance away.

• The seismic method relies upon changes in acoustic

properties of rock to alter the properties of sound waves

transmitted through the rock.

www.dme.qld.gov.au/zone_files/geoscience_images/seismic3.jpg pubs.usgs.gov/of/2000/of00-304/htmldocs/chap01/images/seismic.gif

Seismic Surveying

• Seismic surveying is largely concerned with the

primary P waves.

• When a wave emanating from the surface

reaches a boundary between two media that

have different acoustic impedance, some of the

energy is reflected back into the upper

medium, and some may be refracted into the

lower medium.

Seismic Surveying

• Reflection and refraction

Seismic Surveying

• The acoustic velocity of a rock varies

according to its elastic constants and density.

The velocity of a P wave:

• k is the bulk modulus Vs = (n/) ½

• n is the shear modulus

• ρ is the density

/)3

4( nkvP

Seismic Surveying

• Factor affecting Velocity:

Density – velocity typically increases with density

– (k and n are dependant on ρ and increase more

rapidly than ρ):

Porosity and fluid saturation-

• Increasing porosity reduces velocity.

• Filling the porosity with fluid increases the velocity.

Seismic Surveying

• The acoustic impedance is the product of the rock’s

density and the velocity (ρ x ν), and is characterized by

the reflection coefficient, R:

•

• Where ρ is the density, and v is the P wave velocity.

• The greater the R the stronger the reflection.

• The quality of reflectors and hence the ability to define

successions of rocks and their characteristics depends

initially on the natural variations in the rock.

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Seismic Surveying

Reflection seismic survey:

Seismic Surveying

• The reflections generated

from many sources delivering

signals to many layers in the

subsurface, and collected at

many receivers, are compiled

to yield seismic cross-

sections in 2D and seismic

volumes in 3D.

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Seismic Surveying

3D surveys: collect data on a grid.

Seismic Surveying

• 2D seismic cross-sections and 3D seismic

volumes are most commonly displayed by

linear x and y coordinates as measured on the

Earth's surface and z measured in time

beneath the Earth's surface.

• If the average acoustic velocity of the rock is

known, then it is possible to calculate the depth

(D) to the interface.

• D = vt/2

• v is the acoustic velocity

• t is the two-way travel time

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Seismic Surveying

• Seismic acquisition:

• Land: Dynamite and Vibroseis are the most

common sources of energy for land-based

seismic surveys.

• Vibroseis comprises a heavy all-terrain vehicle

that can lower a steel plate onto the ground

surface.

• Other energy sources such as weight dropper

and Dinoseis (explosion of a propane/air

mixture in a chamber mounted below a truck).

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Seismic Surveying

• Guns, Weight dropper and Vibroseis

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Seismic Surveying

• The returning acoustic waves

are recorded on Geophones.

• The signals are transmitted

from the geophones along

cables to the recording truck

and records on magnetic tapes.

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Seismic Surveying

• Marine: The energy source for such surveys is

almost exclusively the air gun. An air gun

discharges a high-pressure pulse of air into the

water.

• The air guns can emit energy sufficient to

generate signals at between 5 and 6 s two-way

travel time.

• Depending on interval velocities, these signals

may penetrate to over 5 km.

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Seismic Surveying

• The reflected signals are recorded by

hydrophones on a cable towed behind the ship.

• The cable (streamer) runs several meters

below sea level and may be between 2 and 5

km in length.

• The reflected signals are transmitted

electronically from groups of hydrophones

along the cable to the recording unit on the

survey ship.

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Seismic Surveying

• Seismic ship and air gun.

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A seismic ship shooting a 3D marine survey. The four streamers under tow create the wake patterns seen

at the edges of the photo. Immediately to either side of the ship's wake is an air-gun array. Each array contains four

strings of air guns. With the two sources firing in an alternating pattern, eight lines of seismic data were acquired at

once (Western Geophysical). The Leading Edge 2005; v. 24; no. Supplement; p. S46-S71;

Seismic Surveying

• Seismic processing:

• The aims are to enhance

the interpretable (useful)

seismic information relative

to the noise in the signal

and place the seismic

reflectors in their correct x,

y, z space.

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Seismic Surveying

• Here is a brief description of

some of the processing steps:

• Editing and Muting: Manually

cleaning up the data.

• • Remove dead traces

• • Remove noisy traces

• • Switch polarity on reversed traces

• • “Cut” out unwanted signal e.g.

pre-arrival noise, direct arrival,

ground roll.

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Reflected Ray Paths Geometry

• If t1 & t2 are travel time and x1 & x2 are offset,

so:

Δt = t2 – t1 ~ (x22 – x1

2 )/ 2V2to

(3)

If one geophone at shotpoint (or at x1=0),

Δt known as

normal moveout (NMO), Δtn

then,

Δtn ~ x2 / 2V2to (4) The importance of NMO:

• Having determined the layer velocity, we can use the predicted quadratic

shape to identify reflectors

• Then correct (shift traces) and stack to enhance signal to noise

Seismic Surveying

• Convolution/deconvolution

processes, which are designed

to allow determination of the

effect of the Earth on the seismic

signal.

• The seismogram recorded at the

surface (S) is the convolution of

the two

• S = W * R

• W: source wavelet, R: reflectivity series

• Deconvolution: undoing the

convolution to get back to the

reflectivity series.

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Seismic Surveying

• Common depth/mid point (CMP)

stacking, which involves the

arrangement of component data

for a single depth point side by

side.

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Single horizontal reflector

QuickT ime?and aTI FF (Uncompressed) decompressor

are needed to see this picture.

NMO Correction

• Before and after NMO

www.ocean.slb.com/docs/seabed/Public_Webreport_2010_1

Seismic Surveying

• Migration: The process of trying to move reflections

back to their point of origin. When beds dip steeply, the

wave returns from the reflector from a point not

immediately beneath the surface location midway

between the shotpoint and each individual geophone but

from a point up-dip from this position. The data must be

migrated to correct this effect.

• In consequence, migration is designed to restore

seismic reflectors to their proper x—y position

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Pre-migration Migrated stack

Note improvement of data/image quality

Seismic Surveying

• Amplitude Variation with Offset (AVO)

• Variation in seismic reflection amplitude with change in

distance between shotpoint and receiver that indicates

differences in lithology and fluid content in rocks above

and below the reflector.

• AVO is a seismic technique that uses pre-stack seismic

data, to detect the presence of hydrocarbons in the

reservoir.

• In reservoir rock, AVO response is dependent on the

velocities of P- and S-waves and on density to define

the pore space and fluids within the rock matrix.

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Seismic Surveying

• AVO analysis is a technique by which geophysicists

attempt to determine thickness, porosity, density,

velocity, lithology and fluid content of rocks.

• A gas-filled sandstone might show increasing amplitude

with offset, whereas a coal might show decreasing

amplitude with offset.

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Seismic Surveying

• Seismic interpretation:

• Objective - to generate a coherent geologic story from

an array of seismic reflections.

• Involves tracing continuous reflectors across 2D grids

of seismic lines or throughout 3D data volumes.

• Three-dimensional seismic datasets are usually

interpreted on a workstation.

• The computer files contain the whole seismic volume,

which can be viewed or sliced in any direction.

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Seismic Workstation

Seismic Surveying

• Salt dome

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