PRAKLA-SEISMOS INFORMATION No. 3 /\ PRAK1A·SEISMDS

Areal Reflection Seismies -V V

! BUilding-EJlock Syste~'s, a selection of operational

3-D data acquisition systems

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General Earlier than expected by the industry, seismic 3-D tech­niques have arrived on the scene of hydrocarbon and mineral prospecting. PRAKLA-SEISMOS' contribution to this field was introduced to the public in its main brochure "Land Seismic Surveys", late in 1976, presenting the concepts and patterns of three distinct areal seismic surveys. This brochure is intended to render a more comprehensive view of our contribution to the present state of this art.

System A 75: 96 channels, 6 fold coverage

8 parallel geophone lines are recorded simultaneously from SP's arranged on traverses perpendicular to the lines. Roll-along switch box is employed as in conventional line surveys.

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The essence of the areal concept consists in providing a narrow grid of information on subsurface structure. This information is stored in a data bank, being at disposal for • furnishing quasi-conventional sectional displays in any

desired direction, • 3-D migration processing, likewise in any desired

direction. Superior signal-to-noise ratio and strength of multiple­reflection attenuation are the most significant properties of 3-D migration sections (see examples on page 5) . The large number of differently composed CDP-families ensures effective multiple suppression.

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red end bl ut' spiders :

Iwo a fbllrory 6- fol d CDP's .... ,11 th Ine rps pt>C! lvt' dis ionces

bt' t we.n shot I rece lver POIr S

subsurtacE' coordmote x surfac . coord lnot t>

.. sho t on trOY "'f St' 6 5 .. st'lo t on tra ve rsE' 85 .. sho t on troversE' 10 5 .. shot on trovpr se 125 scal~

Subsurface Coverage in X-Component (3-fold) o 100 200 300 ' DOm

Shot-Rece iver configuration and respective subsurface coverage

2

The incentive for areal surveying was initiated by the German coal-mining industry. The amazing success of the first areal survey late in 1975 was followed by aseries of further areal surveys, each one being adapted in its pattern to actual problems. Thus, some specific types of survey arrangements were developed, classified in so-called Building-Block Systems. A representative selection of these is presented in this brochure .

Building-Block Systems are the way we attack the problems of areal seismic detailing and reconnaissance surveys. They provide : • adaptability to the time range of main prospective

interest, • economic solutions of detailing problems through

restriction and concentration to the area in question, • easy later completion work at full coverage degree when

an areal extension is wanted, • adaptability to the terrain conditions encountered in

detailing as weil as reconnaissance prospection .

System C 76: 96 channels, 8fold coverage

This is the prototype of our Building-Block Systems. 20 shots at positions within and surrouding the block layouts are recorded on 96 seismic channels. No trace switching is applied. The single compartments of the subsurface

block produced (shown in red), being different in their coverage degrees, are filled up to final 8fold by specific overlaps of adjoining subsurface blocks.

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CDP's

Red figures indicate degree of coverage in the respective compartment s .

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0--0--0- geophone cable and stations

EIl SP's for twin-block 2/3

+ SP's for other blocks

Multileg spiders show 2 CDP- families and their shot - receiver configurations

The display on the cover shows a cut-out of astation location map of the survey system C 76 and the 8fold CDP's of the areal coverage.

3

System B 76 . Rhine Survey: 96 channels, 8fold (mean) coverage

A 6 km-section of the meandering River Rhine was covered by an areal survey within one week. The mean degree in coverage was 8 fold . Comprehensive knowledge on the subsurface structure was thus obtained in a fo rmer "wh ite zone" in seismic mapping. The field technique applied : Airgun pops fired in 25 m in tervals from a ferry along traverses across the river were

4

recorded by two 48-channel recording instruments, one on each river side. Four parallel geophone lines in 200 m intervals and with 12 stations each on both river sides were recorded from each SP-traverse, the traverse separations being 100 m. The total number of profiles produced was 113, each being 1 km in length, the area covered thus being 5.75 km2.

Because of the bended river course the total length to be surveyed had to be subdivided into three slightly over­lapping blocks. The location map of Block 111 shows the configuration described and the traces of the seismic profiles produced and stacked. The red lines perpendicular to the profile traces indicate the positions of the sequence of cross-sections displayed on page 5. These cross-sec-

tions are based on are-arrangement of the respective traces of the primary sectional stacks. The left column is the line-up of the stacked traces, the others are the result of the respective 3-D migrations along these lines; the right-hand column shows an interpretation of the Cretaceous basis and of the faulted Carboniferous top.

Sequence of re-arranged sections

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by courtesy of Ruhrkohle AG, Essen

5

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System G 77: 48 channels , 6fold coverage

The pattern on this page shows a variant of our Building­Block Systems suited to a VIBROSEIS* survey in the desert, where a flank of a heavi ly faulted structural high zone with a number of productive wells was to be covered by an areal survey of 5 km in length (strike direction) and 2.5 km in width.

This design was adapted to the surface-source method by applying suitable lateraloffsets and avo id ing physical interference of the vibl-ators with the areal receiver layout. The performance attainable: two blocks per day of 48 stations each , recorded from 40 vibrator stations .

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System J 77 . Framework- (Fachwerk-) Shooting: 48 or 96 channels , singlefold coverage. A seismic survey system operating along the perimeters of inaccessable areas to be prospected and designed for singlefold coverage .

On this page another variant of the Building-Block Systems is introduced which provides areal seismic cover­age below those areas where regular layout of shot and receiver stations is not feasible (due to topography, settlements, prohibited access, lack of permits etc .). The underground coverage density depends on the aptitude of the arrangements of shot-receiver configurations and

on receiver and shot spacings. Deviations from strictly geometrical subsurface gridding caused by survey operations along somewhat irregular perimeters - are not prohibitive to later data processing, as long as the midpoint positions between emitter and receiver stations are weil defined.

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7

System F 77: 48 or 96 channels, singlefold coverage

This figu re demonstrates the pattern of a geometrically regular system which provides closely spaced UP's (under­ground points) in 12..5 m spacing and singlefold coverage, in spite of shot-and receiver spacing in the order of 100 m. This pattern was designed for more ambitious high-resolu­tion demands in 3-D migration .

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receiver spacing : in x - direct ian 100m in y -directian 50m

maximum shot I receiver spacing : - 900m 125m

375-425m

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CDP-spacing : in x-and y

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Note: the areal concept of the Building-Block Systems can be transferred to marine and shallow-water seismic surveys.

f\ PRAKlA·SEISMDS

"\J V

PRAKLA-SEISMOS GMBH· HAARSTRASSE 5 . P.O.B. 4767 . 0-3000 HANNOVER 1 PHONE: 80721 . TELEX: 922847· CABLE: PRAKLA . GERMANY

© Copyright PRAKLA-SEISMOS GMBH, Hannover 1977

3000577

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