Some considerations on shallow seismic reflection surveys q

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1 Ž. Journal of Applied Geophysics Some considerations on shallow seismic reflection surveys q M. Feroci a, L. Orlando a,), R. Balia b, C. Bosman a, E. Cardarelli a, G. Deidda b a Dip. Idraulica, Trasporti e Strade-UniÕersita ` La Sapienza di Roma, Area Geofisica, Via Eudossiana 18, Roma, Italy b Dip. Georisorse e Territorio-UniÕersita` di Cagliari, Cagliari, Italy Received 4 June 1998; accepted 26 May 2000 Abstract High-resolution shallow seismic reflection surveys require more attention to the choice of source and configuration, receivers and recording geometry for optimizing data acquisition than conventional oil exploration surveys. Moreover, some standard processing techniques to increase signalrnoise Ž SrN. ratio need special accuracy Žfor example, surgically precise removal of early-time coherent noise and iterative, small time shift static corrections.. This paper compares results obtained using different sources at two test sites: explosive, cap, shotgun, hammer and weight drop. Data from experiments using geophones with different natural frequencies and using various acquisition geometries are also compared. In data processing, it is demonstrated how increasing the SrN ratio for high-resolution results requires special consideration in some common processing steps Ž F K filter, first arrivals muting, elimination of air wave and static corrections.. The comparison, based on shot gathers and stack sections, shows that attenuation of high frequencies by the earth is the most significant influence on the spectral properties of the data, as expected the source itself also does have some influence on frequency content, depending to some extent on surface conditions. The high-velocity explosive sources produced the highest frequency reflections and best SrN ratio, because they have higher energy related to higher burnrblast velocity and source containment then the other sources and they are used in hole Ži.e. below ground surface where the air wave energy is more attenuated. but the shotgun also an explosive source was reasonably comparable to high explosive when used in hole. Special care must be taken during processing to insure artifacts are distinguished from real reflection events. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Shallow seismic reflection; Sources; Processing q This report is mostly from the PhD research of M. Feroci, whose contribution is the largest. The authors R. Balia and G.P. Deidda gave useful suggestions on data acquisition and worked on the field for the site of Cagliari. The other authors gave their contributions in the entire project but particularly E. Cardarelli for data acquisition, C. Bosman for processing and L. Orlando for both. Manuscript preparation and editing are by M. Feroci and L. Orlando. ) Corresponding author. Fax: q Ž. address: orlando@dits.ing.uniromal.it L. Orlando r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. Ž. PII: S

2 128 ( ) M. Feroci et al.rjournal of Applied Geophysics Introduction In the last 20 years, the growing interest in engineering and environmental problems has increased the use of seismic reflection surveys in the study of shallow targets Žhydrogeological, engineering, environmental, archaeological, and geotechnical problems.. The most important consideration connected with these methods is recording reflections with broad bandwidth Ž spectra shifted towards high frequency. and to attenuate as much coherent noise Žair wave and ground roll. as possible. To obtain that, it is necessary to choose carefully the sources, geophones, geometry of acquisition, processing to apply to the data etc. Shallow seismic reflection surveys should not be considered routine, but one requiring special equipment and parameters for each site and target. Consequently, many authors have concentrated their studies on the problems connected with the method. In particular, Hunter et al. Ž 1982., Hunter et al. Ž 1984., Pullan and Hunter Ž 1990., and Steeples and Miller Ž focus on data collection techniques designed to optimize shallow reflections. Knapp and Steeples Ž discuss instrumentation issues, i.e. as the dynamic range must be high Ž bit. to record the low energy of reflection signal and as the importance to apply the analog filters before ArD signal conversion for lower dynamic range. Widess Ž 1973, 1982., Kalweit and Wood Ž and Knapp Ž refers to vertical resolution, as it depends on bandwidth, on the frequency content and on the phase of the signal. The high-resolution goal in shallow seismic surveys puts special requirements on the choice of source to use ŽSingh, 1984; McCann et al., 1985; Miller et al., 1986; Pullan and MacAulay, 1987; Miller et al., In fact, it is not possible to record high-frequency data Ž )80 Hz. if the source does not generate and propagate high frequencies. Furthermore, it has been pointed out that the source affects not only the frequency content of the record, but also the quantity of energy generated and, above all, the signalrnoise Ž SrN. ratio. Miller et al. Ž 1986, 1992, have made field comparisons of various sources placed in sites with different geology and have come to the conclusion that the quality of recorded data depends greatly on the depth of the water table and on near surface geology. With their experiments, they have demonstrated that filling the shot hole with water allows a higher SrN ratio in the records due to containment and improved coupling. Pullan and MacAulay Ž observed that the source is influenced from the soil. Meekes et al. Ž refer as the superficial sources produce stronger air wave and ground roll compared to the hole-source. Experiments conducted with high explosives ŽZiolkowski and Lerwill, demonstrated that the resolution decreases as the energy of the source increases. The question of choosing a source is still critical since it is not always possible to use an invasive source Ž shot holes.: because of location in populated areas with utility and contamination issues and because shot holes are difficult and costly to install. Therefore, continued experimentations in different geologicrhydrologic settings can provide us with a broader experience base to help determine the optimum source and configuration. The importance of the geophones and geophone plants is also to be taken into account as pointed out by Palmer Ž and Maxwell et al. Ž It is clearly possible to increase the SrN ratio through data processing. But, as Steeples and Miller Ž have pointed out, in high-resolution seismic prospecting, some standard processing operations require special attention. For instance, static corrections must be very carefully determined, since comparably small time shifts can lead to greater than 1r2 l phase shifts in high-frequency data and misalignment during stack can severely affect the resolution of the final section. Refracted events can be interpreted as reflections on the stack section unless they are correctly identified and carefully eliminated in the initial processing phase. The application of filters also requires great atten-

3 ( ) M. Feroci et al.rjournal of Applied Geophysics tion: if not eliminated or carefully tracked through the processing coherent noise can be aliased or aligned leading to apparent coherency in the stacked section ŽSteeples and Miller, Steeples and Miller Ž also point out the importance of an accurate velocity model since it can vary rapidly in the horizontal and vertical direction in the shallow subsurface. In consideration of the above, this paper analyzes the possibility of increasing the SrN ratio initially during data collection, and later during processing. The results obtained from experiments with unique, mostly easy-to-use engineering Ž low energy. sources, in geological situations where the water table is less then 3 m, are reported here. The results have been analyzed by qualitatively comparing the records obtained with the various sources. 2. Data collection The experiments were conducted at two sites with different lithological situations. The first site, situated near Cagliari Ž Italy., is a dry lake-basin where the surface soil is clayey silt with sandy intercalation, and subhorizontal layering. The detailed stratigraphy of the site encompasses lacustrine clay and silt Ž from 0 down to 6 8 m., loose sands Žbetween 6 8 and %20 m., intercalation of sandstones and marl Ž between 20 and %50. and finally marl Ž m.. These formations rest on Miocene sandstone bedrock. The following sources were used at this site: GEL-1 Ž 35 g. explosive, seismic cap only, Minibang shotgun Ž 8-gauge. wlike the Betsy seisgun described by Miller et al. Ž 1986.x, hammer Ž 7 kg. used on a steel plate, weight drop Ž Dynasource. Ž Miller et al To enhance the high frequency for all these sources, the data were recorded with 100-Hz geophones along a 140-m profile using the same type of off-end geometry, geophone interval 2 m, minimum offset 6 m, maximum offset 52 m. Some data were also recorded with geophone interval 0.5 m using the Minibang Žminimum offset 1.5 m, maximum offset 13 m.. All profiles have 1200% coverage. The most important noise problem encountered using surface sources was the dominant Ž. presence of the air wave Figs. 3 and 5, which Fig. 1. Ž. a Shot gather from the Cagliari site. The data were collected using explosive source and 100-Hz geophones, offend geometry Ž minimum offset 6 m, maximum offset 52 m. and 2-m geophone interval. Gain Ž trace balance. is applied for displaying. Ž. b Frequency spectra of shot of Ž. a related to 1 8 traces Ž indicated with 1., 9 16 traces Ž indicated with 2. and traces Ž indicated with 3.. The spectra show absolute value in linear scale.

4 130 ( ) M. Feroci et al.rjournal of Applied Geophysics Ž. Ž. Fig. 2. a Shot gather from the same site of Fig. 1, collected using cap source. b Frequency spectra as in Fig. 1. has high energy and spectra overlapping the reflection spectra. Therefore, some experiments were conducted to find ways to minimize air waves during the recording phase. In particular, the Minibang shotgun used had its plate modified by the authors, so its barrel and a small part of the plate itself is buried in a 60-cm shot hole. To attenuate the air wave, experiments were also conducted using an array of six 100-Hz in-line geophones. The pattern was chosen following the formula suggested by Verna and Roy Ž 1970., using an air wave velocity of 340 mrs. Records were obtained using the Minibang source and the same geometry as the other test records. The second site, located near the Fiumicino Airport Ž Rome, Italy. has as target the deltaic series, the first 100 m of which are characterized by sub-horizontal layering. The experiments at this site were all conducted along the Ž. Ž. Fig. 3. a Shot gather from the same site of Fig. 1, collected using Minibang shotgun source. b Frequency spectra as in Fig. 1.

5 ( ) M. Feroci et al.rjournal of Applied Geophysics Ž. Ž. Ž. Fig. 4. a Shot gather from the same site of Fig. 1, collected using weight drop Dynasource source. b Frequency spectra as in Fig. 1. same 140-m profile with 100-Hz geophones and different types of hammers, namely two iron hammers of 7 and 0.8 kg, respectively, and a wooden hammer of 4 kg Ž 25=14 cm.. The first hammer was used on steel plate and the last two hammers were used on wooden plate. Off-end geometry with minimum offset 3 m and maximum offset 26 m were used with geophone interval 1 m. To define the influence of geophone interval on the record, data were collected with the Minibang and the 7 kg hammer with geophone interval 2 m, minimum offset 6 m, and maximum offset 52 m. In all profiles, fold is 12. No high explosives were allowed at this site. A Geometrics ES channel, 15-bit ArD, Instantaneous Floating Point Ž IFP. conversion seismograph with a dynamic range of 114 db was used at both sites. All shots were recorded with a sample interval of 0.2 ms and a Ž. Ž. Fig. 5. a Shot gather from the same site of Fig. 1, collected using hammer source. b Frequency spectra as in Fig. 1.

6 132 ( ) M. Feroci et al.rjournal of Applied Geophysics Fig. 6. F K cumulative spectra of 25 shots acquired with Minibang buried 60 cm below the surface Ž. a and Minibang used normally Ž. b.

7 ( ) M. Feroci et al.rjournal of Applied Geophysics record length of 409 ms with no analog filters applied. To make static corrections and determine the interval velocity of the shallow layers, a refraction profile was also recorded at both sites. For processing the program Seistrix 3 Žby Interpex. was used. 3. Records analysis Records for each site were analyzed by comparing the frequency spectra obtained with the different sources. The results from each site are analyzed separately. Fig. 7. Shot gathers acquired along the same line using single-geophone Ž. a and six-geophone array Ž 100 Hz.Ž. b traces 1 6 connected to single geophones and traces 7 12 connected to strings Ž 50-cm geophone spacing.. Ž c. Frequency spectra of geophones 7 12 of Ž.Ž. a. d Frequency spectra of geophones 7 12 of Ž. b. The source was Minibang and the channel spacing was 2 m.

8 134 ( ) M. Feroci et al.rjournal of Applied Geophysics First test site: Cagliari Figs. 1a 5a show, as an example, a shot gather for each source after gain application Ž trace balance.. Some differences, based on qualitative observations can be pointed out: the record for the explosive Ž Fig. 1a. has a good reflection at 45 ms, compared to the others, probably because of its greater SrN ratio. If we consider a mean velocity of 2000 mrs, it could identify the limit between intercalation of sandstone and marl and solid marl, located at depth of m. There is, however, a high amplitude ground roll. The cap record, instead Žsee spectra in Fig. 2a., is richer in high-frequency content, allowing better detection of the two events immediately following the first arrival Ž between 35 and 50 ms.; however, the SrN ratio Ž based on continuity of reflections. is lower in some parts. In the shotgun Ž Fig. 3. and dynasource ŽFig. 4. records, there is a strong disturbance from the air wave. The hammer shot gather Ž Fig. 5. has low coherent signals compared with the other sources. It can be noticed that low energy sources, such as cap, minibang and hammer, at short offsets, refracted and air waves dominated and superimposed to reflected wave. A frequency analysis has been performed combining the spectra of traces 1 8 Žcloser to the shot point., 9 16 Ž mid range. and Ž long offset.. Figs. 1b 5b show the spectra for the various sources. For all sources, most of the energy of the traces closest to the shot point is concentrated at frequencies of about 50 Hz and can be attributed to the ground roll. The peak in the long-offset traces is around 120 Hz and can be attributed to both the air and reflection waves; the air wave is less evident in the case of the explosive, but very strong with both the shotgun and the hammer. At frequencies above Hz, all sources show low energy. It can also be noted that for all sources, except the cap, even the traces that are farthest from the shot point have a remarkably low frequency content compared to the mid range traces. This analysis shows that most of the energy is attributable to the coherent noise Ž ground roll and air wave.. For the sources used in the experiments and the type of soil at the sites, frequency content above 200 Hz is negligible. Whereas it is possible to eliminate most of the ground roll through filtering, because most of its spectrum is below 80 Hz, it is not possible to filter the air wave because of its spectral overlap with reflections remaining strong, especially for the Minibang Fig. 8. Shot gathers collected in Fiumicino Airport with different types of hammers: Ž. a iron hammer Ž weight 7 kg., Ž. b small iron hammer Ž weight 0.8 kg. and Ž c. wooden hammer Ž weight 4 kg.. Geophone interval 1 m.

9 ( ) M. Feroci et al.rjournal of Applied Geophysics and Dynasource records. Where records are collected using a geophone interval of 2 m, the air wave is also spatially aliased in F K domain Ž Fig. 6b., making it difficult to eliminate it from the shot gathers using velocity filters easily produces artifacts in the alias filtered record. Fig. 6 compares the F K spectra of the shots by the Minibang buried Ž a. and used in normal Fig. 9. Amplitude spectra of the reflected signal of shot gathers in Fig. 8, after it was chosen with a window on the record: Ž. a average amplitude spectra of big iron, small iron and wooden hammers, Ž. b iron hammer Ž weight 7 kg., Ž. c small iron hammer Ž weight 0.8 kg. and Ž d. wooden hammer Ž weight 4 kg.. The spectra show relative values in db.

10 136 ( ) M. Feroci et al.rjournal of Applied Geophysics setup Ž b.. Note that the aliased air wave, which is very strong in spectrum Ž b., is considerably reduced in spectrum Ž a.. Therefore, the use of the buried Minibang Ž modified. allows a considerable improvement in the quality of the records for the attenuation of the air wave as pointed out by Miller et al. Ž We tried to attenuate the air wave during acquisition with the use of six-geophone arrays arranged according to Verna and Roy Ž 1970., taking into consideration that the air wave has a very wide frequency spectrum, with dominant frequencies between 100 and 350 Hz and a velocity of 340 mrs. On this basis, the field records were collected along three coinciding lines of 12 shots each, using in-line strings of six 100-Hz geophones spaced 40, 50 and 60 cm. The shot gathers, shown in Fig. 7b, had traces 1 6 connected to single geophones and traces 7 12 connected to strings of geophones each spaced 50 cm. For comparison, Fig. 7 shows one record obtained with single-geophones Ž a. and one record obtained with strings Žb only traces 7 12 connected to strings.. The use of strings considerably attenuates specific frequency components of air wave, but no significant difference was noted in using three different distances. As a further effect, there is a considerable attenuation of the ground roll, linked to the fact that the used pattern attenuates low frequencies Ž Fig. 7c and d. in events with a velocity of mrs such as Hz, the dominant frequencies of the ground roll filtered by our geophones Second test site: Fiumicino Fig. 8 illustrates the records obtained at the site near Rome, Fiumicino Airport, with various types of hammers. Note that also here ground roll and air wave dominate the records. Identifiable reflections can only be seen within the first 70 ms of Fig. 8a. Fig. 9 shows the frequency content of the traces within windows as shown in the figure Žno coherent noises as air wave and ground roll were included.. The maximum frequencies recorded are between 300 and 350 Hz. It can be noted that all three sources have dominant frequency of reflection around 150 Hz and records collected with the 0.8-kg steel hammer have a lower energy in the highfrequency band. The comparison between the records obtained with the shotgun and the iron hammer at Ž. Ž.Ž. Fig. 10. Shot gathers collected in Fiumicino site : a iron hammer weight 7 kg ; b Minibang. Geophone interval 2 m.

11 ( ) M. Feroci et al.rjournal of Applied Geophysics Fig. 11. Amplitude spectra of the reflected signal of shot gathers collected in Fiumicino site Ž Fig. 10., after it was chosen with a window on the record: iron hammer Ž weight 7 kg. and Minibang. Geophone interval 2 m. The average amplitude spectra Ž relative values in db. are also shown. a geophone interval of 2 m was also made ŽFig The reflected signal was isolated with a window on the record and the spectra are shown in Fig. 11. The reflections show dominant frequencies between 100 and 200 Hz and the Minibang source shows more energy in the Hz range, probably due to air wave, compared to the hammer. 4. Conclusions Comparing the frequency content of the sources used at the first site, it can be noted that there are no substantial differences between them. The explosive and, in particular, the cap shows a greater high-frequency content. In general, the amplitude of the reflected signal is low compared to the coherent noise with all the sources that have been used here. The modification of the Minibang plate and the burying of the barrel has reduced the air wave considerably. Moreover, shooting into a shot hole, getting the sources below the first 60 cm of the highly absorbent weathering layer, helps improve the quality of the records. Nevertheless, field operations for routine prospecting in these conditions are more expensive, and sometimes,

12 138 ( ) M. Feroci et al.rjournal of Applied Geophysics especially in urban and contaminated areas, it is not possible to make holes at all. Regarding the comparison between different types of hammers at the second site, there are not substantial differences in frequency content, but significant SrN difference. Reflection frequency were dominant around 150 Hz higher than at the first site confirming the fact that the response is highly dependant on the ground. The records made with the iron hammer have however a better SrN ratio. When the geophone spacing was increased to 2 m Žfrom 1 m for the hammer comparison tests., the dominant frequency with both the hammer and shotgun source drops due to the greater distances traveled through the ground. Therefore, it seems that the sources do not influence the response much in terms of frequency content for which the ground filter plays a very important role but that they nevertheless can dramatically effect the SrN ratio, especially in consideration of the air wave. The use of an array of geophones allows considerable attenuation of the coherent noise and involves, perhaps, less work on the field compared to the buried Minibang; nevertheless, part of the useful signal may also be eliminated along with the noise, the higher frequencies in particular. In fact, there may be time shifts among the geophones of the string caused by both the apparent velocity of the reflected wave and by the static variations in the weathering between one geophone and the other in the same string, especially if the weathering layer is very inhomogeneous. In conclusion, the type of ground and the thickness of weathering exert the greatest influence on the quality of the results. On the basis of the experiments above, it can be said that the best results are obtained with sources in shot holes, which allow a greater transmission of energy and considerable noise reduction. Surface sources are more practical and economical and, at times, are the only acceptable solutions Ž e.g. in inhabited areas.. In these cases, special data collection techniques Žsuch as arrays of geophones. can be used to improve the SrN ratio. Moreover, a targeted processing can improve the quality of the results, even if often at the expense of the depth of investigation and fold multiplicity. Acknowledgements The authors wish to thank Prof. M. Bernabini for his useful suggestions during the entire project, and Prof. D. Steeples for the final manuscript review. A special acknowledgement is for Prof. E. Brizzolari, who suggested and started this research but unfortunately died before the conclusion. The authors wish also to thank referees Anonymous, S. Pullan and R. Miller for their critical revisions and useful suggestions. References Hunter, J.A., Burns, R.A., Good, R.L., MacAulay, H.A., Gagne, R.M., Optimum field techniques for bedrock reflection mapping with the engineering seismograph. Curr., B, Geol. Surv. Can. 82-1b, Hunter, J.A., Pullan, S.E., Burns, R.A., Gagne, R.M., Good, R.L., Shallow seismic reflection mapping of the overburden bedrock interface with the engineering seismograph some simple technique. Geophysics 49 Ž. 8, Kalweit, R.S., Wood, L.C., The limits of resolution of zero phase wavelets. Geophysics 47 Ž. 7, Knapp, R.W., Vertical resolution of thick beds, thin beds, and thin-bed cyclothems. Geophysics 55 Ž. 9, Knapp, R.W., Steeples, D.W., High resolution common-depth-point seismic reflection profiling: instrumentation. Geophysics 51 Ž. 2, Maxwell, P.W., Faber, K., Edelman, H.A.K., Modern geophones: do they meet the demands of shallow seismic measurements? SEG 64th Annual Meeting. Los Angeles Technical Program: Expanded Abstract. pp McCann, D.M., Andrew, E.M., McCann, C., Seismic sources for shallow reflection surveying. Geophys. Prospect. 33, Meeks, J.A.C., Schieffers, B.C., Ridder, J., Optimization of high-resolution seismic reflection parame-

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