GEOFISIKA

Dr. Sri Mulyaningsih

Tugas I

• Apa yang anda ketahui dari geofisika eksplorasi?

• Apa perbedaan dari metode magnetik dan gravity?

• Apa prinsip dasar dari metode density?

• Apa prinsip dasar dari metode seismik?

Seismic as Rays

• The concept of rays are used to analyse the path of the seismic wave in the ground. 

• This is an infinitesimally thin cone of the wave that is perpendicular to the wavefront.

Seismic Velocity

• The speed or velocity at which a seismic wave travels through rocks varies dependant upon the property of the rock

• It depends upon the axial modulus and density of the rock.

• In general it can be thought of as being related to hardness. 

• Seismic waves travel faster through hard rocks and slower through soft rocks. 

• Gas in sandstone also slows down seismic waves.

Seismic Velocity

• Interval Velocity is the Difference in Depth divided by the Difference in Time for ray paths to the same reflector from different source distances.

• V2T2 – V1T1      T2-T1

• VnTn-V(n-1)T(n-1)

Tn-T(n-1)

Wavelength and Frequency • The wavelength is the distance between the successive

compressions and dilations, • The distance between one crest of a wave and the next. • The frequency is the number of time is a second that a

wave advances by one wavelength. • i.e. if two compression (waves) move past the same

position in 1 second, that wave has a frequency of 2 cycles per second. 

• The SI unit is the hertz or Hz (1 Hz = 1 cycle –1) • Wavelength and frequency are related by the following

equation : Vp = fl  – where l = lambda  f = frequency Vp = velocity

• The frequency bandwidth of seismic waves generated by a reflection source is approximately 1 – 1000 Hz.  10 – 70 Hz is the most useful.  

• Once generated the frequency of a wave cannot vary.

Geometrical Spreading and Absorption

• As seismic wave spreading out from a source will have a wavefront that is a sphere

• If the radius of the sphere is r  the wave energy is spread out over an area 4pr2 , the surface of the sphere

• The wave energy will decrease with distance from the source r -2  • Amplitude is proportional to the square root of energy, thus

amplitude decreases with distance as r-1 • This means that a seismic wave has half the amplitude at 200m as it

does at 100m.  This is called geometric spreading (or divergence) • This is why the image from deeper with in the earth’s crust becomes

weaker and less easy to interpret • To investigate the deeper earth’s crust need a much bigger energy

source (i.e. earthquakes which generate a few billion times as much energy!)

• Absorption is the other reason for loss of energy.  This is the transfer of wave energy to heat.  The loss through absorption is the same for each wavelength travelled.  Thus longer wavelengths suffer less proportionally. Long low frequency waves travel farther

Reflection Amplitude (strength)

• Acoustic impedance is the contrast (difference) in rock properties (hardness) on either side of a boundary (interface)      Density: Lmst   2.8      Shale  2.3  

•   Velocity: effectively the compressibility of the rock      Soft  =  Low Velocity      Hard =  High Velocity

• Reflection Coefficient  R is the ratio of the amplitude of the reflected wave to the amplitude of the incident wave.               

• For an angle 0 the equation is R =   p2v2 – p1v1         p2v2 + p1v1

A reflected wave will only occur if there is a change in acoustic impedance across the

boundary Soft to Hard = +ve RC  Hard to Soft = -ve

RC

REFLECTION

• The limitation of seismic is its resolution, which is controlled by the size of the seismic wave. 

• In thin beds, the reflected waves will interfere and overlap and it may not be possible to resolve the bed as a clear separate seismic wavelet

• The size(thickness) of bed that can be distinguished will depend on the length of the seismic wave.  This will vary, but as a wave propagates into the earth, the higher frequencies are absorbed, leaving the lower longer frequencies, and thus reducing resolution. 

Multiple Reflections • The assumption is made that rays travel down from the source and

then back up from the reflector (primary reflectors) • Many rays do this, but some are also reflected internally from one or

more interfaces These are multiple reflectors. • Multiples have lower amplitudes as they have been reflected by

more  than one interface and have longer travel distances • Two type of multiples have high amplitudes, Water layer multiples

(sea bed and the sea surface), which tend to mimic the sea floor and should be easy to recognise. 

• Ghost multiples travel from the shot up to the ground surface and then are reflected down.

• These thus arrive slightly after any real reflector. Other common multiples are peg leg multiples.

• It is important to recognise multiples to prevent them from being interpreted as true reflectors. 

• Seismic Data Processing

Static Corrections

• This is to allow for the different heights or depths of the source and geophones.

• It also accounts for the effects of the near surface weathering layer, which often has a very low velocity, causing severe time delays.

Dynamic Corrections • All the traces with the same CMP Common Midpoint are put together, the

CMP Gather. • They show different travel times to a reflector because of the different length

of the ray paths. • A horizontal interface will have reflections times that increase in a hyperbolic

fashion. • To improve the data quality the traces will be summed up, but to do this a

correction for the reflection times must be made, this is termed normal moveout. 

• A Velocity Analysis is carried out on the CMP gather which gives valuable information on the velocity of the rocks for later use in depth conversion. 

• Each trace is corrected by the determined velocity function and then summed up.

• This stacking process increases the Signal to Noise ratio, as the noise is random, but the signal is not, this stacking a number of records from different geophones (6, 12, 18, 48, 72 fold) improves the quality of the seismic signal.

Frequency Filtering

• This is another technique to increase the signal to noise ration further.

• Frequency filtering removes frequencies that are known to be not generated by the source. 

• Time variant filters can be used, to remove certain frequencies from a part of the section, whilst retaining them in others.

Deconvolution

• Frequency filters can remove noise that has different frequencies from the seismic signal, but not noise that has the same frequency. 

• A different type of filter can be used, deconvolution. 

• This reduces the effect of 1/ a long wave train of the source signal, 2/ absorption 3/ multiple reflections.

Migration

This analytical process is applied to seismic to correct the seismic image where the point of reflection is not immediately below the CMP, which is the case in all situation where the reflector  is dipping. 

Migration is a computational process which aims to restore the image to account for this.

It is a complex process, but in principle simple. 

Three main  problems with Seismic Data

• Assumes the reflection come from directly below the mid point of source and recorder.  With dipping reflectors this is not the case.  Need Migration of data.

• Reflection may come from “out of the plane” of the section.  To evaluate and correct this need 3D Data.

• Seismic is in TIME and not DEPTH.  Velocity variation within the earth may have a big effect on the image.  Require velocity information to depth convert the seismic.

Latihan:

Seismik refraksi

• Metoda seismik refraksi mengukur gelombang datang yang dipantulkan sepanjang formasi geologi di bawah permukaan tanah.

• Peristiwa refraksi umumnya terjadi pada muka air tanah dan bagian paling atas formasi bantalan batuan cadas.

• Grafik waktu datang gelombang pertama seismik pada masing-masing geofon memberikan informasi mengenai kedalaman dan lokasi dari horison-horison geologi ini.

• Informasi ini kemudian digambarkan dalam suatu penampang silang untuk menunjukkan kedalaman dari muka air tanah dan lapisan pertama dari bantalan batuan cadas

Seismik refraksi

Teknik Pengukuran

• An impulsive seismic source creates a seismic wave (sound wave) which travels through the earth.

• Several options are available for the impulsive source. • A sledge hammer as an energy source may be effective

if the bedrock is not deeper than 20 or 25 feet, and if the overburden is sufficiently consolidated.

• A higher energy impulsive source, such as 8-gauge seismic shells or small charges of a two-component explosive, may be required if the overburden is loose and poorly consolidated, or if the bedrock interface is significantly deeper.

Teknik Pengukuran• The impulse source generates a seismic wave

that travels through the subsurface. • When the wave-front reaches a layer of higher

velocity (e.g. bedrock) a portion of the energy is refracted, or bent, and travels along the refractor as a “head wave” at a velocity determined by the composition of the refractor (bedrock).

• Energy from the propagating head wave leaves the refractor at the “critical angle” of refraction and returns to the surface.

• The angle of refraction depends on the composition in the refractor and the material it is in contact with (Snell’s Law).

Teknik Pengukuran• When the seismic wave returns to the surface its arrival is detected by a

series of geophones (seismic spread) and recorded on a seismograph. • Each seismic refraction “spread” consists of a series of 12 or 24

geophones placed along the line at a set distance or “geophone interval.” • The geophone interval is generally 10 to 50 feet depending on the

desired resolution and the desired depth of exploration. • Due to the geometry of refraction (governed by Snell’s Law), it is

necessary for the length of the seismic “spread” to be approximately 3 to 5 times the depth of the overburden in order to detect the primary refractor (i.e., the bedrock).

• A series of 5 to 7 “shots” are initiated for each spread, one at each end, one or more beyond the ends (“off end”), and one or more along the spread.

• These additional “shotpoints” allow dipping interfaces, changes in overburden materials, and intermediate layers to be identified and resolved.

• Hence the additional intermediate shotpoints increase the accuracy of the depth-to-bedrock interpretation.

• Several spreads may be put together to form a longer refraction profile line.

Kegunaan

• determine depth to water table • determine depth to bedrock • locate fractures zones in bedrock • Bulk, shear, and Young’s Moduli, and Poisson’s ratio • contour mapping of bedrock • estimate earth rippability • measure thickness of aggregate deposits • determine depth to base of backfilled quarries • determine depth of landfills • determine thickness of overburden • map topography of groundwater

Refraction tomography (P-Wave) is used to delineate stratigraphy, identify fracture zones in bedrock (low velocity zones), and provide velocity profile for rippability studies. The example section below represents a P-Wave refraction tomography section showing shallow rock stratigraphy and shallow channel subsidence zone below a known collapse sinkhole.  Similar tomography studies can be performed using S-Wave refraction survey techniques to delineate aquifer formations for hydrogeologic studies, and provide P and S wave velocity information to determine vertical and horizontal variations with elastic properties.

Aplikasi

• In geotechnical engineering: • it is used to determine depth to and rippability of

bedrock for design and cost estimates of road cuts, pipelines, and other civil engineering projects.

• Depth to and rippability of bedrock are also important in aggregate investigations.

• Groundwater applications include mapping bedrock channels,

• identifying faults and fracture zones, and delineation of geologic boundaries to constrain hydrogeologic models.

Earth’s Magnetic Field

Sumber: DMTC (2002)

The Earth's magnetic

field appears to come from a giant bar magnet, but with its south pole located up near the Earth's north

pole (near Canada).The magnetic

field lines come out of the Earth near Antarctica and enter near

Canada.

Magnetometer

Magnetic field• The direction of the cross

product can be obtained by using a right-hand rule: FINGERS of the right hand point in the direction of the FIRST vector (v) in the cross product, then adjust your wrist so that you can bend your fingers(at the knuckles!) toward the direction of the second vector (B); extend the thumb to get the direction of the force.

Magnetic Susceptibility

• Magnetic strength of a mineral or rock is a function of two things:  – the amount of iron, nickel or cobalt, – the amount of alignment which takes place 

• The measure of magnetic strength of a mineral or rock is called the “magnetic susceptibility”

• This can be measured with a simple magnet by testing the “pull”, or it can be measured with very sensitive, highly sophisticated instruments 

• The susceptibility of a completely nonmagnetic substance is equal to 0

• The susceptibility of a highly magnetic mineral like magnetite is about 20

Rock/Mineral Magnetic Susceptibility

• Batuan   – Garam 0 – 0.001– Slate 0 – 0.002– Batugamping 0.00001 –

0.0001– Granulit 0.0001 – 0.05– Rhyolit 0.00025 – 0.001– Batuhijau 0.0005 – 0.001– Basalt 0.001 – 0.1– Gabbro 0.001 – 0.1– Dolerite 0.01 – 0.15   

• Mineral  – Pyrite 0.0001 – 0.005– Hematite 0.001 – 0.0001– Pyrrhotite 0.001 – 1.0– Chromite 0.0075 – 1.5– Magnetite 0.1 – 20.0

GEOLISTRIK

• Disebut juga galvanic electrical methods• Untuk menentukan kondisi geologi dan

hidrogeologi dalam ataupun dangkal:– Menentukan zona2 rekahan, sesar, karst, dll– Jalur aliran airtanah/kontaminan;– Menentukan lensa2 batulempung dan alur batupasir;– Menentukan zona perched water dan kedalaman

airtanah; – Kadang2, untuk menentukan sejumlah besar residu

dan produk floating-nya• Alat yang sering digunakan adalah Wenner,

Schlumberger, dipole-dipole

Dasar Teori

• Utamanya, arus listrik dialirkan melalui sepasang electroda.

• Sepasang electroda kedua digunakan untuk mengukur voltase listrik yang dihasilkan

• Nilai terbesar dari jarak terjauh kedua elektroda adalah hasil investigasi yang lebih akurat

• Di alam: komponen batuan bervariasi untuk mengetahui variasi lateral dan vertikal batuan yang ada di bawahnya

Hukum Ohm

Gaya Lorentz medan magnet

Skematika pengoperasian alat geolistrik (Warner)

Metode Warner

• Electroda harus kontak langsung dengan tanah/batuan

• Jika tertutup aspal / beton, maka dibuat lubang dulu untuk menancapkan electroda

• Untuk tujuan investigasi dalam, electroda harus lebih panjang.

• Jarak antar elektroda terpasang minimal 4-5 kali kedalaman investigasi

Nilai Resistiviti beberapa batuan, mineral dan unsur kimia

Survei Geolistrik: 20 electroda, 14 (20 - 2x3) pengukuran dengan sepasi “2a”

Warner

Warner-Schlumberger

Dipole-dipole