Analisis anomali gravitasi untuk menafsirkan struktur bawah permukaan di daerah pegunungan kapur utara bagian barat, Jawa Tengah

AVISENA, Novi, Dr. H. Wahyudi

2007 | Tesis | S2 Ilmu Fisika

Telah dilakukan penyelidikan geofisika dengan metode gravitasi di Wilayah Pegunungan Kapur Utara bagian Barat propinsi Jawa Tengah mencakup area penelitian seluas 33,8 x 49 km2 dengan 146 titik ukur. Alat yang digunakan adalah Gravimeter LaCoste & Romberg tipe G-1118 MVR. penentuan posisi dan ketinggian digunakan GPS Trimble Navigation 4600 LS. Pengolahan data standar dilakukan pada data. Penentuan densitas Bouguer menggunakan metode analitik Nettleton dihasilkan densitas Bouguer 2,67 gr/cm3. Anomali Bouguer lengkap pada topografi diproyeksikan pada bidang datar dengan pendekatan deret Taylor pada ketinggian rata-rata 446 m. Anomali lokal-regional dipisahkan dengan metode kontinuasi ke atas dengan ketinggian 9000 m diatas spheroida acuan. Interpretasi anomali regional dan lokal secara kuantitatif menggunakan metode inversi kompak 2-D menghasilkan model regional berupa sinklin dan antiklin dan model lokal menunnjukkan adanya sesar yang diinterpretasikan sebagai kenampakan sesar Lasem.

Geophysical investigation by gravity method has been carried out in Northern Karst Mountain western area of Central Java. The survey consisted of approximately 146 gravity stations spaced at 3-km to 4.5-km intervals distribution in the area 33.8 x 49 km2. This work utilized a LaCoste-Romberg gravimeter which its accuracy of 0.005 mgal. Geographical position and elevation points were measured by Trimble Navigation 4600 LS GPS. Standard field corrections were applied to the data. Bouguer and terrain corrections were made using a density determined from a gravity profile along a traverse of marked topographic relief using Nettleton analytical method, and achieved Bouguer density of 2.67 gr/cm3. Complete Bouguer anomaly at irregular surface was achieved after rigorous reduction of observed data. The complete Bouguer anomaly was projected from topographic surface to horizontal plane using Taylor series approach at average elevation 446 m above mean sea level. Finally, a regional trend was defined and removed by upward continuation method at elevation 9000 m above mean sea level. Interpretation of regional Bouguer anomaly using 2-D compact inversion method yields sincline and anticline structures, density of the body anomalies are 2.67 gr/cm3 and 3.1 gr/cm3 interpreted as basaltic rock. Interpretation of local Bouguer anomaly using 2-D compact inversion method yield a thrust fault structure and interpreted as Lasem fault.

Kata Kunci : Anomali Gravitasi,Struktur Bawah Permukaan,Pegunungan Kapur, Northern Karst Mountain, subsurface structure, Bouguer anomaly