APPENDIX A
Technical requirements
for 3D seismic acquisition on the FIELDS
TAKOUAZET WEST AND TAKOUAZET EAST
Block 245-south Gara TIsselit
Algeria
C O N T E N T S
1. Subjects and Objectives of the STUDY
2. Data Acquisition System
3. supplementary operations
3.1. STATIC CORRECTIONS
3.2. TOPOGRAPHY
3.3. SITE CLEARANCE
4. QUALITY CONTROL
4.1. GENERAL PROVISIONS
4.2. MINIMUM PROCESSING GRAPH
4.3. APPARATUS TESTING
4.4. Data recording
5. TOPOGRAPHY
6. MVS (Micro-velocity survey, UPHOLE TIME MEASUREMENTS)
7. DOCUMENT SUBMITtal AND TIME terms
7.1. DOCUMENTs be SUBMITTed
7.2. TIME terms FOR DOCUMENT SUBMITTAL
7.3. DOCUMENTS AND model REPORTS be submitted BY CONTRACTOR
1.Subjects and Objectives of the study
In the territory of discovered fields Takouazet West and Takouazet East one proposes to conduct seismic survey CDP-3D in the area 360 km2 (full coverage fold area being 250 km2) to obtain greater detail on geological structure of identified HC traps and choose points for spudding wells for delineation of the oil-gas-condensate accumulations (Fig.1).
Fig.1. Schematic map of the area
Seismic survey 3D is proposed to be run in the zone of Wells TAKW-4, TAKW-5, TAKE-4, TAKE-6, TAK-1. The zone outlines are chosen so as to study in detail reservoirs mapped by previous surveys CDP-2D (2002-2003). Being capable to provide greater detail, the 3D survey will permit a more accurate tracing of faults throughout the area, especially those of small extent and throw. As a result of combined interpretation of the CDP-3D seismic and drilling data it will become possible to study lateral distribution of reservoir properties. Moreover, the 3D survey will permit to delineate local small-extent sedimentary bodies which may serve as reservoirs for hydrocarbon accumulations.
The goals of the proposed 3D seismic acquisition are as follows:
-detailed study of relief of interfaces in the Devonian and Ordovician sections and prediction of reservoir properties of productive reservoirs (F6 C1, F6 C3, Unit III-2 and Unit IV-1);
-detailed structural mapping of small and complex-shape highs;
-mapping of basement surface as well as identification and tracing of faults which may provide tectonic control for accumulations in Devonian and Ordovician deposits.
The study area relief is a plateau with absolute (a.s.l.) elevations varying from 500 to 580 m. The southern part of the Area is crossed in the latitudinal direction by a valley up to 3 km wide with abrupt margins.Terrain level in the middle of the valley is by 60-80 m lower than in elevated zones. The valley margins are very steep, so one expects skipping of shot-points here.
The surface seismic-geological conditions in the planned study area are essentially variable. Bed rocks are often covered with sand deposits unfavorably affecting the seismic energy generation. In some zones at the surface there occur compact high-velocity in places cavernous marls which also create conditions unfavorable for the seismic signal generation and reception.
One of the main factors controlling efficient solution of geological problems is the necessity to acquire seismic data with maximum ratio signal/noise and high resolution of the signal in time intervals associated with target horizons. On seismograms CDP-2D recorded in the study area in 2002-2003 one can see a powerful background of various wave-noises, such as sound, surface, multiple, refraction and secondary generation waves. The level of these noises is particularly high in the southern part of the Area (on the southern cliff) where (according to drilling data) the high-velocity marl layer is particularly thick (up to 20 m). Therefore, prior to conducting field operations one has to organize pilot investigations to substantiate the field data acquisition methodology. Higher ratios signal/noise are known to depend on both optimum conditions for energy generation and choice of efficient system of observations, so the pilot work program must include operations as follows:
- choice of optimum parameters for sweep-signal;
- obtaining of test seismic materials by varying accumulation of sweep-signals to determine the degree of efficiency of the chosen data acquisition parameters. The proposed spread for signal reception is schematically shown in Fig. 2.
Fig.2. Active spread including 1680 channels. System «cross-type», 14 receiver lines 120 channels in each (120 Х 14). Receiver line - source line spacing is 250 meters. Source-receiver far offset is 3439 m
2.Data Acquisition System
Survey Type /CDP-3D
Seismic station / I\O ImageNumber of active channels / 1680
Bin size / 25 х 25
Recording Parameters
Total number of channels / At least 4000
Geophone type / SM-24/SM4-LD
Number of geophones in a group / 6 х 2 = 12
Connection of geophones / Parallel-series
Geophone spacing / 4.17 m
Geophone group base / 45.87 m
Sampling rate / 2 ms
Record length / 3 s
Recording medium / 3480 and 3592 cartridge tapes, capacity 60 Gigas.
Recording format / SEG-D
High-pass filter / open channel
Low-pass filter / 0.5 Nyquist frequency, minimum-phase
Data Acquisition Technique
Spread geometry / cross-type
Fold of coverage / 84
Fold along X axis / 12
Fold along Y axis / 7
Near offset, m / 35.3
Far offset, m / 3449
Far offset along X axis, m / 2975
Far offset along Y axis, m / 1725
Number of receiver lines per swath / 14
Receiver line spacing / 250 m
Receiver spacing on receiver line / 50 m
No of recording channels per receiver line / 120
Source line spacing / 250 m
Source spacing on source line / 50 m
No of SP per source line for one block / 5
Receiver line shift after swath shooting / 1
Energy Source Parameters
Source type / Vibrators I\O AHV-IV or equivalent.
Number of vibrator units / (4+1)x2
Peak force / 30 t
Number of sweep signals / 4
Sweep signal length / 12 s
Sweep signal frequency / 10-82 Hz (to be test-determined)
Vibrator unit arrangement / 4 vibrators in line
at 15 m spacing(to be test-determined)
Dynamic grouping / Vibrators to be moved up by 5 m after each vibration action (to be test-determined)
Coordinates of the 3D area corners (Clarke 1880 coordinate system):
Point № / X-coordinate / Y-coordinate1 / 404200.0 / 3141000.0
2 / 400900.0 / 3150700.0
3 / 434400.0 / 3162400.0
4 / 437700.0 / 3152700.0
3. supplementary operations
3.1. STATIC CORRECTIONS
- field calculated model: defined at the work start
- design parameters:
- DP = + 450 m
- Filling velocity- to be determined based on test
3.2. TOPOGRAPHY
Topographical survey is carried out with the help of system GPS (Trimble 4000 SSI, 4700 real time DGPS). Besides the control geodesic points situated in area, it is necessary to do survey of at least 5 points distributed in all the work area and mark them on the terrain.
All benchmarks, logs of old lines and wells shall be positioned with the help of GPS system.
3.3. SITE CLEARANCE
At least 2 bulldozers to clear out 4-meter wide strip for source points.
4. QUALITY CONTROL
4.1. GENERAL PROVISIONS
Recording methods, filters and other parameters indicated in this document may be subject to changes upon Client’s supervisor written approval only.
Equipment: Field processing system for 3D data processing.
- Control over various apparatus tests;
- Field records visualizing;
- Below-mentioned processing graph application;
- Calculation and application of static corrections making use of adjustments derived from first arrivals and tie-up the static corrections on at least 2 MVS (UTM) wells;
- Repeated recording of topographic data, static corrections, preliminary sums and operator reports on cartridges;
- Verify compliance of operator reports;
-Noise level control;
- RMS amplitude for each excitation.
4.2. MINIMUM PROCESSING GRAPH
- Geometry description.
- Geometry control.
- Amplitude correction for geometrical divergence.
- Obtaining preliminary sections with a priory static and kinematical correction. Updating the priory static corrections, whenever required.
- Suppression of medium velocity distortions (whenever required).
- Deconvolution and band filtering (with Deconvolution tests attached).
- Equalizing seismic record amplitudes.
- Obtaining temporal sections with a priory static and kinematical correction.
- Analysis of stacking velocities.
- Static corrections adjustment.
- Obtaining time sections with updated static and kinematical corrections
In the process of conducting the preliminary processing certain amplitude-frequency evaluations of signal (spectrum) are required before and after deconvolution, which upon completion of preliminary processing shall be handed over to the Client together with final sections resulted from the preliminary processing
This processing graph may be subject to changes by Client’s supervisor, who may also request to conduct additional tests.
4.3. APPARATUS TESTING
As described below, one should periodically conduct a complete testing of apparatus with tape-recording to verify its correspondence to specifications indicated in registers and confirm its manufacturing quality:
- at survey start and completion;
- once a month with the survey progress.
All apparatus test results shall be submitted to Client’s Supervisor in the field on the day of testing.
Moreover, the test results shall be processed and analyzed by service personnel.
Whenever necessary, the check-up and base tests of the systems shall be conducted each recording day in accordance with the equipment manufacturer instructions.
Contractor shall submit to Client’s Supervisor all such equipment/instrument documentations.
4.4 Data recording
The list of tests and order and timing of their performance shall be agreed with Client’s supervisor prior to commencement of field operations. This includes daily noise-test with operating vibrators and evaluation of noise level by Operator and Client’s supervisor.
To determine maximum number of faulty channels one has to follow instructions as follows:
Total number of recorded channelsper excitation point / Maximum tolerable number
of faulty channels per excitation point
Up to 120 / 1
121 – 240 / 2
241 – 320 / 3
321 - 480 / 4
For 1680 channels of one complete spread the number of faulty channels shall not exceed 4, the number of adjacent faulty channels not exceeding 2 and the faulty trace being separated by bat least three quality traces.
Terrain elevations of geophones of one channel should not differ from each other by more than 3 m. On steep slopes one should place the geophone in point, making respective note in the operator report.
Receiver lines shall be tested daily prior to routine work commencement and located within the above indicated limits.
If during the day the recording was interrupted for the excessive noise level due to wind or any other reason, which may cause the parameters to exceed permissible limits, all the recording system shall be checked with the aim to guarantee the resumption of work within specified limits.
а)CABLes aND geophones
а/1All cables, geophone groups and telemetry modules shall be numbered so as to exclude erasing for the purpose of facilitating scheduled maintenance operations. One shall keep a register book ofinspection and repair operations for each numbered apparatus unit.
All geophones shall be checked for polarity, faults and integrity of insulation at least one time a month. All cables shall be checked for faults and integrity of insulation at least once a month.
а/2In Contractor’s monthly report there shall be summarized data on all completed servicing operations concerning cables and geophones. Twisting is not permissible when treating cables and geophone groups. Damaged telemetry cabling shall be replaced. Cable and geophone group insulation resistance shall be at least 1 Mohm.
а/3Geophone polarity is to correspond to Standard SEG. Geophone misphasing shall be less than 1 (one) millisecond at 20 Hz for geophones with own frequency equal or inferior to 20 Hz.
All equipment must be standardized.
b/VIBRATor units
b/1Similarity tests on cable shall be conducted within the following time frame:
- at survey start and end;
- once a week;
- after each vibrator repair.
b/2Radio channel similarity tests shall be conducted within the following time frame:
- every time new vibrator is put into operation;
- every time sweep parameters are changed;
- twice daily at work start and end.
b/3Client’s Supervisor may request the similarity tests of cable and radio channel to be conducted more often depending on operation conditions and equipment specifications.
All similarity tests of cables shall be recorded on working tape and analyzed as soon as possible or at Client’s Supervisor request.
b/4Maximum tolerance by phase between signal «Ground Force» and pilot signal shall be + / – four (4) degrees.
b/5Phase tolerance between vibrators shall be less than or equal to 5 degrees (absolute value).
b/6Maximum tolerance by phase between signal «Ground Force» and pilot signal is one (01) millisecond below 30 Hz. All characteristics of vibrosources shall comply with standard SEG.
b/7THD is less than or equal to 25%.
5. TOPOGRAPHY
Main objective is to identify coordinates X, Y, Z of centers of geophone groups and excitation (energy) source groups.
Special attention is to be paid to mapping of obstacles and any possible hazardous zones, which may hamper normal progress of operations.
а/Accuracy of positioning
а/1General requirements:
In agreement with Client one should choose system of coordinates (WGS84, Clarke 1880 or other) to be used in all operations.
All the intervals between reception and excitation points shall be determined in projection to horizontal plane. Distance error shall not exceed accuracy of portable apparatus (Trimble SSI) used to mark Lines.
а/2Excitation lines and points.
Contractor may not deviate from excitation points and lines specified in the form of theoretical coordinates, except at unfavorable terrain conditions or access, when contractor may curve the excitation/reception point lines without breaching the below-indicated limits. All and any possible actions shall be taken in order to ensure that excitation and reception point lines are routed in compliance with their project (theoretical) position.
Skipping of project excitation points because of impossibility for vibrator access should not decrease the coverage fold in the survey full- coverage zone by more than 10%.
Transverse offset of excitation point for the obstacle reason must not exceed 20 m. However, Client’s supervisor, if necessary, may authorize such an increase.
Zones where positioning of excitation points is impossible should be surveyed by topographers in a good time and the Quality control group of the Contractor has to prepare a plan for the shooting with recalculation of the coverage fold map.
Manager of field operations shall observe that the clearing of excitation lines (and receiver lines, if necessary) with bulldozers is conducted in optimum manner.
Numbering ofexcitation lines and points shall be done from East to West and from South to North starting with point 101.
а/3Geophone group
Geophone groups shall be positioned so as to put a group center as close to survey peg as possible, the distance between them not exceeding one (1) meter.
b/GPS USE IN RTK-DGPS MODE
* CALIBRATION: At the beginning of survey GPS equipment shall be calibrated according to methods and rules commanded by its manufacturer.
DGPS base station shall be installed in location approved by Client’s representative.
* IN-FIELD QUALITY CONTROL
On daily basis prior to commencement of seismic activities and any relocation of base station one shall conduct vibrator positioning test with tolerance of +/- 1m.
b/1Contractor shall submit to Client’s representative for approval the GPS control grid closure (or equivalent) diagrams. Antenna height values shall be measured independently in meters and recorded into each station card.
Summary data of GPS base lines processing and grid correction shall be systematized and included into final report.
b/2Positioning of excitation and reception points
Differential system GPS (DGPS) is used for positioning sources and receivers (400 SSI, DSM-12R).
b/3 DGPS base station shall be placed in a location known, reconnaissanced and approved by Client’s representative. Position and elevation of the location shall be determined by way of multiple observations.
GPS base receiver at a base station shall calculate correction on differential pseudo-distance for all GPS satellites observed.
These corrections shall be transmitted by radio to portable station receivers to adjust pseudo distances determined at portable station prior to calculation of its position.
Distance between base station and portable receivers as well as distance to the tracking station shall be as small as possible and not exceed 15 km.
Contractor shall make sure that selected points do not have multiple channels errors. All computer hardware and software to be used at the base station, portable station and tracking station must be of one and same model and version.
b/4Portable station
The portable station shall be complete at minimum with items as follows:
- GPS receiver capable of registering pseudo-distances measured with at least eight (8) satellites simultaneously via physical channels;
- radio communication system allowing for reception and demodulating differential correction signal emitted by the DGPS base station.
Aerials shall be positioned vertically over survey point and have no obstacles for direct visibility to horizon all around 360°. The aerials shall be mounted far from any electrical equipment which might impair aerial operation and, if possible, at a height of at least two (2) meters above any horizontal surface.
b/5Post-processing: Topographic unit computer shall ensure the following functions:
- download data package on positioning from data receiver on the same day;
- adjustment of DGPS positions (base GPS) to datum level and their further conversion to projection grid coordinates;
- reducing the height of the DGPS portable station aerial expressed in meters above the WGS84 steroid to standard datum of MSL;
- when performing calculations one has to take into account vertical GPS aerial displacement and geoid undulation;
- comparison of points by theoretical data and survey data;
- quality control of planimetry and altimetry by means of graphic image;
- by the end of each day a summarized statistical information is issued: correction quality indices, number of available satellites, quantity of data registered and mean PDOP for the observation time period;
- all the above mentioned data on quality control shall be reviewed jointly by representatives of contractor and client and final coordinates shall be agreed upon before starting seismic data recording.
b\6Checking
At the start of survey the portable stations shall he located at a second known point to be controlled from base station.
Allowance (permissible deviation) is equal to 15 cm for planimetry and altimetry at the survey start and at each line end or at the commencement and end of each recording day, and also after each base station replacement. The DGPS system shall be checked up by comparing position corrected by differential method from each portable station with initial values based on survey data or adopted values. Position and altitude allowance shall be one (1) meter.