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 .
geophones 7–12 of a . d Frequency spectra of geophones 7–12 of b . The source was Minibang and the channel spacing was 2 m.
3.1. 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 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
Ž .
between 35 and 50 ms ; however, the SrN 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
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 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
. small iron hammer weight 0.8 kg and c wooden hammer weight 4 kg . Geophone interval 1 m.
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 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 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 .
Ž . Ž
. Ž . a average amplitude spectra of big iron, small iron and wooden hammers, b iron hammer weight 7 kg , c small iron
Ž .
Ž . Ž
. hammer weight 0.8 kg and d wooden hammer weight 4 kg . The spectra show relative values in dB.
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 . Fig. 10. Shot gathers collected in Fiumicino site : a iron hammer weight 7 kg ; b Minibang. Geophone interval 2 m.
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
