PEMETAAN PERMEABILITAS MAGNETIK PERMUKAAN BERBASIS CITRA SAR POLARIMETRIK DENGAN PENGUKURAN IN SITU DI LAPANGAN GUNUNG API

Asep Saepuloh, Edo Kharisma Army, Zaki Hilman

Abstract


Pemetaan parameter fisis berupa permeabilitas magnetik permukaan pada zona lemah yang terkait dengan aktivitas tektonik maupun vulkanik perlu dilakukan seiring dengan meningkatnya aktivitas kegempaan maupun kegunungapian dalam beberapa tahun terakhir. Pendeteksian keberadaan zona lemah umumnya dilakukan berdasarkan parameter kelurusan struktur geologi, reflektansi spektra, dan temperatur permukaan dengan memanfaatkan citra satelit optis. Makalah ini mengolah parameter yang berbeda berupa permeabilitas magnetik permukaan (μr), yaitu parameter fisis yang diturunkan dari citra satelit Polarimetric Synthetic Aperture Radar (PolSAR) dengan polarisasi penuh (quad-polarization) dan diverifikasi dengan pengukuran in situ. Parameter μr permukaan merupakan salah satu parameter yang digunakan untuk mengidentifikasi keberadaan zona lemah jalur fluida termal dari bawah sampai ke permukaan gunung api aktif. Aktivitas hidrotermal mengakibatkan degradasi nilai magnetik batuan di sekitarnya, sehingga ekstraksi nilai permeabilitas magnetik permukaan μr dianggap sangat efektif untuk mengidentifikasi jalur fluida termal di permukaan. Modifikasi pada metode dielectric from Polarimetric Synthetic Aperture Radar (dPSAR) menggunakan citra PolSAR dilakukan untuk mendapatkan parameter permeabilitas magnetik permukaan μr, yaitu kemampuan permukaan tanah dalam menerima respon medan magnet, disamping permitivitas dielektrik εr. Pengukuran μr in situ di lapangan menggunakan alat ukur Ferromaster Magnetic Permeability Meter dilakukan untuk menganalisis tingkat akurasi μr dari hasil inversi dPSAR. Berdasarkan uji validasi, nilai koefisien determinasi R2 yang diperoleh cukup tinggi, yaitu sekitar 0,73 antara μr hasil dPSAR dan pengukuran lapangan. Selain itu, hasil uji normalitas menunjukkan nilai residual antara keduanya terdistribusi normal. Hal ini mengindikasikan bahwa pendeteksian μr dengan metode dPSAR memiliki potensi untuk dikembangkan dan diaplikasikan lebih lanjut dalam pemetaan fisis geologi di permukaan.

ABSTRACT - Surface Magnetic Permeability Mapping Using Sar Polarimetric and In Situ Measurement at Volcanic Field. Physical parameters mapping of surface magnetic permeability in weak zones associated with tectonic and volcanic activity is necessary due to the increasing seismic and volcanic activity in recent years. Typically, weak zones are detected from lineament related geological structures, reflectance spectra, and surface temperature visually on optical satellite images. This paper processed a different physical parameter of surface magnetic permeability (μr) derived from the Polarimetric Synthetic Aperture Radar (PolSAR) quad-polarization satellite image that was verified by field measurement. The μr was used to identify the weak zones connected to the fluid path of the subsurface hydrothermal system of an active volcano. The activity of hydrothermal fluids through the weak zones is responsible for the degradation of the magnetic permeability of the host rocks so that extracting μr is effective in identifying fluid path at the surface. A modification has been applied to the dielectric from Polarimetric Synthetic Aperture Radar (dPSAR) method using the PolSAR image by inverting the μr, in addition to εr parameters, that served as capability of the ground surface responds to the magnetic field. Measurement of μr in the field using Ferromaster Magnetic Permeability Meter was carried out to analyze the accuracy of the inverted μr from the dPSAR method. According to the validation test of μr that derived from dPSAR and field measurement, a high coefficient determination R2 of 0.73 was achieved. In addition, the normality test showed that residual values were distributed normally. Therefore, the μr inversion using dPSAR method has the potential to be developed and applied for advanced physical surface geological mapping.



Keywords


PolSAR, dPSAR, geology, magnetic, thermal

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DOI: http://dx.doi.org/10.14203/risetgeotam2020.v30.1080

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