Checkshot Survey

What Checkshot survey?

Checkshot survey adalah kegiatan pengukuran waktu tempuh gelombang seisimik dengan posisi source berada di permukaan dekat dengan lubang bor, dan posisi receiver berada di lubang bor.

A type of borehole seismic data designed to measure the seismic traveltime from the surface to a known depth. P-wavevelocity of the formations encountered in a wellbore can be measured directly by lowering a geophone to each formation of interest, sending out a source of energy from the surface of the Earth, and recording the resultant signal. The data can then be correlated to surface seismic data by correcting the sonic log and generating a synthetic seismogram to confirm or modify seismic interpretations. It differs from a vertical seismic profile in the number and density of receiver depths recorded; geophone positions may be widely and irregularly located in the wellbore, whereas a vertical seismic profile usually has numerous geophones positioned at closely and regularly spaced intervals in the wellbore.

The seismic reference survey (SRS), often called a seismic check shot survey, is designed as a calibration mechanism for reflection seismic data. In such a survey, seismic velocities are measured in the borehole by recording the time required for a seismic pulse generated by a surface energy source to reach a geophone anchored at various levels in the borehole.

What Checkshot survey?

Why Checkshot survey?

Checkshot dilakukan untuk mendapatkan data hubungan waktu tempuh dengan kedalaman (TD Curve) yang sangat berguna untuk konversi waktu ke kedalaman, mengkoreksi sonic-sonic corrected check-shot *) untuk keperluan pembuatan seismogram sintetik, memperbaiki kecepatan seismic (velocity scaling), estimasi Q, dll.

Hal ini dikarenakan data sumur berupa kedalaman dan data seismik berupa domain waktu (TWT) sehingga diperlukan data checkshot untuk konversi domain data depth ke time dan melakukan well seismic tie.

Data sonic log dan checkshot memiliki kelemahan dan keunggulan masing-masing. Kelemahan data sonic diantaranya adalah sangat rentan terhadap perubahan lokal di sekitar lubang bor seperti washout zone, perubahan litologi yang tiba-tiba, serta hanya mampu mengukur formasi batuan sedalam 1-2 feet.

Sedangkan kelemahan data checkshot adalah resolusinya tidak sedetail sonic. Untuk ‘menutupi’ kelemahan satu sama lain ini, maka kita melakukan koreksi dengan memproduksi ‘sonic corrected checkshot’. Besarnya koreksi checkshot terhadap sonic disebut dengan ‘DRIFT’.

*) Sonic corrected checkshot (data sonic dikoreksi oleh check shot): hal ini dilakukan karena diantaranya terjadi perbedaan orientasi ray path (jejak sinar). Jejak sinar seismik pada survey checkshot relatif tegak lurus terhadap perlapisan batuan sedangkan sonic cenderung sejajar. Perbedaan ini menghasilkan perbedaan waktu tempuh yang kita kenal dengan drift curve

sebagai penerjemah domain kedalaman data-data sumur ke dalam domain waktunya data seismik.

Sebenarnya penerjemahan domain kedalaman ke dalam domain waktu dapat dilakukan oleh data sumur yaitu log sonic. Log sonic berupa pengukuran transit time yang disingkat DT dapat diubah menjadi log kecepatan sonic. Kecepatan sonic inilah yang mampu menerjemahkan domain kedalaman ke dalam domain waktu. Akan tetapi, kecepatan sonic dalam well seismic tie mempunyai beberapa kelemahan sehingga masih diperlukan data kecepatan lain yang diperoleh sebagaimana data seismik diperoleh yaitu data checkshot.

Adapun kelemahan data sonic sehingga masih diperlukan data checkshot adalah:• data sonic mengukur volume batuan tidak sebagaimana data seismik mengukur• kandungan frekuensi data sonic jauh lebih tinggi dibandingkan data seismik• resolusi vertikal data sonic jauh lebih tinggi dibandingkan data seismik

Karena dua point terakhir di atas, sebelum membuat seismogram sintetik biasanya interpreter melakukan smoothing terlebih dahulu pada log sonic (dan juga log densitas).

Why Checkshot survey?

The recorded travel times are used to calibrate the sonic log, which then becomes the basic seismic calibration reference. A time versus depth plot is produced from these data. The calibrated sonic and the density logs (Figures 21.11 and 21.12) are used to construct a synthetic seismogram, which allows identification of reflecting horizons by reference to the seismic response at the wellbore.

Why Checkshot survey?

Checkshot and

computed result

The curve is used to define the geological formation tops on the seismic line at the well location for seismic mapping. Also you can use this time depth data to create synthetic seismogram by using sonic and density logs of the well.

kurva waktu tempuh dan

kedalaman yang di-overlay dengan first break Check-Shot

sonic corrected checkshot

Sinthetic seismogram seismik

Why Checkshot survey?

When Checkshot survey?

Checkshot dilakukan untuk mendapatkan data hubungan waktu tempuh dengan kedalaman (TD Curve) yang sangat berguna untuk konversi waktu ke kedalaman, mengkoreksi sonic-sonic corrected check-shot *) untuk keperluan pembuatan seismogram sintetik, memperbaiki kecepatan seismic (velocity scaling), estimasi Q, dll.

Where Checkshot survey?

How Checkshot survey?

Sumber gelombang biasanya berupa air-gun (500 cu in). Jika survey-nya di darat, air gun disimpan di dalam sebuah bak air atau kolam. Air-gun digunakan karena lebih repeatable dibandingkan dinamit. Sedangkan alat perekam sendiri biasanya berupa geophone accelerator 3 komponen (X,Y,Z) dan berada pada 4 lokasi (4 shuttles) untuk satu rangkaian alat.

Perekaman dilakukan pada beberapa titik kedalaman lubang bor baik sebelum ataupun setelah dipasang casing. Interval kedalaman biasanya sekitar 100m atau sesuai dengan kebutuhan.

The purpose of a velocity survey is to produce a down-going seismic wavelet at the surface near a well and then to measure the time required for that wavelet to travel to a known depth where a seismic receiver is positioned in the well. This borehole receiver is locked successively at several different depth levels, and the vertical traveltime to each level is measured.[1] Each measurement of the source-receiver traveltime is a checkshot, and the compilation of all of the traveltime measurements into a time-depth calibration function is referred to as a checkshot survey.

How Checkshot survey?

Ordinarily, the borehole receiver is first lowered to the deepest checkshot level, and the traveltime to this deepest receiver position is measured for one or more surface shots. The receiver is then moved upward a distance of 200, 500, or 1000 ft (61, 152, or 305 m) to record the checkshot, or vertical traveltime, at successively shallower levels

The tool lowered into the borehole consists of: - velocity sensitive geophones - amplifier circuits - hydraulic anchoring systemAt the surface, there will be: - air guns - air compressor - reference hydrophone - extra surface hydrophones if required - high speed recorder (self developing film) - control panel (amplifiers, filters) - digital tape recorder

The anchored geophone permits releasing cable tension, thus eliminating transmission of much of the surface generated noise. This allows the use of an air gun as a power source thereby obviating explosives and all the attendant safety hazards and logistical complications.

How Checkshot survey?

Di sini kita akan menghitung bagaimana data checkshot diperoleh (perhatikan Gambar 1). Parameter yang sudah diketahui adalahoffset: jarak antara sumur dengan source

TVD-SRC: kedalaman receiver dengan ketinggian source terhadap MSL sebagai datumnyaFB: waktu first break yaitu waktu tempuh gelombang langsung yang ditangkap oleh receiver

How Checkshot survey?

How Checkshot survey?

How Checkshot survey?

QC Checkshot survey?

QC utama yang harus dilakukan pada rekaman Check-Shot adalah kejelasan first break yang bersih dari noise baik untuk komponen X,Y ataupun Z dan peningkatan waktu tempuh sejalan dengan penambahan kedalaman (jika posisi lubang bor vertikal atau miring). Hal ini akan berbeda jika pada horizontal well. Kualitas data dipengaruhi oleh kesehatan alat, coupling antara alat dan lubang bor, kehadiran gerowong yang berada dibalik casing, dll.

Checkshot di bandingkan dengan survei seismic lainnya

Checkshot• Measure only first break event• Larger interval/spacing• Records are in short duration• Record have fixed gain• Single receiver recording system• Single sources• Low resolution data• In check shots, we are focused on only

the first break time which is equal to the one way travel time between surface and check points. First break times are combined with sonic velocity and density logs to calculate synthetic seismogram.

VSP• Measure first break and reflected

event• Smaller interval• Records are in longer duration and

record full waveform• Record have variable gain• Multiple sources• High resolution data (more accurate)• The vertical distance between VSP

traces should not exceed one-half of λmin, where λmin is the shortest wavelength contained in the recorded VSP wavefield.

VSP's use both the first break to get (T-D

data) and the remaining waveform from the shot. The waveform contains reflectivity (upgoing and downgoing) and

other information which can be used in

synthetic seismic generation and

modeling of multiples

VSP source-receiver geometries

Several types of VSPs can be recorded by altering the position of the energy source relative to the receiver. The term offset is used to describe the horizontal distance between the source and receiver. If the receiver is directly below the source, the recorded data are called a zero offset VSP. If there is a significant horizontal distance between the source and receiver, the recorded data are referred to as anoffset VSP. Examples of offset and zero offset geometries are shown in Figure 4. A common misuse of the term offset is in describing the horizontal position of the energy source relative to the wellhead rather than the position of the source relative to the location of the subsurface receiver. For this reason, the geometry in Figure 4(d) is an offset VSP, not a zero offset VSP.

Zero offset VSP

Zero offset VSP

Offset VSP Offset VSP

Posisi source A baik digunakan untuk receiver pada posisi Z1 karena arah

gelombang tegak lurus

Posisi source B baik digunakan untuk receiver pada posisi Z2 karena arah

gelombang tegak lurus

The time-depth calibration function and velocity analyses that can be calculated from checkshot measurements are more reliable if each source-receiver travel path is a vertical straight line rather than an oblique, refracted path.[2]

Consequently, if a well is deviated, then the surface position of the source should be readjusted each time the downhole receiver is moved to a new depth level, as shown in Figure 2, so that the travel path always remains as vertical as possible.

One of the travel paths is usually a better approximation of a straight line than the other. For example, in Figure 3, source position A is preferred when the receiver is at depth Z1 but source position B is the better choice for a receiver at depth Z2. Usually, the traveltimes measured for sources A and B are simply averaged at each receiver depth because the structural dips and formation velocities are rarely known with enough precision to predetermine which travel path is the better approximation of a straight line.