This document provides an overview of geophysics and its various applications. It discusses how geophysics studies the physics of the Earth and its atmosphere. Key methods described include seismic reflection and refraction techniques to map subsurface structures. These methods make use of the travel times of seismic waves to determine depths and detect features like faults and folds. The document also outlines how geophysics has various applications in mineral and oil/gas exploration to locate deposits and structures below the surface using physical property measurements.

1. TECHNICAL ENGLISH - I
Introduction to Geophysics
Technical English I – Fall Term 2013-2014
Department of Geophysical Engineering

2. GEOPHYSICS
GEOPHYSICS, AS ITS NAME INDICATES, HAS TO DO
WITH THE PHYSICS OF THE EARTH AND ITS
ATMOSPHERE. GILBERT’S DISCOVERY THAT THE
EARTH BEHAVES AS A GREAT AND RATHER
IRREGULAR MAGNET AND NEWTON’S THEORY OF
GRAVITATION MAY BE SAID TO CONSTITUTE THE
BEGINNING OF GEOPHYSICS. THE CONTINUED
EXPANSION IN DEMAND FOR METALS OF ALL
KINDS AND THE ENORMOUS INCREASE IN THE
USE OF PETROLEUM PRODUCTS HAVE LED TO
THE DEVELOPMENT OF MANY GEOPHYSICAL
TECHNIQUES OF EVER-INCREASING SENSITIVITY
FOR THE DETECTION AND MAPPING OF UNSEEN
DEPOSITS AND STRUCTURES.

3. BECAUSE THE GREAT MAJORITY OF
MINERAL DEPOSITS ARE BENEATH THE
SURFACE, THEIR DETECTION DEPENDS ON
THOSE CHARACTERISTICS THAT
DIFFERENTIATE THEM FROM THE
SURROUNDING MEDIA. METHODS BASED
ON VARIATIONS IN THE ELASTIC
PROPERTIES OF ROCKS HAVE BEEN
DEVELOPED FOR DETERMING
STRUCTURES ASSOCIATED WITH OIL AND
GAS, SUCH AS FAULTS, ANTICLINES, AND
SYNCLINES SEVERAL KILOMETERS BELOW
THE SURFACE.

4. THE VARIATION IN ELECTRICAL
CONDUCTIVITY AND NATURAL CURRENTS
IN THE EARTH, RATES OF DECAY OF
ARTICIAL POTENTIAL DIFFERENCES
INTRODUCED INTO GROUND, LOCAL
CHANGES IN GRAVITY, MAGNETISM AND
RADIOACTIVITY – ALL THESE PROVIDE
INFORMATION ABOUT THE NATURE OF
STRUCTURES BELOW THE SURFACE, THUS
PERMITTING GEOPHYSICISTS TO
DETERMINE THE MOST FAVORABLE
PLACES TO SEARCH FOR MINERAL
DEPOSITS.

5. GEOPHYSICS DEALS WITH ALL ASPECTS OF THE
PHYSICS OF THE EARTH, ITS ATMOSPHERE, AND
SPACE. GEOPHYSICAL MEASUREMENTS WERE
MADE BY THE MAN LANDED ON THE MOON, AND
THE ATMOSPHERES, MAGNETIC FIELDS, AND
OTHER PROPERTIES OF PLANETS ARE
STUDIEDUSING GEOPHYSICAL DATA OBTAINED
BY UNMANNED SPACECRAFT. THE PRINCIPAL
SUBDIVISIONS OF GEOPHYSICS ARE AS
FOLLOWS; SOME OF THESE HAVE BEEN
INVESTIGATED FOR MANY YEARS SIMPLY
BECAUSE OF THEIR SCINTIFIC INTEREST:

6. SEISMOLOGY
THERMAL PROPERTIES OF EARTH
TERRESTRIAL MAGNETSM
TELLURIC CURRENTS
GEODESY AND GRAVITATION
RADIOACTIVITY OF THE EARTH, SEA, AND
ATMOSHERE; COSMIC RAYS
ATMOSPHERIC ELECTRICITY
METEREOLOGY
OUR KNOWLEDGE OF EARTH HAS BEEN
DEVELOPED BY COMBINING
INFORMATION FROM ALL THESE DATA.

7. APPLIED GEOPHYSICS IN THE SEARCH
FOR MINERALS, OIL, AND GAS MAY BE
DIVIDED INTO FOLLOWING METHODS OF
EXPLORATION:
GRAVITATIONAL
MAGNETIC
SEISMIC
ELECTRICAL
ELECTROMAGNETIC
RADIOACTIVITY
WELL LOGGING
MISCELLANEOUS CHEMICAL, THERMAL,
AND OTHER METHODS

8. GEOPHYSICAL PROSPECTING
GEOPHYSICAL PROSPECTING IS THE
ART OF SEARCHING FOR CONCEALED
DEPOSITS OF HYDROCARBONS (OIL
AND GAS) OR USEFUL MINERALS BY
PHYSICAL MEASUREMENTS FROM THE
EARTH’S SURFACE. ORDINARILY,
THESE MEASUREMENTS YIELD
INFORMATION ON THE PHYSICAL
PROPERTIES OF MATERIAL WITHIN
THE EARTH.

9. SUCH INFORMATION, WHEN PROPERLY
INTERPRETED, CAN BE USED TO LOCATE
MINERAL DEPOSITS HAVING ECONOMIC
VALUE. TO BE EFFECTIVE FOR THIS
PURPOSE, THE DATA FROM GEOPHYSICAL
SURVEYS MUST BE EXPRESSED IN TERMS
OF GEOLOGY. THE RELIABILITY OF THE
GEOLOGICAL PICTURE THUS DERIVED,
WHICH IS GREATER FOR SOME
TECHNIQUES THAN OTHERS, DEPENDS ON
THE QUALITY OF THE DATA AND THE SKILL
DISPLAYED IN THEIR INTERPRETATION.

10. EVER SINCE GEOPHYSICS WAS FIRST
USED IN EXPLORATION, THERE HAS
BEEN CONTINUAL IMPROVEMENT
BOTH IN INSTRUMENTS AND
TECHNIQUES IN METHODS OF
INTERPRETATION. THESE
IMPROVEMENTS HAVE TAKEN FULL
ADVANTAGE OF OUR RAPIDLY
ADVANCING TECHNOLOGY
SPECIFICALLY INCREDIBLE HURDLES
IN DIGITAL ELECTRONICS.

11. GEOPHYSICAL METHODS HAVE
BECOME MORE EFFECTIVE BUT THEIR
INCREASED CAPABILITIES HAVE NOT
QUITE KEPT PACE WITH THE
INCREASING DIFFICULTY OF FINDING
NEW OIL AND MINERALS AS SOURCES
EASIEST TO LOCATE AT ANY TIME ARE
PROGRESSIVELY FOUND AND
EXPLOITED. GEOPHYSICISTS NOW
FACE THE DISCOURAGING, PROBLEM
OF HAVING TO RUN FASTER AND
FASTER TO STAY IN THE SAME PLACE.

12. GEOPHYSICS IN THE OIL
INDUSTRY
WHEN THE EARLIEST CREWS, USING
THE TORSION BALANCE AND
REFRACTION SEISMOGRAPH, LOOKED
FOR SHALLOW SALT DOMES ALONG
THE GULF COAST OF THE UNITED
STATES AND MEXICO DURING THE
MIDDLE OF 1920’S, THEIR EFFORTS
WERE SPECTACULARLY SUCCESSFUL.

13. LARGER POOLS THAT WERE EASY TO
DISCOVER BY GEOPHYSICS HAVE
BEEN MOSTLY FOUND, SO THAT THE
INDUSTRY IS WILLING TO DRILL ON
MORE MODEST PROSPECTS AS THE
GOOD ONES HAVE BECOME SCARCER.
THUS, A GREATER NUMBER OF
GEOPHYSICAL CREWS IS NECESSARY
TO LOCATE THE SAME NUMBER OF
BARRELS OF OIL EACH SUCCESSIVE
YEAR SO THAT EXPLORATION COSTS
PER BARREL INCREASE.

14. MANY OIL COMPANIES HAVE BEEN
MEETING THIS PROBLEM BY
EXPANDING THEIR OPERATIONS
WORLDWIDE. OTHERS HAVE BEEN
CONCENTRATING ON OFFSHORE
EXPLORATION, WHERE GEOPHYSICS
HAS BEEN NOTABLY SUCCESSFUL.
FOREIGN OPERATIONS ARE QUITE
EXPENSIVE AND OFTEN INVOLVE
POLITICAL RISK WHILE OFFSHORE
LEASING AND PRODUCTION ARE VERY
COSTLY.

15. GEOPHYSICS IN MINING
EXPLORATION
BY THE GREATEST PORTION OF THE
ACTIVITY IN GEOPHYSICAL PROSPECTING
HAS BEEN IN THE SEARCH FOR OIL AND
GAS. ONLY A SMALL FRACTION OF THIS
EFFORT HAS GONE INTO THE SEARCH FOR
SOLID MINERALS. YET GEOPHYSICAL
METHOD WERE USED TO EXPLORE FOR
ORES SEVERAL CENTRUIES BEFORE THERE
WAS ANY OIL INDUSTRY AT ALL.

16. ALTHOUGH THE TOTAL EXTENT OF
GEOPHYSICAL WORK IN THE MINERAL
INDUSTRY IS SMALL COMPARED TO
THAT IN THE PETROLEUM INDUSTRY,
GEOPHYSICAL SURVEYS RESULTED IN
SOME SPECTACULAR DISCOVERIES OF
ORE DEPOSITS. THERE ARE A NUMBER
OF REASONS WHY GEOPHYSICS HAS
LESS WIDESPREAD APPLICATION IN
MINERAL PROSPECTING THAN OIL
AND GAS EXPLORATION.

17. IN THE FIRST PLACE, THE PHYSICAL
PROPERTIES OF MANY ORE BODIES DO NOT
SHOW SUBSTANTIAL CONTRASTS WITH
THE CORRESPONDING PROPERTIES OF THE
COUNTRY ROCK WHICH SURROUNDS
THEM. THUS THERE ARE MANY DEPOSITS
WHICH ARE NOT INTRINSICALLY
PROMISING AS GEOPHYSICAL TARGETS.
MOREOVER, MINING COMPANIES TEND TO
BE SMALLERAND MANY CANNOT AFFORD
THE NECESSARY OUTLAY FOR
GEOPHYSICAL WORK.

18. EXPLORATION GEOPHYSICS AND
THE BASIC SCIENCES
ONE OF THE MOST RECENTLY
ESTABLISHED BRANCHES OF APPLIED
SCIENCE, EXPLORATION GEOPHYSICS
IS ACTUALLY AN OFFSHOOT OF
SEVERAL BASIC DISCIPLINES, SUCH AS
PHYSICS, CHEMISTRY, AND
MATHEMATICS.

19. THE VARIOUS TECHNIQUES OF
GEOPHYSICAL PROSPECTING ARE
BASED ON A NUMBER OF
FUNDAMENTAL PRINCIPLES OF
PHYSICS, SUCH AS THE LAWS OF
GRAVITATIONAL AND MAGNETIC
ATTRACTION, THE LAWS GOVERNING
REFLECTION AND REFRACTION IN
OPTICS (AS APPLIED TO SEISMIC
PROSPECTING), AND THE ELEMENTS
OF ELECTRICITY AND
ELECTROMAGNETIC THEORY.

20. ALTHOUGH THESE PRINCIPLES ARE QUITE
SIMPLE, IT IS GENERALLY DIFFICULT TO
APPLY THEM TO THE STUDY OF ROCK
MATERIALS, WHICH ARE SELDOM
HOMOGENEOUS AND WHICH OFTEN HAVE
COMPLEX PHYSICAL PROPERTIES. NEARLY
ALL THE MAJOR METHODS OF
GEOPHYSICAL PROSPECTING EVOLVED
FROM TECHNIQUES ORIGINALLY
EMPLOYED FOR MORE OR LESS ACADEMIC
STUDIES OF THE EARTH’S LARGE-SCALE
FEATURES.

21. GRAVITY PROSPECTING WAS DEVELOPED
AFTER PENDULUM MEASUREMENTS HAD
BEEN CARRIED OUT FOR SEVERAL
DECADES FOR SEVERAL DECADES TO
DETERMINE THE EARTH’S PRECISE SHAPE
FROM CHANGES IN GRAVITATIONAL PULL
BETWEEN DIFFERENT OBSERVING
STATIONS. THE SEISMIC REFRACTION
METHOD MAKES USE OF PRINCIPLES
WORKED OUT EARLY IN THE PAST
CENTURY BY EARTHQUAKE SEISMOLOGIST
WHO DEVELOPED THEM TO UNRAVEL THE
STRUCTURE OF TRHE EARTH’S INTERIOR.

22. MAGNETIC INSTRUMENTS WHICH WERE
BASICALLY THE SAME AS THOSE USED IN
PRESENT-DAY PROSPECTING MADE IT
POSSIBLE TO CHART SOME OF THE
EARTH’S MAGNETIC ELEMENTS ON A
GLOBAL SCALE AS EARLY AS THE
SEVENTEENTH CENTURY. TODAY THERE IS
A MUCH GREATER VOLUME OF ACTIVITY
IN GEOPHYSICAL EXPLORATION THAN IN
BASIC RESEARCH ON THE PHYSICS OF
EARTH.

23. SEISMIC REFLECTION METHOD
WITH THIS TECHNIQUE, THE STRUCTURE OF
SUBSURFACE FORMATIONS IS MAPPED BY
MAKING USE OF THE TIMES REQUIRED FOR A
SEISMIC WAVE (OR PULSE), GENERATED IN THE
EARTH BY A NEAR-SURFACE EXPLOSION OF
DYNAMITE, RETURN TO THE SURFACE AFTER
REFLECTION FROM THE FORMATIONS
THEMSELVES. THE REFLECTIONS ARE
RECORDED BY DETECTING INSTRUMENTS
RESPONSIVE TO GROUND MOTIONS, WHICH ARE
LAID ALONG THE GROUND NEAR THE SHOT
HOLE

24. DEPTHS TO REFLECTING INTERFACES CAN
BE DETERMINED FROM THE TIMES IF
VELOCITY OF SEISMIC WAVES IN THE
OVERLYING THE SECTION CAN BE
MEASURED. THE REFLECTION TECHNIQUE
FURNISHES MORE AND BETTER
STRUCTURAL INFORMATION THAN ANY
OTHER GEOPHYSICAL METHOD, BUT IT
HAS THE DISADVANTAGE OF BEING
SLOWER AND MORE EXPENSIVE.
MOREOVER, THERE ARE NUMEROUS AREAS
WHERE REFLECTIONS CAN BE OBTAINED
ONLY WITH GREAT DIFFICULTY.