[1] Merapi is a high-risk andesitic volcano in Central Java, Indonesia. Very little information is known about the detailed regional density structure around Merapi and its neighbor volcano Merbabu. We compute a subsurface three-dimensional (3-D) model of anomalous density for the volcanoes Merapi and Merbabu in Central Java, Indonesia, by inversion of the gravity field. As input for the inversion methodology, gravity observations from 443 points, whose 3-D coordinates are determined by GPS, are used. The inversion algorithm is based on an explorative approach to fit a least squares condition, including a balancing factor between the minimization of the residuals and the anomalous mass. A mean density about 2242 kg/m 3 for the region of Merapi and Merbabu has been computed by least squares adjustment. Results of the inversion show major low-density contrasts up to À242 kg/m 3 and positive structures about +264 kg/m 3 , referred to the determined mean density. A density anomaly (relative high) with densities up to +264 kg/m 3 is connecting the volcanoes in a 152°course from NW to SE and might be built of older basaltic lava. Low-density contrasts about À242 kg/m could be found in agreement with magnetotelluric and electromagnetic results. Generally, the identified highand low-density bodies are in agreement with the results of other geophysical methods such as electromagnetic and magnetotelluric prospecting or geological formations and structures. A porosity about 21% is derived for the largest negative density bodies about À242 kg/m 3 . Furthermore, the density model gives some new information about the controversial origin of a hill formation near Merapi and is also used to discuss the possible existence of a shallow magma chamber, which is also a controversial subject. Generally, the density model serves as basic information for the interpretation of geodetic and geophysical observations and confirms existing results from magnetotellurics, electromagnetics, and seismic data interpretation.Components: 6265 words, 8 figures.

Typically, the mitigation of coastal erosion is achieved by amending surface conditions using materials, such as concrete. The objective of this study is to evaluate the feasibility of constructing artificial beachrocks using natural materials (e.g., microbes, sand, shell, pieces of coral, and seaweed, etc.) within a short time, and to propose the method as a novel strategy for coastal protection. Initially, a survey on resistivity and a multichannel analysis of seismic waves (MASW) were conducted along the coastal lines to characterize and elucidate the subsurface structure of existing beachrocks in the Southeast Yogyakarta coastal area, Krakal-Sadranan beach, Indonesia. The field survey on natural beachrocks suggested that both resistivity and shear wave velocity were higher in the deeper deposits compared to the underlying unconsolidated sand layer within a depth of approximately 1.5 m and covering an area of 210.496 m 2 for the α-section and 76.936 m 2 for the β-section of beachrock deposit. The results of the sand solidification test in the laboratory showed that treated sand achieved unconfined compressive strength of up to around 6 MPa, determined after a treatment period of 14 days under optimum conditions.

ABSTRAKPengukuran geolistrik telah dilakukan untuk mengidentifikasi litologi dan akuifer berdasarkan resistivitas batuan pada lokasi rencana pembangunan bendung dan terowongan di daerah Sulawesi Utara. Survei tiga lintasan menggunakan konfigurasi dipole-dipole dengan 32 elektroda (a = 5,5 m) dan kabel ekstensi 9 elektroda (a = 20 m). Analisa tomografi resistivitas menunjukan daerah survei tersusun oleh tanah berupa lempung pasiran (resistivitas rendah di permukaaan), breksi Tondano (30 -1000 Ωm), lapukan tufa Tondano (30 -80 Ωm) dan tufa Tondano (< 30 Ωm). Di area bendung pada bagian tepi sungai terdapat tanahtipis, breksi Tondano danlapisan tufa Tondano. Lapisan Tondano daerah terowongan telah mengalami pelapukan menjadi lempung pasiran dengan ketebalan hingga 15 m, sedangkan lapisan dibawahnya berupa breksi tondano yang telah lapuk menjadi pasir kelempungan dengan fragmen kerikil (40 -125 Ωm) hingga bongkah (200 -2000 Ωm) dengan ketebalan 10 -35 meter. Pada kedalaman 50 m ditemukan intrusi andesit, kekar-kekar pada andesit diisi oleh air.Terdapat setidaknya ada lima akuifer dengan kedalaman sekitar 50 meter diantara lokasi rencana surgetank sampai powerhouse.Kata kunci: Tomografi resistivitas, litologi, akuifer, bendungan, terowongan ABSTRACT Geoelectric measurements have been conducted to identify the lithology and aquifers based on rock resistivity at a dam and tunnel site plan in North Celebes using three-lines survey with 32 electrodes (a= 5,5 m) dipole-dipole configuration and 9 electrode extension cables (a=20 m). Based on electrical resistivity tomography (ERT), it can be interpreted that the survey area is composed of soil in the form of sandy clay (low resistivity), breccia Tondano (30-1000 Ωm), weathered of Tondano tuff (30-80 Ωm) and tondano tuff(<30 Ωm). In the area of the dam, there are thin soil, a layer of tuff and breccia tondano on the river banks. Tondano layer in the tunnel area has experienced weathering into sandy clay with a thickness of up to 15 m, while layers below it has took the form of breccia, which has weathered into sandy loam with fragments of gravel (40-125 Ωm) to boulder (200-2000 Ωm) with a thickness of 10 -35 meters. Andesite intrusion was found at a depth of 50 m, with cracks on andesite filled by water. There are at least five of the aquifers at a depth of about 50 meters between the location of surgetank to the powerhouse plan.

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Abstrak -Telah dilakukan penelitian sebaran batuan aspal buton (asbuton) dengan menggunakan metode resistivitas dipoledipole. Penelitian ini bertujuan untuk memetakan sebaran asbuton secara 2-D maupun 3-D seta menghitung cadangan asbuton daerah penelitian. Penentuan sebaran asbuton didasarkan pada perbedaan nilai resistivitas asbuton dengan batuan di sekitarnya. Pengolahan dan pemodelan data resistivitas menggunakan software Res2Dinv, sehingga didapatkan model penampang resistivitas asbuton. Penentuan nilai asbuton daerah penelitian didasarkan pada lintasan yang melewati singkapan asbuton, yaitu antara 90 -600 Ωm. Sebaran asbuton setelah dilakukan pemodelan secara 2-D maupun 3-D banyak terdapat pada Formasi Tondo dengan bentuk asbuton berupa bolder dengan ketebalan bervariasi antara 1 -40 meter. Besar potensi cadangan asbuton dimasukkan dalam 2 kategori yaitu : cadangan terukur dan cadangan tereka. Besar potensi asbuton pada cadangan terukur dengan volume 6,1 juta meter kubik (m3) adalah 17,0 juta ton. Besar potensi asbuton cadangan tereka dengan volume 22,2 juta meter kubik (m3) adalah 62,1 juta ton.Abstract -Rock asphalt of buton (asbuton) research using dipole-dipole arrays resistivity method has been done. The purpose of this reserach is to modelling the distribution of asbuton reserve, based on different resitivity value between asbuton with the surroundings rocks. Processing and modelling resistivity value is using Res2Dinv software and the result is 2-D section resistivity of asbuton. True resistivity value which based on asbuton outcrop in research area is within 90 -600 Ωm in range. Asbuton distribution is known exist at Tondo Formation in boulders with variative thickness between 1 -40 metres in range after inverse modeling. Asbuton potential reserve could be classified into 2 categories, which are : measureable reserve and possibility reserve. Asbuton potential measurable reserve with volume I6,1 millions of metre cubic (m3) in mass is 17,0 millions of tons. Asbuton potential at possibility reserve with 2,2 millions of metre cubic (m3) in volume is 62,1 millions of tons.