Aplikasi Metode Inversi Damped Least-square untuk Pemodelan 1-D Data TEM

Abstract

ABSTRAK

Metode TEM (transient electromagnetic) merupakan metode alternatif yang dapat digunakan untuk mencari sumber air tanah selain metode geolistrik. Prinsip metode TEM adalah menginduksikan energi elektromagnetik ke dalam tanah dengan menggunakan transmitter loop dan receiver koil yang merekam respon bawah permukaan ketika transmitter dalam keadaan mati (tidak ada medan primer). Pemodelan data TEM pada umumnya masih menggunakan model 1-D. Penelitian ini akan membahas mengenai metode Inversi damped least-square untuk pemodelan inversi 1-D dari data TEM. Metode damped least-square membutuhkan parameter model awal untuk selanjutnya dilakukan inversi damped least-square menggunakan teknik SVD (Singular Value Decomposition). Metode ini diaplikasikan terhadap tiga data sintetik. Data sintetik I menunjukkan nilai resistivitas semakin kecil seiring bertambahnya kedalaman. Data sintetik II menunjukkan nilai resistivitas semakin besar seiring dengan bertambahnya kedalaman. Data sintetik III merupakan respon model yang mewakili daerah prospek airtanah, memperlihatkan lapisan konduktif diapit oleh lapisan resistif dan sebaliknya. Metode damped least-square juga diaplikasikan pada data lapangan daerah karst Kabupaten Tuban. Hasil pemodelan data sintetik memperlihatkan kesesuaian yang optimum antara data resistivitas semu observasi dan perhitungan. Sedangkan, hasil pemodelan data lapangan memerlihatkan kesesuaian model dengan kondisi geologi setempat.

Kata kunci: metode damped least-square, singular value decomposition dan transient electromagnetic.

ABSTRACT

Transient electromagnetic (TEM) sounding method is an alternative method other than the geoelectrical method that can be used for groundwater exploration purposes. The principle TEM method is done by the induction of electromagnetic fields at the surface within a transmitter loop and record the subsurface response in the receiever coil when the absence of the primary field (transmitter off). TEM data are usually interpreted in term of 1-D model. The damped least-square method is presented in this paper. The damped least-square method needs an initial model parameter and employs the damped least-square solution with Singular Value Decomposition (SVD) technique. This methods have been applied to three synthetic data sets. The first synthetic data shows decreasing resistivities with depth. The second synthetic data shows increasing resistivities with depth. The third synthetic data are a response of groundwater prospect model, consisting of a conductor layer between two resistors and resistor layer between two conductors. Damped least-square methods was also applied to field data from karst area, Tuban. The results from synthetic data inversions presented good fit between the observed and the calculated apparent resistivity data. The results from field data show models subsurface resistivity consistent with the local geology.

Keyword : damped least-square method, singular value decomposition and transient electromagnetic.