Teknologi Kebumian(MGD 421)

GDG 10

Program Studi Teknik GeologiFakultas Teknologi Kebumian dan EnergiUniversitas Trisakti2011

Materi GDG 10PendahuluanPENGERTIAN GEOLOGI, Proses, Hukum BUMI SEBAGAI PLANETLITHOSFERA - MINERALBATUAN : BT, BEKU BT SEDIMENBT, METAMORP STRUKTUR GEOLOGI P E TATEKNOLOGI KEBUMIAN

Land SeismicOffshore Seismic

Evidence from Seismic Waves

Earths Internal StructureThe CrustThe MantleThe Core

Seismic ReflectionControlled by:

Snells LawElastic properties of the layers

Seismic RefractionRay paths arebent acrossvelocity or seismicdiscontinuities.

Bend away fromboundary normalif V2 > V1

Bend towardboundary normalif V1 > V2V1V2

Modified from: Brown & Mussett, 1981V2V1In this example, V2 > V1

Locating Boundaries By Refraction

Radial Distribution of Ray PathsRays curveupward becauseof generallyincreasingvelocitywith depthVp and Vs

Internal Earth Structure from SeismologyCrust: 0-60 km

Mantle: 60-2900 km

Core: 2900-6370 km

Seismic Moho and LVZ

Summary Crustal Properties

Mantle TomographyBlue = Fast (cold)

Red = Slow (hot)

Deep Earth Velocity StructureNB that velocity is measured from earthquake waves, but density mustBe inferred from inversion of the velocity combined with elastic modulii

P wave Shadow Zone

S wave Shadow Zone

OlivineFe-NiMetal AlloyPallisite meteorites are classified as stoney-irons and are thoughtto be derived from the CMB region of an asteroidal parent body.Non-seismologicalevidence of core - mantleexchange!

From: Garnero, E.J., Ann. Rev. Earth Planetary Sci., 28 , 509-37, 2000.

Isostasy and GravityIsostasy is the gravitational equilibrium of bodies of different density.Isostatically compensated uplift zones are stabilized by low density roots.Isostatic re-adjustment can be caused by many different processes.

IsostasyCompensation depth is arbitrary and selected for convenience. Total column mass (or gravitational acceleration) must be equal.Low densityHigh density

Isostatic Readjustment: Erosion

Post-glacial Crustal Rebound

From: http://www.uni-geophys.gwdg.de/~gkaufman/work/onset/onset_ice3g.html

Isostatic Uplift due to Crustal ThickeningNB that uplift would depend on the amount and density ofaccumulated magma accreted to the base of the crust.

Gravitational Attraction

Gravity Meters Response to Density VariationsLocal gravity high can be related to a high density body within the crust,while a gravity low can be related to a lower than average density.gravityhighgravitylowLow densityHigh density

Gravity Profile in Isostatic Equilibrium

Non-isostatically Compensated Profile

Negative Gravity Anomaly

Chapter 8

Time and Geology

Sir Charles LyellImage source: www.mnsu.edu/emuseum

Simple Geologic Time Scale

Absolute or Numerical AgeIsotopic DatingUses of Isotopic Dating

Combining Relative and Absolute AgesThe Age of the EarthComprehending Geologic Time

Radioactive Decay MechanismsOperates when Z 58

222Rn86 -> 218Po84 + (5.5 MeV) + n0 = p+ + e-

40K19 -> 40Ca20 + (1.3 MeV) + e- + p+ = n0

125I53 -> 125Te52 + (~3 KeV)

87Rbb87SrRadioactive decay and radiogenic IsotopesRadiogenic isotope ratios are functions of both time and parent/daughter ratios. They can help infer the chemical evolution of the Earth.

Radioactive decay schemes87Rb-87Sr (half-life 48 Ga)147Sm-143Nd (half-life 106 Ga)238U-206Pb (half-life 4.5 Ga)235U-207Pb (half-life 0.7 Ga)232Th-208Pb (half-life 14 Ga)

Extinct radionuclidesExtinct radionuclides have half-lives too short to survive 4.55 Ga, but were present in the early solar system.

Uranium 238 Decay Scheme

Instruments and TechniquesMass Spectrometry: measure different abundances of specific nuclides based on atomic mass.Basic technique requires ionization of the atomic species of interest and acceleration through a strong magnetic field to cause separation between closely similar masses (e.g. 87Sr and 86Sr). Count individual particles using electronic detectors.TIMS: thermal ionization mass spectrometrySIMS: secondary ionization mass spectrometry - bombard target with heavy ions or use a laserSample Preparation: TIMS requires doing chemical separation using chromatographic columns.

http://www.es.ucsc.edu/images/clean_lab_c.jpg

From: http://www.es.ucsc.edu/images/vgms_c.jpg

Half-life and exponential decayExponential decay:Never get to zero!Linear decay:Eventually get to zero!

Pt = Po exp - (to t)1st order rate law

Where

Pt

quantity of the parent isotope (i.e. 87Rb) at time t;

Po

quantity of the parent isotope at some earlier time to, when the isotopic system was closed to any additional isotopic exchange;

is the characteristic decay constant for the system of interest, which is related to the half-life, t1/2, by the equation below:

ln 2 / t1/2

t1/2

is defined as the half-life, which is the amount of time required for 1/2 of the original parent to decay and is a constant.

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87Rbt = 87Rbo e -to t)

(Assume that t = 0, for the present)

87Rbo + 87Sro = 87Rbt + 87Srt

(Conservation of Mass, with 87Sro as the initial

concentration and 87Srt as the concentration today)

87Srt - 87Sro = 87Rbt (e to 1)

M1M2M3

Basic Geochronological AssumptionsDecay constants remain constant through geological time. Good reasons to believe this is correct based on nuclear physics.Measurements of decay sequences in ancient supernovae yield the same values as modern lab measurements.System remains closed to addition or subtraction of parent or daughterDepends on the isotopic system and type of rock (mineral) being measured. Careful sample preparation and thorough characterization of the materials is essential to correct interpretation of results.

Half-lives for common parents

Independent Checks on Radiometric AgesCorrelation of erosion with age on Hawaiian Island Chain: Dates increase in age to the NW as does erosion.Annual growth bands in Devonian corals: 400/yr yields date that is similar to radiometric date. Consistent with slowing of Earth rotation with time.Independent determination of Pacific plate motion yields age progression that is consistent with K/Ar dates of the islands.Agreement between magnetic age from deep marine sediments and radiometric ages of tuffs in East African Rift

Relative and Absolute dates combined

Modern Geological Time Scale

The Age of the EarthEarly Methods: Long debated among intellectuals1625 Archbishop James Usher determined the Earth was created 4004 B.C. by counting generations in the Bible. Young Earth was commonly accepted by western countries.Hindus regarded the Earth as very old. 2000 A.D. would be 1.97 million years according to their calendar.1866 Lord Kelvin calculated the age of the earth based on physical principles. Assumed that the Earth was entirely molten and had to cool to become a solid. Estimated the age between 20-40 million years old.Unknown sources of heat - radioactive decayAssumed that all heat was dissipated by conductionEarly Isotopic Methods: 1905 First crude estimates yielded 2 billion year ageMeteorites gave dates of 4.5 to 4.6 billion years oldModern U-Pb methods yield a consensus value of 4.55 By old

Comprehending Geological Time

SIG atau Geographic Information System (GIS) : menyimpan, dan menganalisis objek dan fenomena geografis merupakan karakteristik yang penting dianalisis

Bentuk SIG Komponen SIG

Tol TomangSelat SundaTerminal Sukarno HattaSenayan

GEOLOGIPENGELOLAAN SUMBER DAYA ALAM DARI BIDANG GEOLOGI, PERTAMBANGAN DAN MIGASTAMBANGNON TAMBANGLOGAMMIGASNON MIGASGEOTHERMALVOLKANOLOGILINGKUNGANGEODINAMISTEKNIK SIPILDLL

Eksplorasi Geologi Permukaan TanahFoto Udara / Foto Satelit Geologi Bawah Permukaan TanahPeta Topografi

Peralatan survai well loggingKegiatan survai lapangan, Pengambilan data Geofisika dan Geokimia

Kegiatan Survei Geofisika (seismik) dalam eksplorasi Migas

Kegiatan Pengolahan dan Interpretasi Data Geologi (di Laboratorium)Pengolahan dan Interpretasi Data di Laboratorium

Ahli Geologi meneliti sampel batuan dari inti pemboran (core)KEGIATAN PENGEBORAN SUMUR EKSPLORASIPeralatan survai well logging

PENGEBORAN DI DARAT dan DI LEPAS PANTAI

Energi Alternatif Ramah Lingkungan Geothermal

GALIAN TAMBANG TEMBAGA FREEPORT

Bagaimana di masa depan?? ???Ada di tangan kita !!

Pustaka dan Acuan : - Prodi Teknik Geologi FTKE Usakti- Museum Geologi- Pusat Survei Geologi Pusat Lingkungan Geologi - Berbagai Situs Internet

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