The Application of Wave – Equation Datuming to 3D VSP Processing

Abstrak Vertical Seismic Profile (VSP) memperluas aplikasi seismik lubang bor dari hubungan 1D antara waktu-kecepatan hingga citra 2D atau 3D di sekitar lubang bor. Citra seismik yang dihasilkan dari VSP diharapkan memiliki resolusi vertikal yang lebih tinggi dari data seismik permukaan karena gelombang seismik direkam di dalam lubang bor. Namun, pengolahan data VSP 2D dan 3D memiliki tantangan karena sifat asimetri dari penjalaran gelombang membatasi untuk diterapkannya pengolahan data berbasis Common Mid Point (CMP) seperti analisis kecepatan, Normal Moveout (NMO), dan koreksi statik. Penelitian ini mendiskusikan sebuah pendekatan baru untuk mentransformasikan rekaman gelombang P pantul atau upgoing wavefields ke sebuah datum datar di permukaan berbasis persamaan gelombang. Transformasi tersebut menghasilkan gelombang seismik yang seolah-olah direkam pada pseudo-reveiver di permukaan bumi sehingga pendekatan penglahan data berbasis CMP dapat diterapkan. Konsep ini kemudian diterapkan pada sebuah data VSP 3D yang diakuisisi dengan geophone yang ditempatkan sangat dekat dengan permukaan. Hasil pemodelan elastik 2D menunjukkan bahwa gelombang seismik pantul sangat dipengaruhi zona kecepatan rendah di dekat permukaan. Jarak yang jauh antara reflektor target dengan geophone menghasilkan rasio sinyal terhadap bising yang rendah. Kondisi desain akuisisi ini sangat mempengaruhi hasil akhir dari citra VSP 3D. Walaupun begitu, citra VSP 3D yang dihasilkan berdasarkan hasil transformasi gelombang P upgoing ini menunjukkan korelasi yang cukup baik dengan data seismik permukaan di zona reservoir. Kata kunci: 3D VSP, transformasi datum, persamaan gelombang, gelombang pantul, pemodelan

Fault Related Stress and Fractures Analysis using the Anisotropy Signatures from Azimuthal Amplitude Variation in Lematang Trend, South Sumatera Basin

Evaluasi reservoir terrekahkan harus didasari pengetahuan tentang arah rekah dominan dan stress horizontal yang berlaku pada area studi demi mendapatkan rekomendasi arah pengeboran yang optimal. Namun demikian, informasi tentang stress yang banyak ditemukan adalah pada skala regional, global, maupun skala sumuran. Dalam hal pemodelan pada skala reservoir, data seismic dapat dijadikan penghubung kedua skala yang berjauhan tersebut. Kehadiran rekahan dan stress pada batuan dapat menyebabkan karakter anisotropi di reservoir. Pada skala seismic, sifat anisotropi sebuah media dapat diamati dari perubahan amplitude dan waktu tempuh terhadap azimuth rambat dan pantulan gelombang seismic. Pada makalah ini, kami menggunakan data seismic pre-stack selangkah di depan metode Amplitude Versus Offset (AVO) dan Extended Elastic Impedance (EEI) yang masih mengandalkan perubahan amplitude terhadap sudut pantul, dengan mengamati perubahan amplitude terhadap azimuth (AVAZ). Untuk pencapai tujuan tersebut, kami memproses ulang data seismic 3D dengan menggunakan metode Offset Vector Tile (OVT) sehingga informasi azimuth tersimpan dengan baik. Informasi yang digunakan untuk menginterpretasi orientasi rewkahan dan tress horizontal maximum adalah orientasi dan magnitude anisotropi. Hasil studi ini menunjukkan rekahan dan stress yang bersifat local dapat dengan yang bersifat regional dan global. Hal ini menunjukkan bahwa arah pengeboran optimal tidak harus dikontrol oleh arah stress global.

Karakterisasi Akuifer Berumur Miosen di Daerah Geologi Kompleks Thrust Fault Zona Kendeng dengan Rock Physics dan Tomografi Resistivitas

Kondisi geologi di Zona Kendeng ini cukup kompleks dengan dominasi lapisan yang berumur tua dan struktur sesar naik (thrust fault) yang intensif menyebabkan sulitnya memenuhi kebutuhan air bersih dari akuifer air tanah di daerah ini. Karakter akuifer di daerah ini berupa batuan napal karbonat formasi Kerek berumur Miosen.Analisis akuifer pada penelitian ini meliputi uji resistivity rock physics, pengukuran sudut kontak, dan pengukuran resistivitas tomografi di lapangan.Pengukuran resistivity rock physics menghasilkan nilai resistivitas batuan pada keadaan kering dan pada keadaan tersaturasi air tanah. Nilai resistivitas ini menjadi referensi untuk karakterisasi citra tomografi resistivitas. Sedangkan nilai sudut kontak menunjukkan apakah air akan merambat ke atas ataukan tertekan lebih dalam. Hasil pengukuran sudut kontak yang selalu bersudut kecil memperlihatkan bahwa sistem kapilaritas di akuifer napal karbonat formasi Kerek ini mendukung untuk menjadi akuifer yang baik. Air cenderung terangkat keatas. Karakterisasi terhadap akuifer air tanah dengan metode tomografi resisitivitas berhasil mencitrakan sejumlah lokasi akuifer air tanah yang potensial meskipun dengan struktur yang relatif didominasi lapisan tegak. Permasalahannya karena akuifer napal karbonat ini dikelilingi oleh lapisan clay berumur Miosen yang impermeabel, maka recharge akuifer di daerah ini justru sangat mengandalkan akuifer dengan struktur-struktur berarah tegak.

A Python Based Multi-Point Geostatistics by using Direct Sampling Algorithm

Multi-Point Geostatistics (MPS) is a type of geostatistical method used to estimate the value of an unsampled location by utilizing several data points around it simultaneously. The MPS method estimates it by defining a model based on initial data in the form of a training image, which is a collection of data in the form of a geological conceptual model in the research area with the integration of geological and geophysical knowledge. The MPS method is currently starting to develop because it differs from conventional covariance-based geostatistical methods such as simple kriging and ordinary kriging, which only use a variogram based on the relationship between two points rapidly. In this study, we evaluated the use of the MPS method by using a direct sampling algorithm with Python that will directly sample the training image and then retrieve the data based on the sample data. A braided channel training image is used as the initial model to estimate the distribution of reservoir properties in lithology with sand and shale types. This study shows that MPS could reconstruct geological features better than kriging.

Study of P and S Wave Quality Factor (Qα and Qβ) Around Mt. Jailolo

Mt. Jailolo is a B type volcano that has never erupted after 1600. Seismic activities around Mt. Jailolo have never been recorded until the swarm in November 2015. Several studies have been done to determine thecause of the swarm, but it is not certain whether the cause of the swarm is tectonic or volcanic activities. The study of attenuation characteristics has never been carried out in the area around Mt. Jailolo. Attenuation characteristics are important to provide the medium information which seismic waves pass through and it can also be applied to the volcanic areas as preliminary disaster mitigation. The main objective of this study is to analyze attenuation characteristics often expressed by Quality factor (Q-factor) of P and S seismic wave (Qα and Qβ), which are inversely proportional to attenuation factor (1/Q). Calculations of Qα and Qβ are obtained using coda normalization method. The study area location is around Mt. Jailolo at 127.3◦ - 127.6◦E and 0.9◦ - 1.2◦ N. Data have been collected with 12 Short Period temporary 7G sensors network belongs to GFZ and BMKG. This study uses 147 swarm events from the sensors with a threshold magnitude of Mw< 5.0, during April 2017. The study obtains Qα(f) = 9.61814f 1.12981 and Qβ(f) = 19.10690f 1.22843. The current analysis concludes that the attenuation beneath Mt. Jailolo corresponds to the volcanic swarms which may have been triggered by its deeper layer’s magmatic activity.

Analysis of H/V Ratio Curve to Estimate Seismic Hazard Vulnerability in Lombok Island, West Nusa Tenggara, Indonesia

In 2018, Lombok Island was hit by a series of destructive earthquakes. According to Indonesian Meteo- rological, Climatological, and Geophysical Agency data, about 1,973 felt earthquakes (M > 3) which shaken Lombok were recorded during August 2018 with three earthquakes with the largest magnitude of 6.9 Mw, 6.8 Mw, and 6.2 Mw. National Board for Disaster Management reported about 555 deaths, 1,833 people injured, and 186,010 houses damaged as a result of Lombok earthquake on August 5th, 2018. A number of seismometers were placed on Lombok Island from August 3rd, 2018 to October 19th, 2018 to monitor the aftershock events. There are 17 stations that record seismicity in Lombok consisting of 10 broadband sensors and 7 short period sensors. In this study, we used the Horizontal-to-Vertical Spectral Ratio (HVSR) method to analyze the risk of earthquake in the Lombok region. The basic concept of this method is to do a comparison between the horizontal component spectrum and the vertical component spectrum of a wave, where theoretically the particle movement of the horizontal component is greater than the particle movement of the vertical component on soft ground, whereas on the hard ground both components (horizontal and vertical) will be similar. H/V curve obtained from earthquake record- ings (Earthquake Horizontal-to-Vertical Ratio/EHVR) and H/V curve obtained from microtremor recordings (Microtremor Horizontal-to-Vertical Ratio/MHVR) shows good agreement and high correlation. Empirical correction of EMR (Earthquake-to-Microtremor Ratio) managed to decrease the difference of estimation of predominant frequency and amplification factor between EHVR and MHVR. Predominant frequency, amplification, and seismic vulnerability map agree with the geological condition of Lombok Island, where high value of amplification and seismic vulnerability was found on soft and thick ground. This study conclude that the maximum ground acceleration and the construction of the building should also considered when one wants to investigate the effect of an earthquake to the damage occurred, beside the site effects.

Ambient Noise Analysis During Nyepi in Denpasar Using Horizontal-to-Vertical Spectral Ratio (HVSR) Method

Nyepi Day is a unique tradition where outdoor human activities stop 24 hours a day. Denpasar City is the region that has the most significant impact on this change because it is the capital province. This study aims to determine the effect of Nyepi on ambient noise in Denpasar on March 25, 2020. We installed a TDS sensor at the Denpasar Geophysics Station for 3x24 hours ie when Nyepi Day, before and after Nyepi as comparative data. The data is processed by the HVSR method to get the value of dominant frequency and amplification factor every hour. Compared to the day before and after Nyepi, the dominant frequency increased during Nyepi and the amplification factor decreased during Nyepi. Seismic vulnerability index value at Nyepi is 24 where there is a decrease of index 3,904 with a day before and after Nyepi. This is showed that Nyepi Day affected ambient noise in Denpasar.

Coulomb Stress Change of Mw 7.5 Palu-Donggala Earthquake, Sulawesi (28 September 2018)

The Mw 7.5 Palu earthquake that occurred in Palu-Central Sulawesi, Indonesia, on September 28, 2018, accompanied by the tsunami and liquefaction caused casualties and building damage in the city of Palu and its surroundings. One month later, a series of earthquakes swarm occurred in Mamasa, West Sulawesi. In this study, coulomb stress were calculated using a half-space elastic model in a square plane which is assumed to be homogeneous isotropy to analyze whether there is a relationship between earthquakes that occur in Palu and earthquakes swam in the coulomb stress field changes. The results show that the area that experienced a stress reduction predominantly towards the north and south of the mainshock hypocenter, while the aftershocks were at an increase in coulomb stress changes, so that the Mamasa earthquakes swarm probably have been triggered by the Palu earthquake.

Analysis and Zonation of Land Vulnerability Areas in Pekon Karangrejo Ulubelu Tanggamus Using Microzonation Method

Ulubelu Tanggamus is an area that have geothermal energy potential. This region consists of a geological structure in the form of graben and reverse fault formed between Mount Rendingan and Mount Kukusan. Identifications that can be done include surveys to map soil characteristics in shaking responses using microzonation methods. This study aims to analyze the values ​​of dominant frequency, dominant period, Vs30 and amplification. The stages of the research carried out are processing data to obtain dominant frequency, calculating the value of the dominant period, calculating the value of Vs30 and the amplification value, and making a map of the soil vulnerability of the UlubeluTanggamus area. Based on the distribution of the dominant frequency values, the UlubeluTanggamus geothermal area is dominated by a dominant Frequency (F0) >0.5Hz which is expected to be a very thick surface thickness of sediments more than 30m. The distribution of the dominant period value obtained is (T0)>1 where the character of the sedimentary rock type is very soft consisting of alluvial material formed from sedimentation of deltas, top soil, and mud. The distribution of the Vs30 value is dominated by the value of 100 <Vs30<200 and obtains an amplification value of 2<A0<6 times where the area is an area with a moderate risk category to the danger of soil vulnerability.

Study of Calculation of Terrain Correction Using square pattern and sloped triangle Method in Karangsambung Area

Conventional assessments of terrain correction are carried out by laying out transparent paper containing the Hammer chart on topographic maps, then estimating the elevation for each compartment. But this procedure has disadvantages, the number of compartments are too small for area with many topographic variations, and there is a subjectivity from the observer in estimating the compartments height. This research aim to overcome these problems and get more accurate terrain correction value. In this research, estimation of terrain correction carried out using square pattern and sloped triangle method. This method divides the area around the measurement point into a zone containing a square-shaped and triangle compartment. The research start with testing the program by using synthetic data to see the effect of rock bodies on terrain correction value. Then the program was applied to Karangsambung to see the topographic influence around Karangsambung on terrain correction. The program is then applied to gravity data, and the results are compared with calculations using the Hammer chart. Based on the synthetic data test, it was found that the value of terrain correction from a rock body measuring 10 x 10 km with a height difference of 1000 m from the station no longer significantly affects at the distance of 20 km. The topography around Karangsambung in the form of South Seraju Ranges with altitude of 1000 m at distance of 20 30 km gives effect of 0.05 mGal on terrain correction, while the Quaternary Volcano with an altitude of 3000 m at distance of 30 40 km gives effect of 0.1 mGal. The results of applying program at the gravity data show that the use of the square pattern method is able to correct errors from Hammer chart up to 3 mGal. The difference between the calculation of the two methods is getting bigger in the station located at slope area. It happens because estimation of the height difference in slope area is more difficult to do.