NGU - BULL 427, 1995 Eirik Mauring & Jan Steinar Ranning 89

Combined use of common depth point and common offsettechniques in shallow reflection seismics

EIRIK MAURING & JAN STEINAR R0 NNING

Eirik Mauring & Jan Steinar Rann ing, Norges geologiske underseketse, P.O.Box 3006-Lade, 7002 Trondheim, Norway.

Fig. 1. Common events on a walkaway noise test reco rd.

microcomputer for data acquisition and proces­sing. High-resolution shallow reflection seismicscan now be successfully applied to geological,engineering and environmental problems (e.g.Meekes et al. 1990, Pullan & Hunter 1990, Miller& Steeples 1994). The most commonly appliedshallow reflection seismic techniques are com­mon offset (CO) (or optimum offset) (Hunter etal. 1984) and common depth point (COP). In thiscontribution we discuss the pros and cons ofboth methods. We also show how CO-data canbe derived from data recorded with the COPtechnique and how CO- and COP-data both canbe used in the interpretation of reflection seismictime sections.

The common depth point and com­mon offset techniquesChoosing the optimum windowA seismic time section is shown in Fig. 1 (fromHunter et al. 1988). The section was recorded asa walkaway noise test (Knapp & Steeples 1986).The optimum window (Hunter et al. 1984) is thehorizontal range in which reflectors of interestcan be viewed with a minimum of noise interfe­rence. To reduce the amount of surgical muting,the left (near-offset) side of the window is chosenso that noise events from surface waves (groundroll and ground-coupled airwaves) arrive afterthe reflections from the deepest target of inte­rest. There are several factors governing thechoice of the right (far-offset) side of the window;(1) reflections from shallow targets will interferewith the direct wave or refractions at great off­sets; (2) reflections from shallow targets will bewide-angle and suffer from phase- and amplitudechanges according to the Zoeppritz equations(Pullan & Hunter 1985); (3) apparent low fre­quency pulses and loss of resolution will be theresult after time stretching from NMO corrections(see below) of wide-angle reflections.

From the points listed above, it is obvious thatthe choice of the position of the right side of theoptimum window will affect the definition of shal­low reflectors.

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IntroductionThe application of high-resolution shallow reflec­tion seismics is dependent on the ability of thenear-surface material to propagate high-frequen­cy seismic energy in the 100-500 Hz range. Inoverburden, this is normally encountered in are­as with a high water-table and fine-grained surfa­ce materials (Hunter et al. 1984). Under theseconditions, reflectors at depths of ten to severalhundred metres can be mapped. Over the lastten years equipment has been developed totransmit and record high-frequency seismicenergy using in-hole shotguns, high-frequencygeophones and digital seismographs with a highdynamic range. The method has become morecost-effect ive through the introduction of the

90 Eirik Mauring & Jan Steinar Ronning GU · BULL 427. 1995

Fig. 2. Common depth point profiling.

Pros and cons of the COP and COtechn iquesWhi le the COP technique requires cost- andtime-consuming processing , very little proces­sing is needed with the CO technique , allowingthe user to quickly obtain a picture of the subsur­face without having to first determine its velocitystructure . Oata acquis ition efficiency is quitesimilar for the two techniques. Oue to multifoldcoverage of subsurface reflectors , the signal-to­noise ratio is better with the COP techn ique.Shallow reflectors are often ill-defined with theCOP technique due to large offsets and wide­angle reflections. The definition of shallow reflec­tors can in some cases be improved by perfor­ming amplitude versus offset (AVO) analysis toreduce move-out errors (Ursin & Ekren in press).

into COP gathers (Fig. 3). Prior to stacking thetraces in the COP gathers, one has to performnormal move-out (NMO) correction. This correc­tion has the effect of moving the source andreceiver to the location directly above the reflecti­on 'point' . The correction is dynamic, and itsmagnitude is dependent on source-receiver off­set, two-way trave ltime to a reflector and theseism ic velocity of the medium above a reflector.Velocit ies can be found by performing velocityanalysis on COP gathers . The NMO correct ioninvolves time stretching that becomes severe atlarge offsets and for shallow reflectors. Thus,shallow reflectors won't stack or they become ill­defined and blurred. To be able to record bothshallow and deep reflectors with a minimum ofsurgical muting, the optimum window can be qui­te narrow, and the production rate becomessmall.

The extraction of common offset (CO) datafrom multichannel records allows us to broadenthe optimum window . By doing so, there will benoise from surface waves on the near-offset tra­ces late on the COP record that can be removedby surg ical muting. CO-data can be assembledby picking the same channel from subsequentCOP records. CO can also be carried out as astand-alone acquisition technique as shown inFig. 4. Depth conversion of CO records is non­linear.

Examples of COP and CO recordsThe first example is from Vigra, western Norway.The objective of the survey was to map bedrocktopography. Fig. 5 shows the COP record (left)and the CO record. The records are derived fromthe same multichanne l records. Recording timeis 100 ms, distance from source to the near-off­set receiver is 20 m and the receiver spacing is 4

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Fig. 4. Common offset profiling.

Short outline of the COP and COtechn iquesWhen conduct ing a COP seismic reflection sur­vey, it is ideal to place all receivers equal spacedin-line inside the optimum window. Once havingfound the appropriate source-and-receiver spa­cing and the offset from source to the near-offsetreceiver, a COP survey can be conducted accor­ding to Fig. 2. The recorded traces are sorted

NGU • BULL 427. 1995 Eirik Mauring & JanSteinar Ronning 91

spacing is 5 m. The records are dominated byhorizontal reflectors, probably representing laye­red sand/silt (according to Riis, 1992). The eventat 105-110 ms is the bedrock reflector. Eventsbetween 20 and 30 ms on the COP record appe­ar low-frequent and incoherent due to NMO timestretching. Shallow reflectors can more reliablybe detected on the CO record. In the example, aweak overburden reflector can be observed atabout 30 ms (indicated by arrows).

ConclusionsCOP- and CO-data can both be derived frommultichannel records. While COP-data offer ahigher signal-to-noise ratio than CO, it is shownthat using only COP-data can give poor resoluti­on of very shallow reflectors. In the interpretationof seismic reflection data the combined use ofCOP- and CO-data has proven to give moreinformation than by using either COP- or CO­data.

ReferencesHunter, JA , Pullan, S.E., Burns, RA , Gagne, R.M. &

Good, R.C. 1984: Shallow seismic reflection map­ping of the overburden-bedrock interface with theengineering seismograph - some simple techni­ques. Geophysics 49, 1381-1385.

Hunter, JA, Pullan, S.E. & Higgins, R. 1988:Geometrics short course on shallow seismic reflec­tion for engineering and geotechnical studies.Unpublished course notes.

Knapp, R.W. & Steeples, D.W. 1986: High resolutioncommon-depth-point reflection profiling: Fieldacquisition parameter design. Geophysics 51, 283­294.

Meekes, JA C., Scheffers, B.C. & Ridder, J. 1990:Optimization of high-resolution seismic reflectionparameters for hydrogeological investigations in theNetherlands. First break 8, July 1990 , 263-270.

Miller, RD. & Steeples, D.W. 1994: Applications ofshallow high-resolution seismic reflection to variousenvironmental problems. Journ. Appl . Geophys. 3t ,65-72.

Pullan, S.E. &Hunter, JA 1985: Seismic model studi­es of the overburden-bedrock reflection. Geo­physics 50 ,1 684-1688.

Pullan, S.E. &Hunter, JA 1990: Delineation of buriedbedrock valleys using the optimum offset shallowseismic reflection technique. In Ward, S.H. (ed.),Geotechnical and environmental geophysics , vollll:Soc. Expl. Geophys., 75-88.

Riis, V. 1992: Avsetningsmodell og hydrogeologi avHaslemoen. Cand. Scient. Thesis, Univ. Oslo, 168pp.

Ursin, B. &Ekren, B.O. in press: Robust AVO analysis.Geophys ics.

AcknowledgementsThe authors wish to thank Drs, Bo H. Jacobsen, Ole B. Lile andMark A. Smethurst for reviewing this paper.

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Fig. 5. COP (left) and CO records derived from the same multi­channel records . Arrows denote bedrock reflector.

Fig. 6. COP (left) and CO records derived from the same multi­channel records. Arrows denote shallow, overburden reflector.

m. The CO record was assembled by picking thefirst trace from each multichannel record. Theexample illustrates the problem with shallowreflectors on COP records. The shallow bedrockreflector on the CO record (indicated by arrows)cannot be recognised on the COP record due toNMO time stretching and far-offset interferencewith direct/refracted waves.

The second example is from Haslemoen,southeastern Norway. The objective of the sur­vey was to delineate intra-alluvial structures. Fig.6 shows the COP record (left) and the correspon­ding CO record, illustrating the better signal-to­noise ratio that can be achieved with the COPtechnique. Recording time is 300 ms (only a partof the record is shown). Distance from source tothe near-offset receiver is 10 m and the receiver