Exploration Geophysics (2)

7/28/2019 Exploration Geophysics (2)

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1. The first step is to define the problem

2. What is the area of interest?

3. What data are presently available?

4. What are the physical properties of interest

and which geophysical methods will respond

to the physical properties of interest?

5. What is the depth of interest of the target?

6. What is the geometry of the target? What is the

required site coverage, spatial sampling and

resolution to detect the target and which

techniques can provide the desired results?

7. What are the site-specific constraints andwhich geophysical tools can perform well

under these conditions?

8. Which techniques can provide

complementary data?

9. What other data (geophysical or non-

geophysical) are required to interpret and or

constrain the interpretation of acquired

geophysical data and when will it be obtained?

10. What are the expected results?

11.What are the likely limitations?

12. Who will make the decision of the methods to

be used, plan the survey, make the

measurements, analyze the data integrate thegeophysical data with other data to arrive at a

realistic conceptual geologic model and write

the report?

13.Will the overall program likely be technically

successful and cost-effective?

Some of the leading questions

before the explorer



A few terms

Exploration

Overburden

Contrast

Anomaly

Exploration

Searching or prospecting of an area with the objective of

locating hydrocarbons, minerals and ground water is what is

called ‘Exploration’ in geophysical parlance

Overburden

Overburden is the material lying over an ore or valuable deposit.Also, the section above a refractor or above a reflector is also

referred sometimes as the overburden.

Contrast

Contrast is the difference in physical properties between a

geologic body and the surrounding rocks.

•Density contrast is made use of for the gravity methods.

•Susceptibility contrast is made use of for the magnetic methods.

•Velocity contrast (to be more specific, acoustic impedance

contrast) is brought into use for the seismic methods



Geophysical

Anomaly

A dev iat ion f rom uni formi ty in physical properties, especially a deviation fromuniformity of exploration interest for example, a travel time anomaly, Bouguer

anomaly, free air anomaly

A portion of a geophysical survey, such as magnetic or gravitational, which is

dif ferent in appearance from the survey in general.

In seismic usage, generally synonym ous wi th structure . Occasionally used for

unexplained seismic events

A feature that may be associated with petroleum accumulation or mineral

deposits

An induced polarization anomaly is usually pos i t ive and greater than

background (or the normal effect) to be economically interesting. An

interesting resistivity anomaly is generally less than backgro und .



Fields of Exploration Geophysics

• Regional Geophysics

•Oil & Gas Geophysics

•Ore Geophysics

•Groundwater Geophysics

•Engineering Geophysics

•Borehole Geophysics



Regional Geophysics

Types of problems:

•Problems located in the Upper part of the crust and the

Upper mantle (5-70km)

•Problems concerning the the study of the crystalline

basement (within a depth range of approximately 10km)

•Problems aimed at in the sedimentary formations (5-

6km)

Common problems: Structure of geosynclinal areas, platform

regions, tectonic movements etc.

Suggested methods: Magneto-Telluric soundings, Deep seismic

soundings, gravity and magnetic methods

Common problems: Surface

relief of the crystalline

basement, Location of

structures like rifts, faults,

intrusives in the basement

Suggested methods:

Reflection seismic

soundings,electrical

soundings, Telluric and

magneto telluric methods,

Gravity-magnetic methods

Common problems: Structural relief of different

stratigraphic complexes within the sedimentary masses,

Distribution of the facies and lithological variations, character

of the basin edges, large structures like anticlines, synclines,

faults, shear zones, intrusives and some typical problems

associated with the shallow subsurface etc

Suggested methods: Gravity, Magnetic, electrical,electromagnetic, refraction and reflection seismic methods



Oil & Gas Geophysics

Structural

•Folded structures,

anticlines and domes, salt

domes, anticlines over

igneous intrusives and

over buried ridges (folds

due to uplift and due todifferential compaction of

sedimentary layers).

•Faulted structures

including faults in folded

structures and homoclinalstructures

•Homoclinal structures cut

by igneous dykes etc.

•Faults, fissures, joints etc.

of lesser importance.

Stratigraphic

•Features associated

with carbonate reefs.

•Permeability

associated with

erosional truncations

such as pinch-outs.

•Sand bodies such as

lenses or stream

channels surrounded

by impermeable

materials

•Facies changes from

permeable to

impermeable lithology

etc.

Types of problems:

•Structural

•Stratigraphic



Oil Exploration

Schedule for the

Virgin Areas

Methods Information outcome

Airborne magnetic survey

(might be coupled with large

scale regional Magneto-

Telluric survey)

Detailed geological mapping (

from the outcrop study)

Close grid gravity magnetic

ground survey

2D seismic reflection survey of

regional dimension

Close grid 2D seismicreflection survey-more

localized in nature

3D seismic reflection survey

in the complex areas

Few

exploratory

wells drilled

More wells

drilled

Wild cat welldrilled

1. Limits of basins.

2. Depth of basement

3. Basement lineations etc.

1. Basin shape and extent

2. Structural trend. 3.Faultlocations. 4.

Prospect leads etc

1. Nature of sediments

2. Age of sediments.3. Possible source/reservoir

1. Types of structures.2. Structural character

3. Relation of different features

4. Sediment velocities. 5.

Sediments, which flow. 6.

Unconformities. 7.

Direction of sediment source.

8. Elements of geological history.

9. Inferences from reflection

character

1. Inferences of depositional

environment. 2.

Inferences of age. 3.

Problems in mapping. 4.Prospect

leads and prospect definition.

5. Leads as to gas accumulations etc.

1.Highly fractured zone mapping

2.To view slices of 3D objects aboutdifferent planes 3.

More objective reservoir estimation



Ore Geophysics

Diversity in

the deposition

types

Shapes and sizes

Mineral deposits may occur as veins, pockets,lenses, stringer etc. of varying dimensions, or

as bedded deposits

Degree of cohesion

Ore deposits can be massive or disseminated

Association

The mode of occurrence might be related to the

tectonic history of the region and is often

controlled by the subsurface geological

structures. In such cases, they are closely

associated with contacts between different rock

formations, fault zones, shear zones, joints,

fractures , fissures etc.



Suggested geophysical methods for

mineral exploration

Direct

geophysical

approach

(If the physical

property contrast

between the orebody and the

surrounding is

substantial)

High-density ore bodies associated with low-

density surroundings:

Example: Chromites (~4.1 gm/cm³) surrounded by Serpentine (~2.55 gm/cm³)

will show strong positive gravity anomaly. Obviously, gravity method is most

suitable.

Highly magnetic deposits:

Example: A manganese ore body, surrounded by garnet sillimanite

gneisses, shows strong positive magnetic anomaly. Obviously

magnetic method is most suitable

Highly conductive (electrical) objects:Example: Massive pyrite surrounded by schist. In such situations,

electromagnetic methods, both harmonic and transient are very

useful



Suggested geophysical methods for

mineral exploration

Indirect

geophysical

approach

(When the

physical

properties of the

mineral soughtafter does not

have or does not

show appreciable

anomaly)

Mica occurring

in pegmatite

Practically no

difference in the

physical properties of

mica and pegmatite.

By using geophysical

methods, what can be

done at best is to

locate pegmatite and

later on drilling,

trenching and pitting,

mica deposits couldbe proved if present.

Presence of gold in

quartz veins

Usually, the amount of gold

present is so small that it does

not alter the physical property

of the quartz mass. So, the

geophysical methods can at

best be used to locate quartz

veins, and the presence of gold can only be established

by test mining.

Diamonds in volcanic

pipes say Kimberlites

Diamonds cannot be explored

directly by geophysical

methods, the volcanic pipes

which might possibly contain

the diamonds are locate in

granites-since the gamma

activity and electrical

resistivity of volcanic pipes are

lower compared to those of

granites, the pipes might be

characterized by lows

Monazite (a mineral of thorium) exploration

in the beach sands of east coast of India

Interestingly, monazite is not identified by the radioactive

property. As it occurs in very small quantity, appreciable

radioactivity over background response is not identifiable.

However, in the beach sands of the east coast india, it is

invariably associated with magnetite, a highly magnetized

substance, So, by conducting magnetic prospecting,

magnetite and thereby monazites are identified.

Prospecting for niobium and yttrium

mineralization

Interestingly, niobium and yttrium occur as non-radioactive ore

deposits. However, these are associated Para-genetically with

small amounts of radioactive minerals. Hence, by making use of

radioactive methods these minerals can be identified.

Prospecting for asbestos occurring at the

contacts between limestones

Asbestos is not located by making use of its physical

properties. Occurring at the contact between limestones and

the basic sills, asbestos is explored by delineating the

deposition of the sills, which can successfully be carried out

by geophysical methods



Groundwater

geophysics

Common

hydro geological

problems

Determination of the depth of the

unweathered hard rocks and theestimation of the thickness of the

overlying water bearing weathered

layers

Suggested geophysical methods: (a) A combination of electrical ‘sounding’ and

‘profiling’

(b) Seismic methods are used in tracing thebedrock topography, which indirectly helps in

determining the thickness of the overlying

sedimentary formations holding water.

(c) Direct location of water has been attempted by

taking advantage of the water velocity contrast

between water bearing and barren formations.

(d) Use of shear waves and converted waves in

arriving at the water bearing properties of

subsurface formations

Tracing the jointed and sheared zones

which control groundwater occurrence

in hard rocks

Suggested geophysical methods: Micro gravity and micro magnetic methods can be

used for tracing the fractures and determining thedegree of fracturing in hard rocks

Location of Karst zones in limestones

Suggested geophysical methods: It needs a site-specific multiple choice of geophysical

methods

(a) Shallow ground penetrating radar ,

(b) Microgravity,

(c) Resistivity,

(d) Vertical seismic profiling (VSP),

(e) Cross borehole tomography etc.

Delineation of salt-fresh water boundaries

Suggested geophysical methods: (a) Resistivity,

(b) Induced polarization methods

Determination of the average porosity of

sand and gravel formation etc.

Suggested geophysical methods:

(a) Well logging methods,(b) Charged body (mise-a-la-masse) method in boreholes



Figure: Shallow radar data with dipping strata shows

near surface evidence of sinkhole activity prior to

collapse

Geophysical data types for

identifying karst zones

Figure: MASW data acquired with a landstreamer over asphaltFigure: Microgravity and multichannel analysis of surfacewaves(MASW) data over a paleocollapse feature



Figure: Correlation of microgravity data (top) and Geoprobe electrical resistivity

pushes (bottom) locate paleo-collapse feature for further characterization

Geophysical data types for

identifying karst zones



Engineering

Geophysics Problem types

Engineering problems with geological implications:

Here, mainly the study of rock types are needed for the construction of dams, reservoirs, other types of huge

and heavy constructions.

Usual Objectives:

1. Determination of the thickness of overburden and the delineation of the relief, which are helpful in making

proper choice for construction site of dams etc.

2. Location of shear, fractures and fault zones in the basement, which may form outlets from the reservoir.

3. Determination of the depth to the groundwater table etc.

Suggested geophysical methods:Seismic refraction and electrical resistivity methods are commonly used. Spontaneous polarization (SP)

method can be used to determine the condition of filtration, location leakages, estimating the changes in

salinity of water etc. along irrigation channels.

Engineering problems of non-geological nature

Types of problems and suggested methods:

1. Location of buried iron pipes (Magnetic)2. Location of areas of corrosion of buried pipes carrying oil and gas

(Electrical).

3. Detection of regions of mounting high pressures in mines (Piezo-

electrical)

4. Location of underground fire (Geothermal).

5. Location of cavities in stone and masonary construction of dams

and huge structures (Radio wave absorption).

6. Determination of heat losses in power grids ( Geothermal and infra

red scanning)



Borehole Geophysics

Problem Types

Five broad disciplines:

•General Geology (GG)

•Reservoir Geology (RG)

•Geochemistry (GC)

•Petrophysics (PPh) and

•Seismic (Seism)

Problems

1.General identification of

lithological succession (GG)

2. Identification of unusual

lithology say, the presence of

volcanics and evaporites (GG).

3. Identification of different

types of minerals (GG).

4. Correlation of different

stratigraphic units (GG).

5. Study of different facies to

delineate depositional

environments (GG).

6. Identification of fractures

(RG).

7. Identification of overpressure bearing zones

(RG).

8. Identification of source

rocks (GC)

9. To ascertain rock maturity

(GC)

10. Porosity determination

(PPh)

11. Permeability determination

(PPh)

12. Determination of shale

volume (PPh)

13. To find out formation water

salinity (PPh)

14. Determination of

hydrocarbon saturation (PPh)

15. Gas identification (PPh)

16. To find out interval velocity

(Seism)

17. To find out acousticimpedance (Seism)



Utility of different

geophysical logs

Permeabilityindicating logs

Spontaneous Potential (SP)

logs

a. Qualitative:

(i) Separating permeable beds

from the impermeable ones

(PPh)

(ii) Study of facies and the

depositional environments (GG)

(iii) Stratigraphic correlation

(GG)

b. Semi quantitative:

In finding out shale volume

(PPh)

c. Strictly quantitative:

To find out formation water

salinity (PPh)

Natural gamma ray

logs

a. Qualitative:

Most of the problems

related to Generalgeology (GG) (i) To

know general lithology,

(ii) To identify unusual

lithology such as

volcanics,

(iii) Identification of

minerals,

(iv) Stratigraphiccorrelation,

(v) Depositional

environment of different

facies.

b. Semi-quantitative:

(i) To know unusual

lithology such asevaporites (GG)

(ii) Source rock

identification (GC)

(iii) Shale volume

determination

Spectral gamma

ray logs

a. Qualitative:

(i) To know general

lithology (GG),(ii) Facies to study

depositional

environment (GG),

(iii) Fracture

identification (RG)


(i) Unusuallithology such as

the identification of

evaporites (GG),

(ii) Mineral

identification (GG),

(iii) Source rock

identification (GC)

and(iv) Shale volume

determination

(PPh)




geophysical logs

Resistivity logs a. Qualitative:

(i) To know general lithology (GG),

(ii) To identify unusual lithology

such as the presence of

evaporites (GG),

(iii) For stratigraphic correlation

(GG),

(iv) To recognize different types of facies to delineate environmental

deposition (GG),

(v) To isolate saline formation

(PPh),

(vi) To identify gas (PPh)


(i) Over pressure identification

(RG), (ii) Source rock

identification, (iii) Determination

of porosity (PPh)

c. Quantitative:

Determination of hydrocarbon

saturation




geophysical logs

Porosity indicating tools

Sonic logs

Qualitative:

(i) To probe unusual lithology

say, the presence of evaporites

(GG),

(ii) For stratigraphic correlation(GG),

(iii) For gas identification (PPh)

Semi-quantitative:

(i) To know about general

lithology (GG),

(ii) Fracture identification (RG),

(iii) Overpressure identification(RG), and

(iv) Source rock identification

Quantitative:

(i) To know porosity (PPh),

(ii) Interval velocity (Seism) and

(iii) To know acoustic impedance

Density logs

Qualitative:

(i) To know unusual

lithology say the

presence of evaporites

and volcanics (GG),(ii) For mineral

identification (GG),

(iii) To know about

depositional environment

of different facies (GG),

(iv) Overpressure

identification (RG),(v) Gas identification

(PPh)

Semi-quantitative:

(i) To know about general

lithology (GG),

(ii) Fracture identification

(RG),(iii) Source rock

identification (GC)

Quantitative:

(i) To determine porosity

(PPh),

(ii) To find out acoustic

impedance (Seism)

Neutron logs

Qualitative:

(i) To know unusual lithology

say the presence of

evaporites and volcanics

(GG),

(ii) For mineral identification

(GG),

(iii) To know about

depositional environment of

different facies (GG) , (iv)

To know shale volume (PPh),

(v) For gas identification

(PPh)

Semi-quantitative:

To know general lithology

(GG)

Quantitative:

To determine porosity of the

formation




geophysical logs

Miscellaneous logs

Caliper logs

Caliper logs can be used

mainly for the qualitative

interpretation of

(i) Fracture identification

(RG),

(ii) Overpressure

identification (RG) and

(iii) In distinguishing

permeable beds from theimpermeable ones (PPh)

Temperature logs

Temperature logs are needed

for

(i) Qualitative use of gas

identification (PPh) and

(ii) Semi quantitative use of

determining maturity of rocks

(GC)





Some frequently asked questions in

Exploration geophysics

•What types of geophysical methods are tobe used for the given nature of the

geological problems?

•What is the nature and magnitude of

geophysical anomaly?

•What is the nature of the basement? Is it

flat or dipping? Is it highly fractured? Thebasinal deposits are basement guided or

basement independent?

•What is the nature of local and regional

anomalies? What types of processing

techniques are to be adopted for separating

local from regional anomalies?

•What are the resolution criteria for the typeof geophysical method adopted? How to

optimize the performance of the given

instrument to adopt it for high resolution

data acquisition?

•What are the physical factors responsible

for controlling the anomalies?

•What is the nature of noise? What methods

are to be adopted for maximizing

Signal/Noise ratio?

•What types of corrections are needed for

the raw field data?

•What is the nature of ambiguities involved

with the type of geophysical methods used?



Inverse nature

of the

geophysical

problemForward problem

When some of the fundamental

physical properties of a body is

known and its effect at some

arbitrary point is supposed to be

calculated, this is called a Forward

problem

Inverse problem

If for a given anomaly curve (plotted

from the field readings), the possible

geometry of a causative body for the

given value of the physical property

contrast is to be guessed, the

possibilities are many and the

problem is called of the inverse type.

In many situations, some of the

solutions of the inverse problems are

mathematically correct but

geologically absurd. There lies the

role of geological insights.



Fundamental

assumptions in

the subsurface

For the observations on the

ground surface, it is usuallyassumed that the subsurface

in general is homogeneous,

isotropic and semi-infinite.

In the boreholes,

the mediumsurrounding the ‘tool’

is homogeneous,

isotropic and infinite.

Causative bodies or any type of discontinuities are

deviations from the properties mentioned above.



Anomaly detection

for the natural field

based methods

For natural

field basedgeophysical

methods, at

any given

datum,

anomaly can

be detectedwhen there is

change in the

physical

properties of

rocks along

the horizontal

direction only.Otherwise, the

thing that will

be observed

is variation in

the scale

factor.

Enhancement



Enhancement

of the

geophysical

anomaly

For the artificial

source based

methods, whether

the objective is to

probe the

subsurface along

vertical direction or laterally, if the

contrast in the

physical properties

of rocks is very

sharp, there is

proportionate

enhancement of

the anomaly

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Exploration Geophysics (2)