www.elsevier.nlrlocaterjappgeo
Some considerations on shallow seismic reflection surveys
qM. Feroci
a, L. Orlando
a,), R. Balia
b, C. Bosman
a, E. Cardarelli
a, G. Deidda
ba
Dip. Idraulica, Trasporti e Strade-UniÕersita ’La Sapienza’ di Roma, Area Geofisica, Via Eudossiana 18,`
00184 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
Ž . Ž
standard processing techniques to increase signalrnoise SrN ratio need special accuracy for 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
Ž .
processing steps 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
Ž
containment then the other sources and they are used in hole i.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.q2000 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:q39-6-445-850-80.
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E-mail address: [email protected] L. Orlando .
0926-9851r00r$ - see front matterq2000 Elsevier Science B.V. All rights reserved.
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128
1. Introduction
In the last 20 years, the growing interest in engineering and environmental problems has in-creased the use of seismic reflection surveys in
Ž
the study of shallow targets hydrogeological, engineering, environmental, archaeological, and
.
geotechnical problems . The most important consideration connected with these methods is recording reflections with broad bandwidth Žspectra shifted towards high frequency and to.
Ž
attenuate as much coherent noise air wave and .
ground roll as possible. To obtain that, it is necessary to choose carefully the sources, geo-phones, 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
particu-Ž . Ž .
lar, Hunter et al. 1982 , Hunter et al. 1984 ,
Ž .
Pullan and Hunter 1990 , and Steeples and
Ž .
Miller 1990 focus on data collection tech-niques designed to optimize shallow reflections.
Ž .
Knapp and Steeples 1986 discuss instrumenta-tion issues, i.e. as the dynamic range must be
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high 16–18 bit to record the low energy of reflection signal and as the importance to apply the analog filters before ArD signal conversion
Ž .
for lower dynamic range. Widess 1973, 1982 ,
Ž . Ž .
Kalweit and Wood 1982 and Knapp 1990 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
require-Ž ments on the choice of source to use Singh, 1984; McCann et al., 1985; Miller et al., 1986; . Pullan and MacAulay, 1987; Miller et al., 1992 . In fact, it is not possible to record high-frequency
Ž .
data )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
Ž .
all, the signalrnoise SrN ratio. Miller et al.
Ž1986, 1992, 1994 have made field compar-. isons of various sources placed in sites with different geology and have come to the conclu-sion 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
cou-Ž .
pling. Pullan and MacAulay 1987 observed that the source is influenced from the soil.
Ž .
Meekes et al. 1990 refer as the superficial sources produce stronger air wave and ground roll compared to the hole-source. Experiments
Ž
conducted with high explosives Ziolkowski and .
Lerwill, 1979 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
Ž .
source shot holes : because of location in popu-lated 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 config-uration.
The importance of the geophones and geo-phone plants is also to be taken into account as
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pointed out by Palmer 1987 and Maxwell et
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al. 1994 .
It is clearly possible to increase the SrN ratio through data processing. But, as Steeples
Ž .
atten-tion: if not eliminated or carefully tracked through the processing coherent noise can be aliased or aligned leading to apparent coherency
Ž
in the stacked section Steeples and Miller,
. Ž .
1990 . Steeples and Miller 1990 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 ana-lyzes the possibility of increasing the SrN ratio initially during data collection, and later during processing. The results obtained from experi-ments with unique, mostly easy-to-use
engineer-Ž .
ing low energy sources, in geological situa-tions 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.
Ž .
The first site, situated near Cagliari Italy , is a dry lake-basin where the surface soil is
clayey–silt with sandy intercalation, and sub-horizontal layering. The detailed stratigraphy of the site encompasses lacustrine clay and silt Žfrom 0 down to 6–8 m , loose sands between. Ž
.
6–8 and %20 m , intercalation of sandstones
Ž .
and marl between 20 and %50 and finally
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marl 50–70 m . These formations rest on
Miocene sandstone bedrock.
The following sources were used at this site:
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GEL-1 35 g explosive, seismic cap only,
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Minibang shotgun 8-gauge like the Betsy
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seisgun described by Miller et al. 1986 ,
ham-Ž .
mer 7 kg used on a steel plate, weight drop ŽDynasource. ŽMiller et al. 1986 . 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
Ž
m using the Minibang minimum offset 1.5 m, .
maximum offset 13 m . All profiles have 1200% coverage.
The most important noise problem encoun-tered using surface sources was the dominant
Ž .
presence of the air wave Figs. 3 and 5 , which
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Fig. 1. a Shot gather from the Cagliari site. The data were collected using explosive source and 100-Hz geophones, offend
Ž . Ž .
geometry minimum offset 6 m, maximum offset 52 m and 2-m geophone interval. Gain trace balance is applied for
Ž . Ž . Ž . Ž .
displaying. b Frequency spectra of shot of a related to 1–8 traces indicated with 1 , 9–16 traces indicated with 2 and
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M. Feroci et al.rJournal of Applied Geophysics 45 2000 127–139
130
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 modi-fied 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
fol-lowing the formula suggested by Verna and Roy Ž1970 , using an air wave velocity of 340 m. rs. Records were obtained using the Minibang source and the same geometry as the other test records.
The second site, located near the Fiumicino
Ž .
Airport Rome, Italy has as target the deltaic series, the first 100 m of which are character-ized by sub-horizontal layering. The experi-ments at this site were all conducted along the
Ž . Ž .
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
Ž .
wooden hammer of 4 kg 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 maxi-mum offset 26 m were used with geophone interval 1 m. To define the influence of geo-phone interval on the record, data were
col-lected 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 ES2401 24-channel, 15-bit
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ArD, Instantaneous Floating Point IFP con-version 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
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record length of 409 ms with no analog filters applied. To make static corrections and deter-mine the interval velocity of the shallow layers, a refraction profile was also recorded at both
Ž sites. For processing the program Seistrix 3 by
.
Interpex was used.
3. Records analysis
Records for each site were analyzed by com-paring the frequency spectra obtained with the different sources. The results from each site are analyzed separately.
Ž . Ž . Ž .
Fig. 7. Shot gathers acquired along the same line using single-geophone a and six-geophone array 100 Hz b traces 1–6
Ž . Ž .
connected to single geophones and traces 7–12 connected to strings 50-cm geophone spacing . c Frequency spectra of
Ž . Ž . Ž .
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3.1. First test site: Cagliari
Figs. 1a–5a show, as an example, a shot gather for each source after gain application Žtrace balance . Some differences, based on. qualitative observations can be pointed out: the
Ž .
record for the explosive 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 sand-stone and marl and solid marl, located at depth of 50–70 m. There is, however, a high
ampli-Ž tude ground roll. The cap record, instead see
.
spectra in Fig. 2a , is richer in high-frequency content, allowing better detection of the two events immediately following the first arrival Žbetween 35 and 50 ms ; however, the S. rN
Ž .
ratio based on continuity of reflections is lower in some parts.
Ž . Ž
In the shotgun Fig. 3 and dynasource Fig. .
4 records, there is a strong disturbance from
Ž .
the air wave. The hammer shot gather 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 Ž
combining the spectra of traces 1–8 closer to
. Ž .
the shot point , 9–16 mid range and 17–24 Žlong 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 100–150 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 com-pared to the mid range traces. This analysis shows that most of the energy is attributable to
Ž .
the coherent noise 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 possi-ble 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
Ž . Ž . Ž .
Fig. 8. Shot gathers collected in Fiumicino Airport with different types of hammers: a iron hammer weight 7 kg , b
Ž . Ž . Ž .
and Dynasource records. Where records are col-lected using a geophone interval of 2 m, the air wave is also spatially aliased in F–K domain ŽFig. 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 Ž .
by the Minibang buried 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, b iron hammer weight 7 kg , c small ironŽ . Ž . Ž .
Ž . Ž . Ž .
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M. Feroci et al.rJournal of Applied Geophysics 45 2000 127–139
136 Ž .
setup b . Note that the aliased air wave, which Ž .
is very strong in spectrum b , is considerably Ž .
reduced in spectrum a . Therefore, the use of
Ž .
the buried Minibang modified allows a consid-erable improvement in the quality of the records for the attenuation of the air wave as pointed
Ž .
out by Miller et al. 1994 .
We tried to attenuate the air wave during acquisition with the use of six-geophone arrays
Ž .
arranged according to Verna and Roy 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 Ž . one record obtained with single-geophones a
Ž
and one record obtained with strings b — only .
traces 7–12 connected to strings . The use of strings considerably attenuates specific fre-quency components of air wave, but no signifi-cant difference was noted in using three
differ-ent distances. As a further effect, there is a considerable attenuation of the ground roll, linked to the fact that the used pattern attenuates
Ž .
low frequencies Fig. 7c and d in events with a velocity of 200–250 mrs — such as 50–70 Hz, the dominant frequencies of the ground roll filtered by our geophones.
3.2. 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. Identifi-able reflections can only be seen within the first 70 ms of Fig. 8a. Fig. 9 shows the frequency content of the traces 17–24 within windows as
Ž
shown in the figure no coherent noises as air .
wave and ground roll were included . The maxi-mum 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 high-frequency band.
The comparison between the records ob-tained with the shotgun and the iron hammer at
Ž . Ž . Ž .
Ž .
Fig. 11. Amplitude spectra of the reflected signal of shot gathers collected in Fiumicino site Fig. 10 , after it was chosen
Ž .
with a window on the record: iron hammer weight 7 kg and Minibang. Geophone interval 2 m. The average amplitude
Ž .
spectra relative values in dB are also shown.
Ž a geophone interval of 2 m was also made Fig.
.
10 . The reflected signal was isolated with a window on the record and the spectra are shown in Fig. 11. The reflections show dominant fre-quencies between 100 and 200 Hz and the Minibang source shows more energy in the 200–450-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
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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 fre-quency 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
geo-Ž
phone spacing was increased to 2 m from 1 m .
for the hammer comparison tests , the dominant frequency with both the hammer and shotgun source drops due to the greater distances trav-eled through the ground.
Therefore, it seems that the sources do not influence the response much in terms of fre-quency 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 consid-erable attenuation of the coherent noise and involves, perhaps, less work on the field com-pared 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 influ-ence 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. Sur-face sources are more practical and economical and, at times, are the only acceptable solutions Že.g. in inhabited areas . In these cases, special.
Ž
data collection techniques such as arrays of
.
geophones can be used to improve the SrN ratio. Moreover, a targeted processing can im-prove 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 pro-ject, 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 be-fore the conclusion. The authors wish also to thank referees Anonymous, S. Pullan and R. Miller for their critical revisions and useful suggestions.
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