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 Ž
. Ž
from 0 down to 6–8 m , loose sands between .
6–8 and 20 m , intercalation of sandstones Ž
. and marl between 20 and 50 and finally
Ž .
marl 50–70 m . These formations rest on
Miocene sandstone bedrock. The following sources were used at this site:
Ž .
GEL-1 35 g
explosive, seismic cap only, Ž
. w Minibang shotgun
8-gauge like the Betsy
Ž .x
seisgun described by Miller et al. 1986 , ham- Ž
. mer 7 kg used on a steel plate, weight drop
Ž . Ž
. Dynasource
Miller 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
Ž . 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
Ž .
17–24 traces indicated with 3 . The spectra show absolute value in linear scale.
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 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
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 .
Fig. 3. a Shot gather from the same site of Fig. 1, collected using Minibang shotgun source. b Frequency spectra as in Fig. 1.
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 Ž
. 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
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.
M. Feroci
et al.
r Journal
of Applied
Geophysics
45 2000
127 –
139 132
Ž . Ž .
Fig. 6. F–K cumulative spectra of 25 shots acquired with Minibang buried 60 cm below the surface a and Minibang used normally b .
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
