GEOFISIKA DASAR (TKG 231)

GEOFISIKA DASAR (TKG 231)TEKNIK GEOLOGI UNDIP2014

Kuliah Pertemuan ke-21Is the measurement of contrasts in the physical properties of material beneath the surface of the earth and the attempt to deduce the nature and distribution of materials responsible for these observationGEOFISIKA (Geophysics)

2We treat the earth and subsurface as ideal subjectSubsurface is constituted by body of constant thickness with planar contactDipping bed is only with constant inclinationThe body is homogeneous, lateral variation pictured as abrupt vertical boundaryEarth surface always horizontalThe vibration from wind and traffic or induced current by electric line are never ilustrated

Some Fundamental Consideration(Suryanto, 2012)3Understand the fundamentals of various exploration methodsBefore planning the data acquision stage : Determine what information already exist (geology, drilling log, etc)Acquisition designDefining Objectives(Suryanto, 2012)4Lack of sufficient contrast in physical propertiesNon-uniqueness of many interpretationsResolutionNoise effectLimitation(Suryanto, 2012)5Often specific survey objectives cannot be met by applying only one geophysical method

(Suryanto, 2012)6AktifSeismik RefraksiSeismik Refleksi

Gelombang gempabumiMikroseismik (microseismic)Gayaberat (gravity)Geolistrik (resistivity)Induced PolarizationPole dipole Self PotentialGeomagnet ElektromagnetMagnetotelurik (magnetotelluric)PASIFMacam-macam Metode Geofisika7seismik8Gelombang permukaan (surface wave)MerusakTerdiri dari gelombang Love dan RayleighTidak merusakTerdiri dari gelombang primer (P) dan Sekunder (S)Gelombang badan (body wave)Sifat-sifat Gelombang Seismik9P wave : Dan Russell animations A wave pulseAnimation courtesy of Dr. Dan Russell, Kettering University http://www.kettering.edu/~drussell/demos.html

10

Compressional Wave (P-Wave) Animation Deformation propagates. Particle motion consists of alternating compression and dilation. Particle motion is parallel to the direction of propagation (longitudinal). Material returns to its original shape after wave passes.11S Wave:Dan Russell animations- Transverse waveAnimation courtesy of Dr. Dan Russell, Kettering University http://www.kettering.edu/~drussell/demos.html

12

Shear Wave (S-Wave) AnimationDeformation propagates. Particle motion consists of alternating transverse motion. Particle motion is perpendicular to the direction of propagation (transverse). Transverse particle motion shown here is vertical but can be in any direction. However, Earths layers tend to cause mostly vertical (SV; in the vertical plane) or horizontal (SH) shear motions. Material returns to its original shape after wave passes.13Rayleigh Wave (R-Wave) Animation

Deformation propagates. Particle motion consists of elliptical motions (generally retrograde elliptical) in the vertical plane and parallel to the direction of propagation. Amplitude decreases with depth. Material returns to its original shape after wave passes.14

Love Wave (L-Wave) AnimationDeformation propagates. Particle motion consists of alternating transverse motions. Particle motion is horizontal and perpendicular to the direction of propagation (transverse). To aid in seeing that the particle motion is purely horizontal, focus on the Y axis (red line) as the wave propagates through it. Amplitude decreases with depth. Material returns to its original shape after wave passes.15Dan Russell animations Rayleigh waveAnimation courtesy of Dr. Dan Russell, Kettering University http://www.kettering.edu/~drussell/demos.html

16Dan Russell animations The people waveAnimation courtesy of Dr. Dan Russell, Kettering University http://www.kettering.edu/~drussell/demos.html

17Perambatan Gelombang (Wave Propagation)

18

19DensityElastic moduliiViscocityPlasticity

Mechanical properties20Young Modulus (E)is the stress needed to compress the solid to shorten in a unit strainPoisson Ratio ( )Poissons measures the relativity of the expansion in the lateral directions and compression in the direction in which the uni-axial compression applies

21

22Bulk Modulus (K)Imagine you have a small cube of the material making up the medium and that you subject this cube to pressure by squeezing it on all sides. If the material is not very stiff, you can image that it would be possible to squeeze the material in this cube into a smaller cube. The bulk modulus describes the ratio of the pressure applied to the cube to the ampunt of volume change that the cube undergoes. If K is very large, then the material is very stiff, meaning that it doesnt compress very much even under large pressure. If K is small, then a small pressure can compress the material by large amounts. For example, gases have very small Bulk Modulus. Solids and liquids have large Bulk Modulus

23Shear ModulusThe shear modulus describes how difficult it is to deform a cube of the material under an applied shearing force. For example, imagine you have a cube of material firmly cemented to a table top. Now, push on one of the top edges of the material parallel to the table top. If the material has a small shear modulus, you will be able to deform the cube in the direction you are pushing it so that the cube will take on the shape of a parallelogram. If the material has a large shear modulus, it will take a large force applied in this direction to deform the cube. Gases and fluids can not support shear forces., That is, they have shear modulii of zero. From the equations given above, notice that this implies that fluids and gases do not allow the propagation of S waves

24Seismic Velocities related to material properties

25

26Seismik Refraksi27Seismik Refleksi28terimakasih29