Tag: tanggulangin

Polisi kukuh menyimpulkan bahwa semburan lumpur di Sidoarjo disebabkan oleh kelalaian dalam pengeboran. Pernyataan ini pernah disampaikan ke DPR oleh Kepala Badan Reserse dan Kriminal Komisaris Jenderal Bambang Hendarso Danuri, yang kini menjabat Kepala Kepolisian RI. Tapi, menurut Bambang, jaksa meminta tambahan saksi ahli yang menyatakan semburan itu sebagai bencana alam.

Itulah yang membuat berkas kasus lumpur Lapindo terombang-ambing. Polisi dan jaksa memiliki pandangan berbeda. Masalahnya, kompromi bukan pilihan terbaik karena hasilnya justru akan mengendurkan jerat hukum yang telah dipasang polisi.

Kengototan jaksa sungguh aneh karena para ahli geologi dunia pun cenderung menyatakan semburan lumpur dipicu oleh pengeboran. Inilah pandangan yang dominan dalam konferensi American Association of Petroleum Geologists di Cape Town, Afrika Selatan, baru-baru ini. Hanya sedikit ahli yang sepakat bahwa semburan itu merupakan dampak gempa di Yogyakarta.

Pengeboran yang dilakukan Lapindo berbahaya karena tidak menggunakan casing secara penuh. Badan Pemeriksa Keuangan, yang pernah mengaudit kasus ini, juga mendapat keterangan penting dari Dinas Survei dan Pengeboran BP Migas. Intinya, proses pencabutan pipa dan mata bor dari kedalaman 7.415 kaki, sehari sebelum semburan terjadi pada 29 Mei 2006, menyebabkan “well kick” terlambat diantisipasi. Peralatan pengeboran pun sering rusak. Menurut auditor BPK, kontraktor yang ditunjuk Lapindo diduga menggunakan beberapa peralatan bekas atau tidak memenuhi standar kualitas.

Jaksa mestinya berpegang pada fakta seperti itu. Mengarahkan kasus lumpur Lapindo ke perdebatan apakah semburan itu berkaitan dengan gempa atau tidak hanya akan mengaburkan persoalan. Sepanjang ditemukan bukti yang cukup adanya kelalaian dalam pengeboran, kasus ini layak dibawa ke pengadilan.

Persoalan ini tak akan berlarut-larut andaikata Presiden Susilo Bambang Yudhoyono segera turun tangan. Perbedaan pendapat antara jaksa dan polisi seharusnya bisa diatasi karena kedua institusi ini di bawah kendali langsung Presiden. Jangan sampai khalayak menilai pemerintah sengaja membiarkan kasus ini menjadi terkesan rumit, sehingga akhirnya tak jelas siapa yang mesti bertanggung jawab atas penderitaan warga Sidoarjo.[Editorial Koran Tempo, 31/10]

Things were a bit different from the other sessions at this conference: photographers and cameramen were roaming the room, and on my way in I had a press release from Lapindo Brantas thrust into my hands, informing me that most of the previously reported findings were based on ‘incorrect data’, and that this would be the first time ‘official data’ were presented to the scientific community. It transpired that this was also stretching the truth a bit; as the session unfolded it became apparent that this was more an issue of which data (specifically, which of the various measurements and estimates of borehole pressure) should be used, and how to interpret them. Contrary to their own press office, then, Lapindo Brantas – to their credit – seem to have been quite open about sharing their records with any interested party, even if they might disagree with how it is then used.

The debate consisted of two talks from the pro-earthquake camp, followed by questions, then two talks from the pro-drilling camp, followed by more questions and a general discussion, ending with a poll of the audience. Below I’m summarising from notes I took during the session; as such, there may be some unintentional inaccuracies.

First up was Adriano Mazzini, who reviewed his arguments that the eruption was the result of the Yogyakarta earthquake reactivating a pre-existing fault that runs through the area. Main points:

• There is clear evidence of displacement along this fault after the eruption, and other small mud eruptions also occurred along its trace at the time Lusi erupted. This does indeed signal that this fault acted as a conduit for upwelling fluids.
• However, the question of what triggered this trigger – the ultimate cause of the fault suddenly becoming an open pathway for mud – is still open. The timing of the fault reactivation is not known, so it could just as easily have been the result of stresses induced by a nearby well blow-out as by an earthquake. In support of the latter, pressure losses in well at the time of the earthquake might indicate that there were measurable stresses being induced in the subsurface, more than is assumed by the pro-drilling camp.
• Lusi’s eruption has been abnormally lengthy; natural mud volcano eruptions tend to last a few days (and in this respect it seems that Lusi also differs from the other eruptions in the area he talked about, which were further from the borehole). It might be something to do with this being an entirely new system, but it could just as easily be a sign of unnatural influences on the triggering of the eruption.
• This area of Indonesia is “ideal”, both in terms of prevailing tectonic, structural and lithological conditions, for the formation of mud volcanoes, and the existence of a highly suitable ‘piercement structure’ (the fault) in the area meant that an eruption was only a matter of time. This last point would become important in the later discussion.

The next speaker, from Lapindo Brantas, was the person who oversaw the borehole, which mades his slightly defensive tone entirely understandable. The talk aimed to use data from the borehole to demonstrate that a blow-out had not occurred.

• The most important claim was that there was no evidence of the well being connected to Lusi’s plumbing, which you would expect if a well blow-out was the trigger. For example, following the eruption of Lusi they performed an “injection test”, where drilling mud was pumped into the borehole. As they did this, the pressure in the well slowly built up, which can only happen if the well is sealed; if there was a breach in the well, the fluid would escape.
• The pressure losses at around the time of the earthquake talk prompted the decision to case the unsealed part of the borehole before drilling deeper. As they were puling out the drill string to do so on the 28th May, there was a ‘kick’ in the hole – an increase in pressure due to fluid entering from the surrounding rock. The pressure increase from this kick, and whether it was enough to trigger a blow-out, is one of the major sticking points of this whole argument.
• The pro-drilling people have published calculations that indicated that it was more than high enough to trigger a blow-out. It was claimed here that these calculations were performed incorrectly, and should give much lower (and safe) pressures. I can’t claim enough expertise to tell if these criticisms were valid or not.
• Either way, borehole records after the kick are inconsistent with it being the result of a well blow-out.

The first speaker from the pro-drilling camp, Mark Tingay, gave a very nice presentation which actually addressed the pros and cons of both theories, although he was clear which he favoured.

• Earthquake reactivation of the fault seems unlikely from the calculated differential shear stresses it induced in the area around Lusi (for more details you can read Maria’s post on this, since she was involved in this study). He had a nice plot to illustrate this point, which I reproduce schematically here.

  

  Basically, the earthquakes which are known to have triggered mud volcanism (green dots) were either larger and/or closer to the induced eruptions than Lusi was to the Yogyakarta earthquake (big red dot), and other seismicity in the area prior to the eruption (orange dots) appears to have induced similar, or in some cases greater, differential stresses. There is of course a lot of uncertainty about how strong the fault was (how large a differential stress you need to cause it to fail), but the point is that there was nothing special about the May 27th earthquake in terms of the force it exerted in the area.

• The apparent disconnection of the well from Lusi could be due to the lower part being sealed off due to it being blocked by rock fragments from the blow-out (which is apparently quite common), or shearing.
• The evidence following the kick is “unclear”, but the stresses that it induced were an order of magnitude larger than those due to the earthquake
• This was a problematic hole, with lots of pressure losses and kicks, and the ‘drilling window’ – the range of pressures where the drilling mud will safely just sit there – was very narrow. This was a result of the well design being based on holes drilled through this sequence quite a distance away, where the underlying carbonates were not overpressured. However, in another well very close by, the carbonates were under extremely high pressure, and they almost lost that well to a blow-out when they hit them. By not taking this into account, it would have been nearly impossible to stop a blow-out following the kick.
• There is a series of linear surface fractures leading between the well and the Lusi vent, which have not erupted any mud but are an indication of sub-surface fracturing.

The final presentation was by Richard Davies, the person whose paper implicating the drilling kicked off my interest in this subject.

• This talk was a review of the events in the well leading up to the Lusi eruption. It was a very good summary, but most of the points have been covered in my accounts of the previous speakers.
• However, in the middle of the talk, proving that for every expert there is an equal and opposite expert, another (non Lapindo Brantas) drilling engineer came up to the stage to show us how all the well pressure data was consistent with the initial kick being due to a blow-out (I think that the main argument was that you had pressure losses after the hole was sealed).

The discussion following the talks emphasised a couple of other points of disagreement, such as the shaking intensity in the Lusi region from the May 27th earthquake: the pro-drillers say I-II, pro-quakers claim IV. Mazzini also argues that the eruption is entirely sourced within the mudstones with the water produced by clay minerals reacting to form illite and water, whereas Davies et al. say that the water is mainly from the underlying carbonates. It is a lot of water, although I guess it would be hard to claim the earthquake would cause the carbonates to breach.

However, more interesting was that several people picked up on the suggestion that this was clearly a prime area for mud volcano formation, and wondered quite loudly why the hell any drilling was being done there in the first place. To sum up the mood, one questioner complimented the second speaker for standing up there to defend the drilling, when it really should be the geologists who told him to drill there in the first place. It’s a good point; perhaps the focus should be less on the drilling practice on this specific well, because whatever your procedures and safety precautions, accidents can and do happen, and more on well siting decisions in general, particularly decisions to drill in “primed” areas like this one, where the margin for error is rather slim.

This attitude might explain why in the closing straw poll, more than 50% of the audience was willing to blame the whole mess on the drilling. Only a tiny fraction thought that the earthquake was solely to blame, with roughly equal numbers of the remaining half either thinking it could have been both, or finding it inconclusive either way. In conclusion, it was a very interesting session, and was conducted in a very fair-minded and respectful manner given the contentious nature of the debate.

Sumber: http://scienceblogs.com/highlyallochthonous/2008/10/aapg_day_2_showdown_at_the_lus.php?

Saya tertarik dengan argumen pribadi Dr. Awang Harun (dari BP Migas)
dan Dr. Andang Bachtiar (ex ketua IAGI), secara terpisah, yang ” sepakat
” (koreksi :Pak ADB justru lebih yakin dipicu pengeboran, koreksi rdp)
merujuk gempa Yogya sebagai pemicu semburan lumpur di Porong.
Disebutkan ada 5 titik semburan, terbentuk pada 29 Mei 1 Juni, dan jika
dihubungkan dengan garis akan embentuk arah barat daya – timur laut.
Orientasi ini earah dengan sesar regional di wilayah ini, dan kalo mau
ditarik lagi lebih jauh juga searah dengan sesar Opak yang jadi
penyebab gempa Yogya. Jika orientasi
barat daya – timur laut ini diperpanjang, akan nampak aris imajiner
yang menghubungkan sesar Opak – Sangiran Dome – Porong. Sehingga
semburan ini ihipotesiskan sebagai likuifaksi, gejala biasa dalam uatu
gempa, seperti yang ditemukan juga di Jetis Bantul) dan Prambanan
(Klaten) dalam bentuk semburan air berlumpur ” (menurut versi penduduk,
seperti ikutip media lokal Kedaulatan Rakyat dan Wawasan). Gempa Yogya
di sebut2 mereaktivasi sesar lokal di Porong, sehingga menghasilkan
semburan lumpur, dan ini adalah murni musibah.

***

Terkait itu, ada beberapa pertanyaan pak Rovicky :

1. Apakah likuifaksi bisa terjadi di tempat yang jaraknya > 200 km dari sumber gempa ?

Sebab, dalam pendapat saya, intensitas di lokasi tersebut sudah
kecil. Jika saya mencoba menghitung dengan menggunakan persamaan
atenuasi intensitas ln I ln Io = k.x dengan koefisien atenuasi (k) =
-0,00387 (berdasarkan titik acuan kota Yogya dan Semarang) erta
intensitas hiposenter (Io) = 8,7 (untuk Mw = 6,3) pada jarak (x) = 200
km intensitasnya 4 MMI dengan percepatan maksimal 2,3 % G), sementara
pada jarak (x) = 250 km intensitasnya menurun sedikit menjadi 3 MMI
(dengan percepatan maksimal 1,4 % G).
Catatan intensitas dari stasiun BMG Surabaya dan Karangkates (Malang)
menunjukkan angka 2 – 3 MMI untuk Surabaya (jarak +/ – 250 km dari
hiposenter) dan 3 – 4 MMI untuk Malang (jarak + / – 230 km dari
hiposenter), artinya tidak berbeda jauh dengan perhitungan.
Fokuskan ke sekitar Surabaya. Dengan intensitas 3 MMI itu, dimana
getarannya setara dengan getaran akibat melintasnya sebuah truk besar
bila kita berdiri di tepi jalan raya, apakah bisa gempa Yogya tadi
menghasilkan likuifaksi disini ? Bila kita merujuk ada kasus gempa Loma
Prieta 1989 (Mw = 6,9) di California, radius terjauh likuifaksi terjadi
adalah sebesar 110 km dari episenter gempa. Kita logikakan saja, dengan
Mw gempa Yogya lebih kecil (6,3) bukankah ” seharusnya ” radius terjauh
likuifaksi < 110 km ?

(catatan : dalam perhitungan saya, jika dianggap koefisien atenuasi
gempa Loma Prieta sama dengan gempa Yogya, dengan kedalaman hiposentrum
17 km, pada jarak 110 km dari episentrum, intensitasnya sebesar 6 MMI
dengan percepatan puncak 12,3 % G, jauh lebih besar dari intensitas di
Porong).

2. Apakah gempa Yogya bisa mereaktivasi sesar lokal di Porong ?

Di sisi timur sesar Opak telah dideteksi ada 74 buah sesar minor
dengan panjang bervariasi antara 1 km hingga 4 km, yang tersebar di
wilayah Gunungkidul – Klaten. Sesar minor terjauh ada di wilayah
kecamatan Bayat (Klaten). Sesar2 minor ini dipastikan merupakan sumber2
afershocks gempa Yogya. Kalo saya menghitung dengan persamaan
empirisnya Ambrosey dan Zatopak (1968, saya kutip dari artikelnya Dr.
George Pararas Carayannis) mengenai hubungan antara panjang sesar (L)
dan magnitude gempa (M) : log L = 1,13 M + K, dimana untuk gempa Yogya
K = – 5,34 (dengan Mw = 6,3 dan L = 60 km), maka jika sesar minor
memiliki panjang (L) 1 – 4 km, gempanya memiliki magnitude (Mw) 4,7 –
5,3.

Masalahnya sekarang, jika gempa Yogya memang mampu mereaktivasi
sesar lokal di Porong, tidak bisa tidak sesar lokal itu harus bergeser
bukan, meski nilai pergeserannya mungkin sangat kecil hingga tidak
menimbulkan retakan di permukaan tanah. Mari kita berandai-andai,
anggaplah pergeseran tersebut meliputi segmen sepanjang 1 km dalam
sesar lokal itu, maka ” seharusnya ” sudah diiringi gempa dengan Mw =
4,7.
Jika segmen yang bergeser hanya 200 m, gempa yang terjadi memiliki Mw =
4,1. Bukankah moment magnitude (Mw) sebesar ini masih bisa dideteksi
dengan mudah oleh seismograf2nya BMG dan USGS. Apalagi USGS memberi
batasan hanya gempa2 dengan Mw > 3,5 saja yang akan
didokumentasikan. Sementara, sejauh yang saya tahu, stasiun2 BMG di
Surabaya dan Karangkates hanya melaporkan adanya guncangan akibat gempa
Yogya saja, namun tidak menyebutkan adanya gempa lain atau aftershocks
dengan episentrum di sekitar Porong.

3. Apakah energi gempa Yogya dirambatkan oleh sesar2 hingga sampai ke Porong ?

Ini
masih terkait dengan pertanyaan no. 2. Mengikuti pendapat pak Awang dan
pak Andang, saya mencoba menarik garis imajiner terusan sesar Opak ke
arah timur laut. Saya juga mencoba menarik garis imajiner yang
menghubungkan sumur Banjar Panji 1 – Purwodadi – Mojokerto – Sangiran,
titik2 dimana terdapat mud volcano atau sumber air asin. Hasilnya bisa
dilihat pada gambar ” situasi bp1 sangiran.jpg “.
Menarik sekali bahwa garis imajiner yang menghubungkan Banjar Panji 1 –
Purwodadi – Mojokerto – Sangiran ternyata menyusuri sisi selatan
Pegunungan Kendeng, dimana menurut van Bemmelen disini terdapat ” sesar
Simo ” yang longitudinal terhadap pulau Jawa.
Sementara garis perpanjangan sesar Opak, justru melintas amat jauh
terhadap Porong. Perpanjangan sesar Opak justru melintasi sesar
pembatas Bawean High – Tuban Graben di Laut Jawa. Menarik juga, bahwa
lintasan perpanjangan sesar Opak di Pegunungan Kendeng dan geosinklin
Jawa utara ditandai dengan banyaknya sesar2 lokal yang orientasinya
sebagian besar paralel dengan sesar Opak.
Dalam pendapat saya, koq tidak ada ya hubungan segaris antara mud volcano di Porong dengan sesar Opak.
Terkecuali jika dikatakan sesar Opak yang berarah barat daya – timur
laut ini bersambung dengan ” sesar Simo ” yang berarah barat – timur,
dimana titik persambungannya ada di sekitar Sangiran. Namun, logikanya,
jika hal seperti itu yang terjadi, seharusnya terdeteksi juga
aftershock di sepanjang ” sesar Simo ” bukan ? Karena energi gempa
Yogya “seharusnya ” merambat di sini.
Apalagi menurut van Bemmelen, sesar Opak adalah bagian dari sesar
transversal yang membelah Jawa dari selatan ke utara. Sesar transversal
ini (saya mengistilahkannya dengan ” sesar besar Jawa Tengah “) menjadi
tempat berdirinya gunung2 api Merapi, Merbabu, Telomoyo, Ungaran hingga
berakhir pada sesar Glagah di utara. Memang sesar besar ini juga
berpotongan dengan perpanjangan ” sesar Simo “, namun titik potongnya
jauh di utara dari sesar Opak, di tempat yang sekarang menjadi kerucut
Gunung Merapi. Sesar Opak justru berpotongan dengan sesar longitudinal
dari sisi utara Pegunungan Selatan (Pegunungan Sewu) di sekitar
Prambanan, dan dari sini saya bisa memahami mengapa sesar2 minor produk
gempa Yogya kebanyakan ada di Gunungkidul utara dan Klaten dengan
sebagian besar berarah arah barat laut – tenggara, sehingga salah satu
daerah yang kerusakannya sangat parah (selain Parangtritis – Prambanan)
adalah Kecamatan Gantiwarno – Wedi – Bayat (sebelah tenggara
Prambanan). Gambaran tentang sesar besar Jawa Tengah ini bisa dilihat
di ” sesar besar jawa tengah.jpg “.

4.
Jika gempa Yogya menyebabkan mud volcano di Porong, mengapa gempa yang
sama juga tidak menyebabkan peningkatan aktivitas mud volcano Bledug
Kuwu atau membangkitkan kembali aktivitas Sangiran Dome ?

Apalagi dua tempat terakhir itu lebih dekat terhadap pusat gempa
dibanding Porong. Dan sejauh ini tidak ada peningkatan jumlah lumpur di
Kuwu ataupun bangkitnya kembali Sangiran Dome. Peningkatan aktivitas
hanya ada di Gunung Merapi dan ini bisa dipahami mengingat dari
Prambanan ke arah utara ada sesar yang langsung menuju ke Merapi.
Sehingga rambatan energi gempa Yogya, setelah melintasi sesar Opak,
sangat mungkin berbelok menyusur sesar tadi,sehingga dapur magma Merapi
menerima tambahan energi.

***

Saya merasa, mengaitkan gempa Yogya dengan mud volcano di Porong
jauh panggang dari api. Gempa memang punya kemampuan likuifaksi, tapi
jangkauannya juga terbatas. Apalagi, merujuk hasil penelitian BMG
seperti dipaparkan Tiar Prasetya, gelombang primer dalam gempa Yogya
tidak merambat homogen ke segala arah, tetapi terkutubkan
(terpolarisasi) hingga seakan-akan membentuk pola bunga melati.
Pengutuban ini menjadi faktor penjelas mengapa kerusakan parah – selain
di sepanjang jalur sesar Opak – hanya dialami sebagian kota Yogya ,
tepatnya mulai dari kompleks kampus IAIN dan Tamansiswa ke arah timur.
Bagian barat kota Yogya, demikian juga dengan kecamatan Gamping, Sedayu
dan Sentolo, relatif mengalami kerusakan ringan.

Jalur kerusakan berat ke barat menghampiri Srandakan – Purworejo dan ke timur melintasi Pacitan. Kalo sumbu
polarisasi ke timur ini diteruskan, posisinya juga jauh dari Porong, pak Rovicky.

Demikian pendapat dan pertanyaan saya pak Rovicky. Matur nuwun atas pencerahannya.

Wassalamu’alaykum

Ma’rufin