PENENTUAN STRUKTUR BAWAH PERMUKAAN BUMI DANGKAL DENGAN MENGGUNAKAN METODA GEOLISTRIK TAHANAN JENIS 2D TESIS

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PENENTUAN STRUKTUR BAWAH PERMUKAAN BUMI DANGKAL DENGAN MENGGUNAKAN METODA GEOLISTRIK TAHANAN JENIS 2D (Studi Kasus Potensi Tanah Longsor di Panawangan, Ciamis)

TESIS Karya tulis sebagai salah satu syarat Untuk memperoleh gelar Magister dari Institut Teknologi Bandung

Oleh

SYAMSUDDIN NIM : 22304009 Program Studi Geofisika Terapan

INSTITUT TEKNOLOGI BANDUNG 2007

PENENTUAN STRUKTUR BAWAH PERMUKAAN BUMI DANGKAL DENGAN MENGGUNAKAN METODA GEOLISTRIK TAHANAN JENIS 2D (Studi Kasus Potensi Tanah Longsor di Panawangan, Ciamis)

Oleh

SYAMSUDDIN NIM : 22304009

Program Studi Geofisika Terapan Institut Teknologi Bandung

Menyetujui Tim Pembimbing Tanggal, 09 April 2007.

Pembimbing Pertama

Pembimbing Kedua

____________ ___________ (Dr. rer.nat. RM Rachmat Sule)

_________________ (Dr. Tedy Setiawan)

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PEDOMAN PENGGUNAAN TESIS Tesis S2 yang tidak dipublikasikan terdaftar dan tersedia di Perpustakaan Institut Teknologi Bandung, dan terbuka untuk umum dengan ketentuan bahwa hak cipta ada pada pengarang dengan mengikuti aturan HaKI yang berlaku di Institut Teknologi Bandung. Referensi kepustakaan diperkenankan dicatat, tetapi pengutipan atau peringkasan hanya dapat dilakukan seizin pengarang dan harus disertai dengan kebiasaan ilmiah untuk menyebutkan sumbernya.

Memperbanyak atau menerbitkan sebagian atau seluruh tesis haruslah seizin Direktur Program Pascasarjana, Institut Teknologi Bandung.

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Dipersembahkan kepada: Istri Tercinda Niswati Jamil, S.Pd dan Anak Tersayang Zahira Salsabila Putrisyam

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UCAPAN TERIMA KASIH

Puji dan syukur penulis haturkan kehadirat Allah SWT, yang senantiasa melimpahkan Rahmat dan Karunia-Nyalah sehingga penulis dapat menyelesaikan penyusunan tesis ini dengan lancar. Adapun judul tesis ini adala PENENTUAN STRUKTUR GEOLOGI DANGKAL DENGAN MENGGUNAKAN METODA GEOLISTRIK TAHANAN JENIS 2D (Studi kasus: Potensi Tanah Longsor di Panawangan, Ciamis). Dalam tesis ini, dibahas tentang kemampuan metoda Geofisika (Geolistrik Tahanan Jenis 2D) untuk mendeteksi bidang gelincir tanah longsor.

Sehubungan dengan selesainya penyusunan tesis ini, maka penulis sepatutnya mengucapkan terima kasih kepada beberapa pihak antara lain: 1. Bapak Dr. rer-nat RM Rachmat Sule, selaku pembimbing pertama dan Dr. Tedy Setiawan, selaku pembimbing kedua yang senatiasa memberikan tuntunan dan arahan mulai dari proses penelitian hingga penyusunan tesis ini. 2. Bapak Dr. rer.nat. Wahyudi W. Parnadi, sebagai ketua sidang dan Dr. Agus Laesanpura, Dr. Darharta Dahrin, dan Dr. Eng. TA Sanny, selaku anggota tim penguji dalam mencapai strata Magister. 3. Bapak, selaku Ketua Program Studi Geofisika Terapan dan Dr. Wawan Gunawan A. Kadir, selaku Pembantu Dekan I Fakultas Ilmu Kebumian dan Teknologi Mineral, serta Prof. Dr. Joko Santoso, salaku pemegang puncak pimpinan di ITB – Bandung 4. Bapak dan Ibu Dosen dalam program studi Geofisika Terapan, yang tak bosan-bosannya memberikan dan memindahkan ilmunya setetes demi setetes sampai penulis menyelesaikan studi di Program Pascasarjana ITB. 5. Para Pegawai program studi Geofisika Terapan, yang senantiasa memberikan informasi dalam memperlancar segala urusan. 6. Saudara Riky, Ari dan Maxi, serta Pak Agus Hidayat, yang membantu penulis dalam proses pengambilan data di lapangan. 7. Teman-teman angkata 2004 (K’ Ramdani, Uni Huriyah, Mba Ade, Mas Ardian, Bang Analiser, Mba Dian, Adik Novri, Pak Edi, Partner Iman),

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serta teman di kosan (K’ Rahim, K’ Ibrahim Puang Iwan, Puang Madi, Enye, Defa, Amrin, Pak Rustan, Pak Zainuddin, dll) yang senantiasa memberikan dorongan dan motivasi sehingga penulis dapat menyelesaikan tesis ini. 8. Yang terhormat Ibunda Yabang dan Ayahanda Malang, yang tak hentihentinya mendoakan dan memberikan dorongan moril maupun materil. 9. Istriku yang tercinta Niswati, yang menjadi motivator utama dan senantiasa berdoa untuk penyelesaian tesis ini. Dan anakku tersayang Zahira Salsabila Putrisyam, yang senantiasa menunggu untuk dibelai. 10. Dan seluruh teman-teman yang tidak sempat disebutkan namanya dalam tesis ini yang turut memberikan bimbingan, nasihat, dukungan, serta saran-saran demi kelancaran penyelesaian studi ini.

Sebagai manusia biasa, penulis menghaturkan maaf karena tesis ini tidak akan luput dari berbagai kesalahan dan kekurangan. Dan akhirnya penulis berharap semoga tesis ini ada manfaatnya bagi yang memerlukannya begitu pula bagi penulis. Terima kasih.

Bandung, 09 April 2007

Penulis

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ABSTRAK PENENTUAN STRUKTUR BAWAH PERMUKAAN BUMI DANGKAL DENGAN MENGGUNAKAN METODA GEOLISTRIK TAHANAN JENIS 2D (Studi Kasus Potensi Tanah Longsor di Panawangan – Ciamis, Indonesia) Oleh Syamsuddin NIM : 22304009 Salah satu jenis bencana alam yang sering terjadi di Indonesia adalah tanah longsor. Bencana tanah longsor yang telah terjadi di beberapa kawasan di Indonesia telah memakan korban yang tidak sedikit, baik jiwa maupun harta benda. Bencana tanah longsor terjadi karena adanya gangguan keseimbangan lereng secara gravitasional yang disebabkan oleh bertambahnya beban material pembentuk lereng. Air yang meresap ke bawah permukaan bumi akan tersimpan di dalam pori-pori batuan, sehingga akan menambah beban material tersebut. Akibatnya daya dukung (gaya kohesi) tanah jadi berkurang. Potensi tanah longsor dapat diteliti dengan memanfaatkan teknologi geofisika. Salah satu metoda geofisika yang dapat digunakan dalam penelitian potensi tanah longsor adalah metoda geolistrik tahanan jenis. Metoda geolistrik tahanan jenis (2D) secara profiling telah digunakan untuk menggambarkan kondisi bawah permukaan bumi, termasuk menentukan bidang gelincir longsoran. Metoda ini mendeteksi sifat kelistrikan bumi dan sangat peka terhadap material yang mengandung air. Konfigurasi elektroda yang digunakan adalah konfigurasi Wenner Alpha dan Wenner Beta. Dari beberapa kasus ”sintetik” dan ”riil” yang telah dilakukan, terlihat bahwa konfigurasi Wenner Alpha cukup sensitif dalam mendeteksi perubahan resistivitas bumi/model secara vertikal. Sedangkan konfigurasi Wenner Beta cukup baik sensitivitasnya, baik ke arah vertikal maupun lateral. Dengan demikian aplikasi kedua konfigurasi tersebut di lokasi penelitian akan mempertajam gambaran bawah permukaan bumi. Pengolahan data 2-Dimensi telah dilakukan dengan menggunakan perangkat lunak RES2DINV. Hasil penelitian menunjukkan adanya bidang gelincir yang ditandai oleh kontras resistivitas yang berkesinambungan membatasi blok material yang memiliki resistivitas tinggi dengan yang rendah. Bidang batas itu diperkirakan sebagai bidang kontak antara breksi vulkanik tua Formasi Cijulang yang lebih kompak dengan breksi vulkanik muda Hasil Gunungapi G. Sawal yang kurang kompak . Kesimpulan ini diperkuat oleh hasil survei GPR yang menggambarkan juga adanya reflektor yang dapat berfungsi sebagai bidang gelincir. Kata kunci: Tanah longsor, bidang gelincir, resistivitas, sensitivitas, dan inversi

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ABSTRACT DETERMINATION OF NEAR-SUBSURFACE BY USING 2D RESISTIVITY METHODS (Case Study of Landslide Monitoring at Panawangan Area – Ciamis, Indonesia) By Syamsuddin NIM: 22304009 One of the natural hazards that is often occurred in Indonesia is landslide. This hazard occurs normally in area which has high density of population. Thus, if they are happened could caused a big number of victims, damage and property loss. Landslide is a geological phenomenon which includes a wide range of ground movement, such as rock falls, deep failure of slopes and shallow debris flows. The hazards will happen if gravity’s action on an over-steepened slope is getting larger, which caused the material fall down from its initial equilibrium. The increase of water content inside earth material is the primary reason for a landslide. The potential of landslide can be detected by applying geophysical methods; one of them is 2D-resistivity method. This method has been used to determine the nearsubsurface structure of the earth including detection of landslide’s slip plane. This Method is an indirect method, in which the results of this method could give an overview of resistivity distribution inside subsurface. There are several kinds of electrode configurations. Two of them, namely wenner-alpha and wenner-beta, have been use in the study area at panawangan area, Ciamis. Base on synthetic study that has been carried out, wenner alpha configuration is sensitive in detecting resistivity changes is vertical direction. Where as, Wenner-beta configuration is sensitive in detecting resistivity changes in both vertical and horizontal directions. The results of inverted resistivity data showed the same characteristics as have been proved in synthetic study. Processing of resistivity data used RES2DINV software. The obtained results of the study show that some contracts between low and high resistivity values in some part of the sections could be the position of slip plane inside subsurface. Those contrasts are interpreted boundary between old volcanic breccias of Cijulang Formation (more compact) and young breccias of Gunung Sawal Formation (less compact). This interpretation has a good agreement with the result of Ground Penetrating Radar (GPR), which show the existence of strong reflection on the same location. Keyword: Landslide, slip-plane, resistivity, sensitivity, and inversion.

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DAFTAR ISI

HALAMAN JUDUL ···············································································

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HALAMAN PENGESAHAN ··································································

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HALAMAN PEDOMAN PENGGUNAAN TESIS ···································

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HALAMAN PERSEMBAHAN ·······························································

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UCAPAN TERIMA KASIH ····································································

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ABSTRAK ·····························································································

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ABSRACT ······························································································

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DAFTAR ISI ··························································································

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DAFTAR GAMBAR ···············································································

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DAFTAR TABEL ···················································································

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DAFTAR LAMPIRAN ············································································

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Bab I

Pendahuluan ·············································································

1

I.1

Latar Belakang Penelitian ····························································

1

I.2

Batasan Masalah ·········································································

4

I.3

Maksud dan Tujuan Penelitian ·····················································

4

I.4

Sistimatika Penulisan ··································································

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Bab II

Metoda Geolistrik Tahanan Jenis 2D ··········································

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II.1

Prinsip Dasar Metoda Resistivitas ·············································

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II.2

Potensial Pada Bumi Homogen Isotropis ···································

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II.3

Potensial Elektroda Arus Tunggal pada Permukaan Medium Isotropis ··················································································

II.4

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Potensial Dua Elektroda Arus pada Permukaan Homogen Isotropis ··················································································

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II.5

Konfigurasi Elektroda dan Sensitivitasi ·····································

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II.6

Model Sintetik ·········································································

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II.7

Hubungan parameter geolistrik dengan parameter gerakan tanah

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Bab III Tinjauan Daerah Penelitian ························································

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III.1.

Fisiografi dan Geomorfologi daerah ·········································

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III.2.

Stratigrafi dan Struktur Geologi ················································

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III.3.

Tata Guna Lahan ·····································································

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Gerakan Tanah ········································································

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Bab IV Akuisisi, Pengolahan dan Interpretasi Data ·································

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IV.1 Lokasi dan Waktu Penelitian ······················································

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IV.2 Peralatan ···················································································

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IV.3 Teknik Pengambilan Data ··························································

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IV.4 Pengolahan Data ·······································································

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IV.5 Interpretasi Data Resistivitas ······················································

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III.4.

Bab V

Korelasi Hasil-Hasil Penelitian Geolistrik Tahanan Jenis dengan Data Pendukung ········································································

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V.1. Hasil Metoda Geolistrik Tahanan Jenis ·········································

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V.2. Hasil Metoda GPR (Ground Penetrating Radar) ····························

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Bab VI Kesimpulan dan Saran ·······························································

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VI.1 Kesimpulan ···············································································

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VI.2 Saran ························································································

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DAFTAR PUSTAKA ··············································································

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DAFTAR GAMBAR

Gambar I.1.

Peta potensi tanah longsor di Jawa Barat dan Banten (Direktorat Vulkanologi dan Mitigasi Bencana Geologi, 2005) ······································································

Gambar II.1

Sumber arus tunggal di permukaan medium homogen isotropis (Loke, 2004) ·····················································

Gambar II.2

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Konfigurasi elektroda dalam eksplorasi geolistrik (Loke, 2004) ·············································································

Gambar II.6

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Kisaran resistivitas beberapa jenis batuan, tanah, dan mineral (Loke, 2004) ······················································

Gambar II.5

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Kisaran rata-rata harga resistivitas spesifik dan permitivitas relatif beberapa jenis batuan.(Schon, 1996) ······················

Gambar II.4

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Dua elektroda arus dan potensial di permukaan bumi homogen isotropis (Musa, 2004) ······································

Gambar II.3

3

Model

sintetik

yang

menunjukkan

sensitifitas

13

tiap

konfigurasi elektroda dalam eksplorasi geolistrik (Darlin & Zhou, 2004) ····································································

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Gambar II.7

Model sintetik satu blok ··················································

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Gambar II.8

Model penampang resistivitas semu konfigurasi Wenner Alpha ·············································································

Gambar II.9

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Hasil inverse dari model sintetik konfigurasi Wenner Alpha. (a) Resistifitas semu pengukuran, (b) Resistifitas semu perhitungan (respon model), (c) Hasil inversi ··········

Gambar II.10

Model penampang resistivitas semu konfigurasi Wenner Beta ···············································································

Gambar II.11

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Hasil inverse dari model sintetik konfigurasi Wenner Beta. (a) Resistifitas semu pengukuran, (b) Resistifitas semu perhitungan (respon model), (c) Hasil inversi ···················

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Gambar II.12

Model penampang resistivitas semu konfigurasi Pole-Pole

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Gambar II.13

Hasil inverse dari model sintetik konfigurasi Pole-Pole. (a) Resistifitas

semu

pengukuran

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(respon

model),

(b) Resistifitas semu perhitungan, (c) Hasil inversi ··········· Gambar II.14

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Model penampang resistivitas semu konfigurasi PoleDipole. (a) Forward Pole-Dipole dan (b) Reverse PoleDipole ············································································

Gambar II.15

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Hasil inverse dari model sintetik konfigurasi Pole-Dipole (a) ke depan dan (b) ke belakang, masing-masing (atas) Resistifitas semu pengukuran, (tengah) Resistifitas semu perhitungan (respon model), (bawah) Hasil inverse ···········

Gambar III.1.

Fisiografi Jawa Barat (van Bemmelen, 1949 dalam Martodjojo, 2003) ···························································

Gambar III.2.

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Kenampakan morfologi daerah penelitian dari citra satelit (Google Earth) ·············································································

Gambar III.3.

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Gambaran morfologi daerah penelitian ditandai dengan rapat-renggangnya kontur (Pusat vulkanologi dan Mitigasi Bencana Geologi Bandung, 2005) ····································

Gambar III.4.

Pola aliran sungai daerah enelitian (Pusat vulkanologi dan Mitigasi Bencana Geologi Bandung, 2005) ······················

Gambar III.5.

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Peta Geologi Daerah Penelitian (Pusat vulkanologi dan Mitigasi Bencana Geologi Bandung, 2005) ······················

Gambar III.6.

31

33

Keadaan daerah penelitian; (a) Morfologi perbukitan dengan berbagai macam tumbuhan, (b) tanaguna lahan sebagai persawahan, (c) kolam atau tambak air tawar sebagai salah satu kegunaan lahan ···································

Gambar III.7.

Jatuhan atau runtuhan batu (Pusat Vulkanologi dan Mitigasi Bencana Geologi, 2007) ·····································

Gambar III.8.

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Slides: a) Gerakan Blok Batu, dan b) Longsoran Rotasi (Pusat Vulkanologi dan Mitigasi Bencana Geologi, 2007) ·

Gambar III.9.

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36

Flows: 1) Longsoran translasi, 2) Aliran bahan rombakan, dan 3) Rayapan tanah (Pusat Vulkanologi dan Mitigasi Bencana Geologi, 2007) ··················································

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Gambar III.10. Beberapa ilustrasi jenis utama tanah longsor oleh Highland L. dan Johnson M. (USGS, 2004) ····································

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Gambar III.11. Gambaran kerusakan infrastruktur di sekitar lokasi penelitian ·······································································

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Gambar III.12. Lokasi stasiun pengamatan GPS (Peta kontur dibuat Pusan Vulkanologi dan Mitigasi Bencana Geologi, 2005) ··········· Gambar III.13. Skema pergerakan titik pantau dengan menggunakan GPS Gambar IV.1

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Alat yang dibutuhkan; (a) Resistivity meter McOHM, (b) GPS portable, (c) Kit connector multi channels ················

Gambar IV.3

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Peta Lokasi Penelitian (Pusat vulkanologi dan Mitigasi Bencana Geologi Bandung, 2005) ····································

Gambar IV.2

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45

Peta lokasi penelitian di dua kampung (a), yaitu kampung Kondang (b) dan Kampung Cirikip (c). (Peta Geologi dibuat oleh Pusat Vulkanologi dan Mitigasi Bencana Geologi Bandung, edisi tahun 2005) ································

Gambar IV.4

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a) Patok kayu dan elektroda yang telah dipasangkan kabel, b) Kit connector dihubungkan dengan kabel dari elektroda, c) Kit connector dihubungkan dengan resistivity meter

Gambar IV.5

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(a) Urutan elektroda untuk Wenner Alpha, (b) Urutan elektroda untuk Wenner, dan (c) Psedusection untuk konfigurasi Wenner Alpha. (Loke, 2004) ·························

Gambar IV.6

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Posisi patok 20 meteran (a) Lintasan L-1 s/d L4 di Kondang, (b) Lintasan L-5 s/d L-7 di Cirikip ····················

49

Gambar IV.7

Diagram Alir Penelitian ··················································

51

Gambar IV.8

Penampang lintasan geolistrik pada daerah persawahan di desa Cinyasag, kec. Panawangan, Ciamis – Jawa Barat. (Darso, 2005) ··································································

Gambar IV.9

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Profil 2D hasil inverse data geolistrik lintasan 1 di Kampung Kondang, Cinyasag, Kecamatan Panawangan, Ciamis – Jawa Barat. (a) Inversi Wenner Alpha, (b) Inversi Wenner Beta ···································································

Gambar IV.10 Profil 2D hasil inverse data geolistrik lintasan 2 di Kampung Kondang, Cinyasag, Kecamatan Panawangan, Kabupaten Ciamis – Jawa Barat (a) Inversi Wenner Alpha,

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53

(b) Inversi Wenner Beta ··················································

54

Gambar IV.11 Profil 2D hasil inverse data geolistrik lintasan 3 di Kampung Kondang, Cinyasag, Kecamatan Panawangan, Ciamis – Jawa Barat (a) Inversi Wenner Alpha, (b) Inversi Wenner Beta ···································································

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Gambar IV.12 Profil 2D hasil inverse data geolistrik lintasan 4 di Kampung Kondang, Cinyasag, Kecamatan Panawangan, Ciamis – Jawa Barat (a) Inversi Wenner Alpha, (b) Inversi Wenner Beta ···································································

56

Gambar IV.13 Profil 2D hasil inverse data geolistrik lintasan 5 di Kampung Cirikip, Cinyasag, Kecamatan Panawangan, Ciamis – Jawa Barat. (a) Inversi Wenner Alpha, (b) Inversi Wenner Beta ···································································

57

Gambar IV.14 Profil 2D hasil inverse data geolistrik lintasan 6 di Kampung Cirikip, Cinyasag, Kecamatan Panawangan, Ciamis – Jawa Barat. (a) Inversi Wenner Alpha, (b) Inversi Wenner Beta ···································································

58

Gambar IV.15 Profil 2D hasil inverse data geolistrik lintasan 7 di kampung Cirikip, Cinyasag, Kecamatan Panawangan, Ciamis – Jawa Barat. (a) Inversi Wenner Alpha, (b) Inversi Wenner Beta ···································································

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Gambar IV.16 Profil 3D Lintasan 1 – 4 di kampong Kondang dengan konfigurasi Wenner Alpha (α) ·········································

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Gambar IV.17 Profil 3D Lintasan 1 – 4 di kampung Kondang dengan konfigurasi Wenner Beta (β) ···········································

60

Gambar IV.18 Sayatan vertical profil 3D Lintasan 1 – 4 untuk konfigurasi Wenner α. (a) Sayatan arah Timur-Barat, (b) sayatan arah Utara-Selatan ··································································

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Gambar IV.19 Sayatan vertical profil 3D Lintasan 1 – 4 untuk konfigurasi Wenner β. (a) Sayatan arah Timur-Barat, (b) sayatan arah Utara-Selatan ·································································· Gambar V.1

Hasil Inversi L-6 yang menunjukkan bidang batas Formasi

xiv

62

Cijulang dan Hasil Gunungapi G. Sawal ··························

65

Gambar V.2

Lintasan pengambilan data GPR ······································

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Gambar V.3

Profil penampang radargram pada lintasan 6 ·····················

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Gambar V.4


67

Gambar V.5

Skema bidang gelincir longsoran pada lokasi pertama

Gambar V.6

(Kampung Kondang) ······················································

68


69

xv

DAFTAR TABEL

Tabel II.1

Setengah kedalaman yang diketahui (ze) untuk bentangan yang berbeda. L adalah panjang bentangan maksimum. Merujuk pada Gambar II.5 untuk konfigurasi elektroda dari bentangan yang berbeda. Faktor geometri untuk nilai “a” 1 meter. ···········

17

Tabel II.2

Hubungan resistivitas dengan porositas ·································

25

Tabel II.3

Kisaran porositas bahan sedimen ··········································

25

Tabel II.4

Diameter ukuran butir rata-rata, densitas dan porositas dari beberapa jenis sedimen; teras kontinen (shelf dan slope); ·······

26

Tabel III.1 Jenis-jenis tanah longsor menurut versi Varnes, 1978 ·············

38

xvi

DAFTAR LAMPIRAN

LAMPIRAN A1 Tabel Koordinat patok 20m lintasan pengukuran Geolistrik ·······················

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LAMPIRAN A2 1. Tabel Data Koordinat Setiap Titik Pantau per Periode ··························

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2. Tabel Pergeseran Posisi Setiap Stasiun/Titik Pantau dari Periode ke Periode ······························································································

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LAMPIRAN B1 Kolom Stratigrafi Daerah Penelitian ·························································

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LAMPIRAN B1 1. Hasil Inversi Wenner Alpha Lintasan L-1 ···········································

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2. Hasil Inversi Wenner Alpha Lintasan L-2 ···········································

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78


79


80


81


82

8. Hasil Inversi Wenner Beta Lintasan L-1 ··············································

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84


85


86


87


88


89

xvii

PENENTUAN STRUKTUR BAWAH PERMUKAAN BUMI DANGKAL DENGAN MENGGUNAKAN METODA GEOLISTRIK TAHANAN JENIS 2D TESIS

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