2. SEISMIC SURVEY DESIGN-carlos

Day 1

SEISMIC SURVEY DESIGN

By Carlos Tarazona

OUTLINE I - Brief Introduction. II - Fundamental Elements of the Design. III - 2D Profiles (Onshore-Offshore)/3D Inline(Offshore). 1 – Objective. a – Type of Play & Depth of the Objective b – Density (Recognisance vs. Detail surveys) c – Orientation (factors affecting the orientation).

2 – Energy Source. a – Energy Source types, Size & arrays. b – Energy Source Signature. c – Energy Source Depth.

3 – Receivers. a – String Patterns (onshore). b– Group Interval (onshore/offshore). c – Split-Spread (onshore) - Single End (onshore/offshore). d – Energy Source and Receivers Gap. e– Nominal Fold/Bin size. f - The recorded signals and the effects of the receiving string.

IV – Field Tests. a – Source depth & size; Geoph. String position and elements spacing,

I - Brief Introduction •

Increasing hydrocarbons demand, is forcing companies to search for it in more challenging areas . In the last decade exploration in deep waters, has increase exponentially in Indonesia.

•

This challenge is demanding innovations in the acquisition techniques as well as in the equipment required to meet them.

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Combination with other non-seismic techniques is increasingly being used to map deep hydrocarbons bearing reservoirs. The most frequently used are Gravity, Electromagnetic and Magneto-telluric.

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This first day of the course is dedicated to review the key elements of the seismic acquisition design, and it should be kept in mind that, each area has their own particularities, both from the geological point of view, as well as from the specific objectives of project.

II - Fundamental Elements of the Design.

• • •

Objective . Energy Source. Receivers.

III - 2D Profiles (Onshore-Offshore)/3D Inlines (Offshore). 1 - Objective. a – Type of Play & Depth of the Objective

• Structural. (Depth penetration, spatial & Time Resolution, fault definition ) • Stratigraphic. (Same as above.) • Structural & Stratigraphic. (Same as above.)

III - 2D Profiles (Onshore-Offshore)/3D Inlines (Offshore). 1 – Objective. b – Density (Reconnaissance vs. Detail surveys). • Depend on the coverage of legacy data and its quality. • Type of “Exploration Plays” the Company is pursuing (structural, stratigraphic or both). • Level of knowledge of the subsurface both, stratigraphy and structure. • Intends to make a “ready to drill prospect” from a lead ?. • Available Budget. • Reconnaissance required in general, a set of 2D lines. • For detail development work, 3D is the common practice today.

III - 2D Profiles (Onshore-Offshore)/3D Inlines (Offshore). 1 – Objective. c – Orientation . (Factors affecting the orientation).

DIP DIRECTION

STRIKE DIRECTION

2D Grid survey

Do not forget to tie the available wells !!!

Factors affecting the orientation • Accessibility.

• Environmental restrictions. • In the marine case , prevalent wind and water currents directions and, water depth.

III - 2D Profiles (Onshore-Offshore)/3D Inlines (Offshore). 2 – Energy Source. a – Energy Source types, Size & arrays. • There are many energy sources that have been developed in the Applied Seismology for the prospecting hydrocarbons however, it would be safe to say that in Indonesia , Dynamite is the king of sources on land as are Air guns in the marine environment. In view of the above, those two types will be our main concern here. • Dynamite comes in cartridges of 500 grams and, they are exploded by sympathy by the explosion of a detonator inserted inside the cartridge. Any specific source size, is made by adding the necessary number of cartridges to complete the desired size. • Larger charges generate more low frequencies and, because the absorption effect of the earth materials (which always like to pray in high frequencies ) these larger sources are suitable for deep targets, as well as, for Refraction Prospecting.

WATER GUNS Operation of a S15 water gun. Air pushes water through the portholes creating cavities that collapse and create a strong high frequent acoustic signal. Copyright: Sercel

Schematic diagram of air gun geometry An array is a geometrical arrangement of seismic sources. This schematic diagram shows an air-gun array towed several hundred meters behind a seismic vessel to provide an energy source for the acquisition of marine seismic data.

AIR GUN SIZES

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Airguns as small as 10 cu in, are used for shallow surveys requiring high frequencies. Such guns have a source levels of 210-220 dB).

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Larger airguns have source levels, in the order of 220 dB but the energy is more concentrated in lower frequency ranges and the main pulse is only a few milliseconds in duration.

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The small guns used in high resolution work are fired from every 7 sec. To as rapidly as every 0.25 sec.

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Arrays used in petroleum industry surveys typically have far field source levels in the range of 240-246 dB for vertical propagation but 10-30 dB lower for horizontal propagation, ( It should be remembered that these are signatures of the notional point source with the same far field signature and that closer to the array, the sound levels do not reach as high as the notional point source ).

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Large arrays are designed to image deep into the earth’s crust and so fire every 8 to 19 sec.

III - 2D Profiles (Onshore-Offshore)/3D Inlines (Offshore). 2 – Energy Source. b – Energy Source Signature. ™ It is very difficult to know the actual signature of a dynamite source. ™ Reasons among others are, variability from shot to shot of the media where the explosion take place, size of the charge and coupling of the charge. ™ It is generally accepted for theoretical reasons that, the generated pulse of a dynamite charge respond to a Ricker wavelet or, to a Minimum delay pulse. These type source signatures although theoretical, are widely used in modelling.

Single Air Gun pulse

Source signature & Amplitude Spectrum of a typical gun array

III - 2D Profiles (Onshore-Offshore)/3D Inlines (Offshore). 2 – Energy Source. c – Energy Source Depth..

• - Dynamite energy sources should be located below the Weathered Layer (WL) to achieve an effective coupling between the energy source and a competent ground. In addition to this there are the ghost effect that require further discussion later. • - Marine energy source arrays should be located at a sufficient distance from the stern of the vessel (typically about 400 – 500 meters). The depth however required as in the Onshore case further discussion.

Shot hole

Ghost effect on Energy sources (simple case) Receiver Air

Air Water

WL

Ghost Source

Source

Ghost Primary Primary

Reflecting interface

Onshore (case A )

Offshore

Gun signature in the marine case

Ghost effect on the source pulse

62 Hz. 50 Hz. 36 Hz.

Shot hole

Receiver

Air

Shot hole

Receiver

Air

Reflec. Coef. = -0.99

WL

WL 7 m.

Source

Reflec. Coef. = -0,2

Source

Ghost

Ghost

Primary

Primary

Reflecting interface

(A)

Reflecting interface

(B)

In case B, if the source is located well below the WL, the generated ghost closest to the primary source, is coming from the base of the WL. This ghost is propagating in a media with higher velocity than that of the WL and therefore, its expected delay with respect to the primary is much smaller than in the previous case and in all probability, the reflection coefficient also smaller than in the WL / Air interface and consequently the generated ghost less destructive.

The dynamite source has 2kg strength and is about 1 meter long. The centre of it is located 6,5 meters below the WL. Shot hole

Receiver

Air WL

7 m. Source

Reflec. Coef. = -0,2

Ghost Primary

Reflecting interface

Onshore (case B )

What have we learned? • From the previous slides, we can see the lengthening of the of the resulting source signature, (this implies a decay of the high frequencies content of the Amplitude Spectrum and therefore diminishing the thickness resolving power). In the marine case, the deeper the gun array is the further lengthening of the of the resulting source signature. • The same comments apply for the onshore case, when the energy source is within the WL or very close to its base.

III - 2D Profiles (Onshore-Offshore)/3D Inlines (Offshore). 3 – Receivers.

• A device used in surface seismic acquisition, both onshore and on the seabed offshore, that detects ground velocity produced by seismic waves and transforms the motion into electrical impulses. Geophones detect motion in only one direction. Conventional seismic surveys on land use one string of geophones connected in series per group, to detect motion in the vertical direction. Three mutually orthogonal geophones are typically used in combination to collect 3C data. • Hydrophones, unlike geophones, detect changes in pressure rather than motion.

Land Geophones

III - 2D Profiles (Onshore-Offshore)/3D Inlines (Offshore). 3 – Receivers. a – String Patterns (onshore).

Geophone String

12 geophones connected in serie

Shot point Group 1

Group 2

Receiver Group

III - 2D Profiles (Onshore-Offshore)/3D Inlines (Offshore). 3 – Receivers. b – Group Interval (onshore/offshore )

The figure illustrate the concept of Group interval, Line array and the Subsurface Reflection Points, also called Subsurface Coverage. Group interval

Recording truck

III - 2D Profiles (Onshore-Offshore)/3D Inlines (Offshore). 3 – Receivers. c – Split-Spread (onshore) - Single End (onshore/offshore).

Split-Spread (onshore) Active recording groups are in both sides from the source (Symmetric or Asymmetric)

Single End (offshore/onshore) Conventional marine seismic data acquisition method using a single vessel to tow one or more seismic source arrays and streamers in a straight line as the vessel records seismic data. In the land case the active recording groups are only in one side from the source (called front end or back end).

III - 2D Profiles (Onshore-Offshore)/3D Inlines (Offshore). 3 – Receivers. d – Energy Source and Receivers Gap.)

The horizontal distance from the source to the first receiver group.

III - 2D Profiles (Onshore-Offshore)/3D Inlines (Offshore). 3 – Receivers.

e – Nominal Fold/Bin size.

FOLD The number of traces that have been added together during stacking is called the fold. • • • •

If we are shooting on every group into a single end spread with N number of groups, the fold that we get is equal N/2. If we are shooting on every other group into a single end spread with N number of groups, the fold that we get is equal N/4. If we are shooting on every group into a split-spread with N number of groups, the fold that we get is equal N/2. If we are shooting on every other group into a split-spread with N number of groups, the fold that we get is equal N/4.

AZIMUTH The angle between the vertical projection of a line of interest onto a horizontal surface and true north or magnetic north measured in a horizontal plane, typically measured clockwise from north.

BIN A subdivision of a seismic survey. The area of a 3D survey is divided into bins, which are commonly on the order of 25 m [82 ft] long and 25 m wide; traces are assigned to specific bins according to the midpoint between the sources and the receiver , reflection point or conversion point. Bins are commonly assigned according to common midpoint (CMP), but more sophisticated seismic processing allows for other types of binning. Traces within a bin are stacked to generate the output trace for that bin. Data quality depends in part on the number of traces per bin, or the fold.

III - 2D Profiles (Onshore-Offshore)/3D Inlines (Offshore). 3 – Receivers. f - The recorded signals and the effects of the receiving string.

Ground Roll and & other noise waves A type of coherent noise generated by a surface wave, typically a lowvelocity, low-frequency, high-amplitude Rayleigh wave. Ground roll can obscure signal and degrade overall data quality, but can be alleviated through careful selection of source and geophone arrays, filters and stacking parameters.

Measuring wavelength on noise records

Let us assume we want to eliminate a surface wave with a 27 meter wave length arriving to a 12 geophone string as shown below.

Geophone String

12 geophones connected in serie

Shot point Group 1

Arriving Surface Wave

Group 2

Response of a string of geophones sin(nsPi) R(s) = --------------n x sin(sxPi) Notation key: R(s) = Array Response ( 0 < R < 1 ) n = number of geophones s = geophone spacing / noise wavelength ( 0 < s < 1 ) Pi = 3,14....

Note: these apply only to waves travelling horizontally

Linear noises nowadays are efficiently eliminated by digital filtering during processing. However, we need to remember that the arrivals at each geophone of an array are electronically summed in the field and therefore, “what is done is done !!!”. In other words, we should be more concerned with what we have done to the reflected signals in the field when trying to Reduce Ground roll or other linear noises.

Angle of emergence Receiver 1

Receiver 2 X2 – X1

a

a = arc sin ((t2-t1) / (x2-x1)) a = angle of emergence t2 – t1 = arrival delay time x2 – x1 = receivers distance

Geophone array of 12 geophones with 5 meters interval 1.2 Arrival wave

1

Total Response

33 Hz Peak freq.

Amplitude

0.8

The sweeping wave across the array is 2500 m/sec. It is assumed there is only a linear moveout across the array.

0.6

27 Hz peak freq.

0.4 0.2 0 0

-0.2 -0.4 -0.6

0.05

0.1

0.15 Time in seconds

0.2

From the previous slide is easy to understand that for a given velocity, the larger the angle of emergence, the bigger will be t2 – t1 and therefore, the stronger the deterioration by narrowing the bandwidth of the arriving wave. Let us see now what happens when the reflecting interface is not horizontal.

Shooting up-dip Shooting down-dip

Surface

Reflector Remember that, standard geophones are only sensitive to vertical ground motion !.

Particle Velocity Vertical component Horizontal component

What happens when there is a non linear moveout across a geophone string?, in other words, what the string array will do to the reflections ? . 0

6

12

18

24

30

36

42

Two way time (sec.)

0.89992 0.89994

Distance (m.) We know the zero offset time. We know the RMS velocity. We know the arrival time at offset 18 m. We can calculate the Va.

0.89996 0.89998 0.9 0.90002 0.90004 0.90006 0.90008 0.9001 0.90012 0.90014

Tangent = dx/dt = apparent velocity, Va

MOVEOUT

What we have learned from the two previous slides ?: 1 – It is better to have short string of geophones. 2 - It is better to have small number of geophones per string (5 or less ?). 3 – It is better to have small distance between geophones (2 to meters or less ?). or: 4 – Would it be better to have just one geophone per group only?. This is

something worth to think about it !

The effect of the streamer depth in the marine case on the recorded signal, is the same to that of the source array depth i.e., the deeper the streamer the further reduction is made to the high frequency content of the recorded signal.

Vessels are typically about 75 m [246 ft] long and travel about 5 knots [9.3 km/hr or 5.75 statute miles/hr] while towing arrays of air guns and streamers containing hydrophones a few meters below the surface of the water. The tail buoy helps the crew locate the end of the streamers. The air guns are activated periodically, such as every 25 m (about 10 seconds), and the resulting sound wave travels into the Earth and is reflected back by the underlying rock layers to a hydrophone and relayed to the recording vessel.

How these parameters are related ?. • • • • • •

Speed of the Vessel Shot interval Recording time Feathering Weather Streamer depth

Let us define first a couple of concepts.

Feathering In marine seismic acquisition, the lateral deviation of a streamer away from the towing direction caused by water currents.

Footprint

The area covered by an array of towed streamers in marine seismic acquisition or, in the onshore acquisition case, a grid of receivers planted in the ground.

9If the speed of the vessel is 9.3 Km/hr and we are shooting every 25 meters it means that, we have a maximum time of 10 seconds between shots and therefore, that is all the time we have to record the incoming signals, record positioning information and the compressor on board to the build up the pressure on the guns to shoot again. 9It seems plenty of time but, if we need higher spatial resolution ,let us say shot every 20 meters then the required shooting time interval will be 8 seconds. 9When strong water currents occurs, feathering could be very severe and, this will force us to increase the speed of the vessel and consequently, reduce the time to accomplish all the tasks we have mentioned above. In this case let us increase the speed from 9.3 Km/hr to 11.3 Km/hr then now, we have only available 6.4 sec. between shots. What happens in this situation when we are shooting in deep water trying to map deep targets ?. I

am sure you can answer this !.

Near and Far Offsets Source

Near Offset

Far offset

Surface

Reflector 1

Reflector 2

For a given offset, notice the decrease of the angle of incidence with the Increase of the depth of the reflector

From the previous slide we know that for a given spread length, the deeper our target is, the smaller become the angles of incidence. Therefore, in the case in which we are seeking deep targets, longer streamers are required to ensure we get a wide range of pre-critical angles of incidence. This can create significant operational problems. Long streamers are more difficult to manage when feathering occurs, turning takes longer also and, take more power towing them. To overcome these difficulties two vessel are used, one carrying guns and, a second one navigating one streamer length behind carrying also guns and a short streamer.

TWO VESSELS CONFIGURATION Vessel 1

Vessel 2 Gun array

spread Gun array

The shooting of the first vessel into the spread towed by the second will generate long offsets while, the shooting of the second vessel will generate the near offsets. This approach required a sophisticated recording and navigation system to synchronize properly such operation.

The fundamental aim of the design of a seismic survey is the imaging of the subsurface with as many as possible of its complexities. In achieving this many methods and techniques have and are being developed and refined. With the more frequent use of 3D surveys (both Onshore & Offshore) it has become clear that a sufficient fold, range of azimuths

and pre-critical angles are key parameters of the design.

IV – Field Tests. a – Source depth & size; Geophone String position and elements spacing.

When not sure what parameters you want, field testing before the survey starts should be done. In the onshore case have a spreads laid out with the group interval that will give you the required subsurface space resolution and pre-critical angles. Have drilled as many shot holes as you will require to test the source size and its depth. Sources Small spreads with different geophones per string

Onshore source testing • Two parameters are important: Depth and Size. For deciding on the depth, the most critical is determining where is the WL. In finding that, a hole should be drilled deep enough to detect a lithology change that may suggest you have reached it. If you have Legacy data, read the previous acquisitions and processing (refraction statics info !.) reports. If these are not available , try to interpret a seismic contact. • As for size is concerned, it will depend on the depth of your target. The deeper the target is, the bigger the source. The higher the resolution you need the smaller the charge. A balance of these two parameters should be determine in order to meet the specific requirements of your project. Each project is a different ball game !!.

Length of the spread • In determining the length of the spread you need to consider three parameters : Fold, Pre-critical angles (azimuths also in 3D) and Subsurface spatial resolution. • The fold will depend on the number of groups on the spread and the shooting interval which in turn, will determine the subsurface space sampling you will get. • As for the angles remember that the deeper your target is, the longer offsets you will need. The best approach to this is to do a ray tracing modelling of your particular objective .

Before you star a seismic survey whether, 2D or 3D, Onshore or Offshore, it is important that you model different approaches that are consistent with the objectives you are looking for. This will help you to decide the optimum parameters to achieve it.

END & THANK YOU !