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 rest- no 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