2019-2020 1

Applied Geophysics Lecture NotesBy

Dr. Fadhil Ali GhaebFourth Semester

2019/2020

Iraqi Kurdistan RegionTishk International University (TIU)Faculty of EngineeringDepartment of Petroleum and Mining-Erbil

Text Books: 1-Keary P. and Brooks, M. (1986, 1991): An Introduction to Geophysical Exploration.2-Reynolds, J. M. (1997): An Introduction to Applied and Environmental Geophysics.3- Dobrin, M. (1974, 1983): An Introduction to Geophysical Prospecting.

Lecture 1

2019-2020 2

THE objects of a geophysical survey are to locate

subsurface geological structures or bodies and where

possible to measure their dimensions and

relevant physical properties. In oil prospecting structural

information issought because of the association of oil with

particular features such as

anticlines in sedimentary rocks. In mining geophysics the

emphasis is on detection and determination of physical

properties. Though mineral ore bodies give distinctive and

measurable geophysical indications they are often of

irregular shape and occur in rocks of complex structure,

making precise quantitative interpretation difficult or

impossible.

2019-2020 3

In site investigation engineers may be interested in both

structure and physical properties. Variations in bedrock

depth are often needed on major construction sites and the

mechanical properties of the overburden may be important

when heavy loads have to be sustained.

A geophysical survey consists of a set of measurements, usuallycollected to a systematic pattern over the earth's surface by land, sea or air, or vertically in a borehole. The measurements may be of spatial variations of static fields of force - the gradients of electrical, gravitational or magnetic "potential"-or of characteristics of wave fields, more particularly of travel-times of elastic (seismic) waves and amplitude and phase distortions of electromagnetic waves. These force and wave fields are affected by the physical properties and structure of the subsurface rocks.

4

By working at different scales, geophysical methods maybe applied to a wide range of investigations from studiesof the entire Earth to exploration of a localized region ofthe upper crust for engineering or other purposes. In thegeophysical exploration methods (also referred to asgeophysical surveying) discussed in this course,measurements within geographically restricted areas areused to determine the distributions of physical propertiesat depths that reflect the local subsurface geology

In many instances more than one method will be used to

survey the same ground. The search for oil may start

with gravity and airborne magnetic work as a

preliminary to seismic shooting in localities determined

by interpretation of the earlier regional surveys.

The survey methods

Geophysical surveying methods are divided into those that make use ofnatural fields of the Earth (passive methods) and those that require theinput into the ground of artificially generated energy (active methods).The first, utilizes the gravitational, magnetic, electrical andelectromagnetic fields of the Earth itself. The second involves thegeneration of local electrical or electromagnetic fields that may be usedanalogously to natural fields, or, in the most important single group ofgeophysical surveying methods, the generation of seismic waves whosepropagation velocities and transmission paths through the subsurface aremapped to provide information on the distribution of geologicalboundaries at depth.Generally, natural field methods can provide information on Earthproperties to significantly greater depths and are logistically more simpleto carry out than artificial source methods. The latter, however, arecapable of producing a more detailed and better resolved picture of thesubsurface geology.

2019-2020 5

2019-2020 6

Applied Geophysics

Comprises the following subjects:1- Determination of the thickness of the crust (which is important in hydrocarbon exploration.2- Study of shallow structures for engineering site investigations.3- Exploration for ground water and for minerals and other economic resources.4- Trying to locate narrow mine shafts or other forms of buried cavities.5- The mapping of archaeological remains.6- Locating buried pipes and cables.

More than one geophysical method are often used to solve geological problems

2019-2020 7

Geology and Geophysics:

Geology involves the study of the earth by direct observations on rocks either from surface exposures or from boreholes and the deduction of its structures, composition and historical evolution by analysis of such observations.

Geophysics involves the study of the inaccessible earth by means of physical measurements, usually on or above the ground surface. It also includes interpretation of the measurements in terms of subsurface structures and phenomena.

Hence, Geophysics is the third dimension of geoplogy

2019-2020 8

? ? ?- Surface geology to be interpreted for subsurface picture.

- Many types of structures can be imagined by geologists.

- Geophysics can end the argument

2019-2020 9

Physical Properties of Rocks

• The physical properties of rocks as used in different geophysical methods are:

- Density---------------------------------------------Gravity method- Magnetic susceptibility------------------------Magnetic method- Elasticity-------------------------------------------Seismic method- Electrical resistively or conductivity-------Electrical and

electromagnetic methods- Radioactivity--------------------------------------Radiometric method-Thermal conductivity---------------------------Geothermal method

Geophysical surveying

applications

Application Appropriate survey methods*

Exploration for fossil fuels (oil, gas, coal) S, G, M, (EM)

Exploration for metalliferous mineral deposits M, EM, E, SP, IP, R

Exploration for bulk mineral deposits (sand and gravel) S, (E), (G)

Exploration for underground water supplies E, S, (G), (Rd)

Engineering/construction site investigation E, S, Rd. (G), (M)

Archaeological investigations Rd, E, EM, M, (S)

----------------------------------------------------------------------------------------------------

* G, gravity; M, magnetic; S, seismic; E, electrical resistivity; SP, self-

potential; IP, induced polarization; EM, electromagnetic; R, radiometric; Rd,

ground-penetrating radar. Subsidiary methods in brackets.

2019-2020 10

Gravity Method

General Review• The basic concept underlying gravity surveying is the variation of the

Earth gravitational field caused by lateral variation of subsurfacerock-densities. In other words, a given rock body whose density isdifferent from its surrounding medium (i.e. geological anomaly)produces a corresponding disturbance (gravity anomaly) in the Earthgravity-filed. The form and amplitude of the created anomaly dependon the subsurface geological anomaly such as a salt dome, graniteintrusion, buried valley, folded or faulted beds.

• Gravity surveying involves measurements of the changes ingravitational acceleration at points over a given area distributedrandomly or in the form of a grid. The observation-data are thensubjected to a series of corrections and some processes in order toreduce them to gravity values measured relative to a datum-plane insuch a way that all gravity values are related to variations in the sub-datum.

2019-2020 11

The corrected and processed field data is then interpreted to what theymean in geology. This gravity-to-geology process (interpretation) formsthe ultimate objective of any gravity-survey project.

2019-2020 12

Applications of Gravity method:Primary; hydrocarbon exploration, regional geological studies.Secondary; explorations for: mineral deposit, Site investigations, hydrogeology, karsts, geodesy, isostasy, archaeology and volcanic monitoring.

The Earth’s Shape

The Ellipsoid

the Earth is approximately ellipsoidal rather than being perfectlyspherical along the mean sea-level surface. This model is called thereference or normal ellipsoid. It is a theoretical surface.

Up to date informationThe equatorial radius (a) = 6378.160 km The polar radius (b) = 6356.775 km The difference (a-b) = 21.385 km The flattening factor (a-b)/a = 1/298.25The angular Rotation Speed (ω) = 7.292 *10-5 radian/sec

(About 460 meters per second)

2019-2020 13