GeophysicsGeology 364

Introduction

Exploration GeophysicsGeology 364

 An Introduction to Geophysical Exploration

Kearey, Brooks, and Hill 3rd Edition, 2002

What is Geophysics?       

• A branch of experimental physics dealing with the earth, including atmosphere, hydrosphere, and lithosphere

Solid Earth Geophysics

• Physical properties of Earth– Gross Structure (materials, heat, pressure)

• Redistribution of Global materials– Plate Tectonics

• EarthScope

Exploration Geophysics

• Measures the physical properties of materials below the earth’s surface to detect or indicate the presence or location of ore bodies or hydrocarbons and geological structures

Environmental Geophysics

• Characterization of surface and ground waters

• Organic and inorganic constituents

• Buried objects or voids

Shallow Subsurface Geophysics

• Void detection (karst, tunnels, etc.)

• Buried utilities

• Construction conditions

Borehole Geophysics

• Determination of materials and conditions in proximity of a borehole (down hole logging)

Divining for minerals

Early Attempts

• Chinese fortune tellers begin using loadstone to construct their divining boards, eventually leading to the first compasses. (Mentioned in Wang Ch'ung's Discourses weighed in the balance of 83 B.C.)

• Divining Rods to find more loadstone• Magnetic compass used for iron prospecting as

early as 1640 (dip needle)

Brief Introduction to Geophysics

• Robert Fox discovered, in 1815, that certain minerals exhibit spontaneous polarization and proposed an exploration method.  

• Conrad Schlumberger, 1912 used basic equipment to map isopotential curves on family estate near Cean, France.

• In 1913 he  used spontaneous potential to locate a new metallic sulfide deposit at Bor  Yugoslavia.  – 26 ore bodies, some dating to 4,500 BC– Source of copper for tools and bronze

Bor • Type: High-sulphidation epithermal

massive-enargite (gold) sulphide deposits, porphyry Cu-Mo and Mo deposits

• Morphology: • Age of mineralization: Upper/Late

Cretaceous, 65 ± 4 Ma• Ore minerals: Enargite, pyrite, chalcopyrite,

bornite, chalcocite, covellite, molybdenite, magnetite, pyrrhotite, galena, sphalerite, grey copper

• Alteration: Silicification, argillic alteration, sericitization, chloritization, carbonatization

• Host rocks: Andesite, volcaniclastic rocks, pyroclastic rocks

• Age of host rocks: Upper/Late Cretaceous, Campanian, 78 ± 4 Ma

• Host rock mineralogy: Quartz, andesite, barite, pyrophyllite, diaspore, alunite, anhydrite, sulphur

• Mining: Surface and underground mining• Metals: Cu, Au, Ag

Conrad Schlumberger• German geophysicist and petroleum

engineer noted for the invention, in 1927, of a method of continuous electric logging of boreholes. Beginning1912, Conrad Schlumberger conceived the idea for electrical measurements to map subsurface rock bodies. He was joined by his brother, Marcel, in 1919. Schlumberger teams conducted geophysical surveys in Romania, Serbia, Canada, South Africa, Belgian Congo and the U.S. Electrical prospecting was used for the first time to map a subsurface oil-bearing structure - a salt dome in Romania. In 1927, the downhole electrical resistivity log was recorded in a well in Pechelbronn, France. Born 2 Oct 1878; died 1936.

Brief Introduction to Geophysics

• Pierre Bouguer, 1735, noted the amount of plumb deflection caused by particular peaks of the Andes.

• Sir George Everest, Surveyor-General of India, in the 1860s had a blown survey of a 365 mile-long, north-south line. there was a difference of 550 feet between the direct triangulation and astronomic methods.  Error caused by the mass of the Himalayas.

• 1915-1916 torsion balance used to delineate salt dome oil field in Czechoslovakia.  Employed in 1922 to locate salt domes in the Gulf Coast Region. By 1929 all of the piercement salt domes had been located by gravity surveys.

Siesmics

Chang Heng, astronomerroyal to the Han Dynasty,invented an accurateseismograph in AD 132 – Itwas a large bronze urnWith eight dragon headsgazing outward in eightdirections. Each dragonheld a ball in his mouth.Around the base of the urn,under each dragon, sat afrog with his mouth open.

Siesmics

• Rober Mallet, 1846, proposed to the Royal Irish Academy a method of obtaining subsurface information by artificial earthquakes

• In two years he developed a simple mercury-bowl seismoscope and measured the P-wave velocity of a granite by refraction.

• The First Principles of Observational Seismology (1862)

Triode Vacuum Tube Amplifier

Basic Triode Vacuum Tubes

1907 - 19291930 - WWII

6L6 and 807

B9A

WWII - 1968

Siesmics• In 1901, the Submarine Signal Company was formed and provided

underwater signaling devices to the United States Lighthouse Service. In 1910, the brilliant Reginald Fessenden joined the company. He invented an oscillator in 1911 that he steadily improved. Within a few years, his massive 250kg transceiver went to sea on the U.S. Coast Guard Cutter MIAMI, and on April 27, 1914 he was able to detect an iceberg over 20km away. While conducting this experiment, Fessenden, who was quite seasick, and his co-workers, Robert F. Blake and William Gunn, serendipitously noted an echo that returned about two seconds after the outgoing pulse. This turned out to be a return from the bottom. "Thus, on just one cruise.... Fessenden demonstrated that both horizontal and vertical echoes could be generated within the sea..." (Bates et al. 1987).

Vines Branch Experiment, 1921

Basic Galvanometer 1905

• Analog Multimeter

Induction Coil

Basic Radio

• Simple Sparker– All frequencies

• Lightning• Sparker transmitters • Early Wireless telegraphy

• Crystal Controlled– Oscillator circuit (fixed frequency)

AM Radio Signal

Siesmics

• 1921 discovery of Orchard Dome, Texas by fan shooting

• 1927 First successful reflection profile

• Seismics were developed in the 1920s     Refraction was most successful in locating non-piercement salt domes during the early 1920s.

• Reflection became routine in 1927.

Very Early Reflection Seismogram

Single Shot Analog Seismogram (1930s)

Military and Early World War II Developments

• Pendulum Gravimeters - U-Boat location system with simple gravity map of Atlantic Ocean.

• Magnetic Mines - disturbance of ambient magnetic field.

• RADAR– Magnetron Tube

• SONAR

The original Mark I Magnetic Mine ('Type GA')

Proximity Fuze- miniature RF          transmitter-receiver-

processor. 

Computers

• Naval Fire Direction computers

• Enigma German code– host of top mathematicians and general

problem-solvers was recruited, and a bank of early computers, known as 'bombes', was built - to work out the vast number of permutations in Enigma settings.

• Magic (Japanese code breaking)

Radar

• RAdio Detection And Ranging

• Magnatron Tube– Ceramic and metal tube focus to produce a

beam

• Microwaves– Wavelength << half the size of object!!!– Reflection and Detection

Early Computers

• ORDVAC• ORDVAC was the first of two computers built under contract at the University of Illinois. ORDVAC

was delivered to US Army Aberdeen Proving Grounds in the spring of 1951 and checked out in the summer. As part of the contract, funds were provided to the University of Illinois to build a second identical computer known as ILLIAC I.

• ILLIAC I• ILLIAC I was the first electronic computer in the United states built and owned by a university. It

was put on service on in 1952. It was built with 2,800 vacuum tubes. ILLIAC I had a 5k main memory and 64k Drum memory..

• ILLIAC II• The ILLIAC II was the first transistorized super computer. It was built by the University of Illinois.

At its inception in 1958 it was 100 times faster than competing machines of that day. It became operational in 1962, two years later than expected.

• ILLIAC II had 8192 words of core memory, backed up by 65,536 words of storage on magnetic drums. The core memory access time was 1.8 to 2 µS. The magnetic drum access time was 7 µS. A "fast buffer" was also provided for storage of short loops and intermediate results (similar in concept to what is now called cache). The "fast buffer" access time was 0.25 µS.

• The word size was 52 bits. Floating point numbers used a format with 7 bits of exponent (power of 4) and 45 bits the mantissa. Instructions were either 26 bits or 13 bits long, allowing packing of up to 4 instructions per memory word.

Transister 1947

Tube vs Transistor Radios

Digital Computation

• Binary Byte is basic unit (1101)

• Right Justified

• Two Bytes (10110011) = 0 to F Hexadecimal

• Two Bytes 1 to 256

• Four Bytes 1 to 65,536

Simple Integers in 2 byte (8 bits)

Range of Numbers

• Integers• 16 bits (4 bytes) -32,768 to 32,767• 32 bits (8 bytes) -2,147,483,687 to

2,147,483,686• Floating Points• Precision is size of gap between numbers• 32 bits • 22 bit mantissa, 9 bit exponent, 1 bit sign

Languages

• Assembly

• Compiler– Operating Systems (DOS, OS1) – Fortran, Basic, C, C++, Cobal, Pascal

• Application

Speed

• Integer calculations 200 to 2,000 times faster than floating point

• Addition twice as fast as subtraction

• Multiplication 100-1,000 time faster than division

• Transcendental functions real slow

• Many programs constrained by old operating systems