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
Ž .
spectra 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 Ž
. 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. Ž
. 1986, 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
Ž .
pointed out by Palmer 1987 and Maxwell et Ž
. al. 1994 .
It is clearly possible to increase the SrN ratio through data processing. But, as Steeples
Ž .
and Miller 1990 have pointed out, in high-res- olution seismic prospecting, some standard pro-
cessing operations require special attention. For instance, static corrections must be very care-
fully 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-
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
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