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GEODESI FISIS
Dwi Arini
PENDAHULUAN
TUJUAN PERKULIAHAN
- Memahami Konsep Dasar Geodesi Fisis.
- Memahami Metode Gravimetrik.
- Memahami Penentuan Geoid dengan pendekatan stokes dan
molodenskey.
- Memahami Sistem Tinggi.
- Memahami Metode Statik dlaam geofis (fungsi kovarian, interpolasi
dan ekstrapolasi anomaly gaya berat.
MATERI PERKULIAHAN
• Konsep Dasar Geodesi Fisis
• Dasar Teori potensial, medan gaya berat bumi, pengertian
gaya berat bumi, gaya berat normal, anomaly potensial,
undulasi geoid dan defleksi vertical.
• Metode Gravimetrik, reduksi gaya berat , reduksi free air,
bouguer dan defleksi vertical.
• Penentuan geoid dengan pendekatan stokes dan
molodenskey, Metode geometric (GPS Highting).
• Sistem tinggi
• Metode static dalam geodesi fisis ; fungsi kovarian,
interpolasi dan ekstrapolasi anomaly gaya berat.
METODA PERKULIAHAN
Tatap Muka
Tugas
Diskusi
Praktikum
PENILAIAN
• Tugas dan keaktifan saat kuliah : 20%
• Ujian Tengah Semester : 30%
• Ujian Akhir Semester : 30%
• Praktikum : 20%
• Absensi 75%
KONSEP DASAR GEODESI FISIS
Geodesi bertujuan untuk mendeterminasi geometric dan bentuk fisik
dari permukaan bumi dan orientasi di angkasa. Cabang geodesi
berkonsentrasi pada determinasi bentuk fisik bumi yang disebut
dengan geodesi fisis.
Geodesi fisisi berbeda dengan disiplin geometrik yang lain dimana
geofis berkonsentrasi pada kuantitas tanah. Skalar potensial tanah atau
gravitasi vector dan medan gravitasi. Kuantitas berkelanjutan, seperti
titik tanah, pixel, jaringan (network), dimana semua kuantitas
terbentuk secara alami.
KONSEP DASAR GEODESI FISIS
Gravity field theory uses a number of tools from mathematic and physic :
- Newtonian Gravitation theory (relativity is not required for now)
- Potential theory
- Vector calculus
- Special function (legendre)
- Boundary Value problem
- Signal Processing
gravity field theory is interacting with many other discipline. The earth
science disciplines are rather operating on a global scale, whereas the
engineering applications are more local. This distinction is not fundamental
KONSEP DASAR GEODESI FISIS
Links to earth sciences
1. oceanography.
The earth’s gravity field determines the geoid, which is
equipotential surface at mean sea level. If the ocean would be at restno waves, no current, no tides- the ocean surface would coincide with
geoid. In reality it deviates by up to 1 m. the difference is called sea
surface topography. It reflected the dynamical equilibrium in the ocean.
Only scale currents can sustain these deviations.
KONSEP DASAR GEODESI FISIS
2. Geophysics.
The earth’s gravity field reflected the internal mass distribution, the
determination of which is one of the tasks geophysics. By itself gravity
field knowledge is insufficient to recover this distribution. A given
gravity field can be produced by an infinity of mass distributions.
Nevertheless, gravity is an important constraint, which is used together
with seismic and other data.
As an example, consider the gravity filed over a volcanic island like
Hawaii, a volcano by itself represent a geophysical anomaly already,
which will have a gravitational signature.
KONSEP DASAR GEODESI FISIS
3. Geology
Different geological formations have different density structures and
hence different gravity signals. One interesting example this is the
Chicxulub Crater, partially on the Yucatan Peninsula (Mexico) and
Partially in Gulf of Mexico. This crater with a diameter 180 km was
caused by meteorite impact. Which occurred at the K-T boundary
(cretaceous_tertiary) some 66 million years ago. This impact is thought
to have cause extinction of dinosaurs. The Chicxulub crater was
discovered by careful analysis gravity data.
KONSEP DASAR GEODESI FISIS
4. Hydrology
Minute changes in gravity field over time- after correcting for other
time variable effect like tides or atmospheric loading-can be attributed
changes in hydrological parameters; soil moisture, water table, snow
load. For static gravimetry these are usually nuisance effect. Nowadays,
with precise satellite techniques, hydrology is one of the main aims of
spaceborne gravimetry. Despite a low spatial resolution, the result of
satellite gravity missions may be used to constrain basin-scale
hydrological parameters.
KONSEP DASAR GEODESI FISIS
5. Glaciology and sea Level.
The behavior of the earth’s ice masses is critical indicator of global
climate change and global sea level behavior. Thus, monitoring of
melting of Greenland and Antarctica ice cap is an important issue, the
ice cap are huge mass loads, sitting on the earth’s crust, which will
necessarily be depressed. Melting causes a rebound of the crust. This
process is still going on since the last ice age, but there is also an
instant effect from melting taking place right now. The change in
surface ice contains a direct gravitational components and an effect,
due to the uplift. Therefore, precise gravity measurements carry
information on ice melting and consequently on sea level rise.
Application in Engineering
1. Geophysical Prospecting.
Since gravity contains information on the subsurface density structure,
gravimetry is a standard tool in the oil and gas industry and other mineral
resources for that matter) it will always be used together with seismic
profiling, test drilling and magnetometry. The advantages of gravimetry over
these other techniques are :
- relatively inexpensive,
- Non dectructive (one can easily measure inside buildings)
- Compact equipment, e.g for borehole measurements
Gravimetry is used to localize salt domes or fractures in layers, to
estimate depth, and in general to get first ide of the subsurface structure.
Application in Engineering
2. Geothecnical Engineering
In order to gain knowledge about the subsurface structure,
gravimetry is valuable tool for certain geotechnical (civil) engineering
project. One can think of determining the depth-to-bedrock for the
layout of tunnel. Or makng sure no subsurface voids exist below the
planned building site of nuclear power plant.
Application in Engineering
3. Geomatics Engineering
most surveying observables are related to gravity field.
1. After leveling a theodolite or total station, its vertical axis is
automatically aligned with the local gravity vector. Thus all
measurement with these instrument are reference to gravity
field. They are in local astronomic frame. To convert them to a
geodetic frame the deflection of the vertical
and
perturbation in azimuth (ΔA) must be known.
2. The line of sight of a levels is tangent to the local equipotential
surface. So levelled height differences. To obtain precise height
differences one should also use a gravimeter.
• Level is tangen to the local equipotensial surface
• 3. GPS Positioning
• 4. Inertial surveying