Study of Basalt Layer in Bebedouro, Paraná Basin with Receiver Function –
Implications for the Seismic Activity Induced by Deep Wells.
Fabio Dias1, Marcelo Assumpção1, Renato Prado1, José Roberto Barbosa1
1 – IAG-USP. Rua do Matão, 1226,05508090, São Paulo, São Paulo, Brasil. fdias@iag.usp.br
Abstract
A study of the seismic structure of the Paraná Basin in Bebedouro, SP, was carried out
with Receiver Functions (RF). Both low frequency (with ~0.5 Hz low pass filter) and highfrequency (~10Hz) RFs were jointly inverted with surface wave dispersion curves.
The results indicate that the basalt layer is 200 to 400m thick, beneath sandstones of
about 50 – 100m thick. The Basin basement was detected between 2.5 – 3.0 km depth,
consistent with expected values from regional bore-hole data in the Paraná Basin.
Beneath several stations a low-velocity zone was detected in the middle of the basalt
pack which is interpreted as a zone of fractured or altered basalt. This anomalous low-velocity
layer was detected near the most seismically active zone in the Andes district. It is also
relatively close to the deep wells with larger outflow. The existence of this low-velocity zone
in the middle of the basalt layer is consistent with the model proposed for the water-well
induced seismicity.
Introduction
Since 2004, the population of the Andes district, Bebedouro-SP, started to feel small
earthquakes. A seismographic network installed in 2005 showed that the earthquakes are
induced by local deep wells used for irrigation (Assumpção et al., 2010). Bebedouro is located
in NW of São Paulo state in the Paraná Basin, where a thin layer of Adamantina sandstones
(50 a 100 m) overlies basalts of Serra Geral formation (300 a 500 m), Botucatu sandstones
(Guarani aquifer) and other formations (Milani el al.,2007).
The thickness of the basalt and the sedimentary layer below are not good estimated
since the local wells reach do not reach the basement. Difficulties in mapping the structures
under the basalt and basement depth using conventional seismic are reported by Rosa et al.
(1982) because great part of energy are reflect in the basalt layer.
A better knowledge of the thickness of the basalt, and its variations in the area, is
important to study the local seismic activity triggered by wells. A better determination of the
basalt thickness and other features of the sedimentary layer under the basalt may help in the
planning and exploration of underground hydro resources, since this exploration increases
every year. The stations used in this study are shown in figure 1.
Figure 1 – The triangles are the stations used in this study. The blue ones presented a lowvelocity zone in the middle of the basalt layer. The circles are the epicenters of the
earthquakes in Andes district. The yellow circles are outflow wells.
Methodology and Results
The first part of the ambient noise processing consists in merging the records of a day
in a unique file. We applied a whitening filter and bit-normalization to remove the effects of
local events in the record. We used time-frequency domain phase-weighted stack (Schimmel
& Gallart, 2005) to stack the days of cross-correlation. After the stacking it is possible
measure the surface wave dispersion.
The Receiver Functions (RFs) basic aspect is that the P-wave incidence in a
discontinuity will produce refracted P and S (Ps) waves. The phase Ps is slower so the time
difference between these refracted phases will measure the depth of the discontinuity. The
teleseismic records have information about the source of event, the path between the source
and seismograph station and the instrument response. A deconvolution of seismic records will
isolate the response of the structure under the station. The inversion was made with linearized
method of Ammon et al. (1990) using RFs and surface waves.
We show in figure 3 the best 16 models for the stations used in this study. Each one
was obtained with a different initial model. Red colors indicate better fit and purple indicates
the worst fit. The black line is the mean model.
Figure 2: S wave velocity profiles obtained using RF and surface wave joint inversion. The
white line represents the velocity of 3.0 km/s and 3.2 km/s.
Discussion and Conclusions
In all stations we note an increase of velocity associated to the basalt layer around 100
m. The basalt thickness varies between 200 to 400 m. A low velocity zone (Figure 3) inside
the basalt layer was identified in BEB4A, BEB4B, BEB22, BEB11, BEB7, BEB19 e BEB1
(blue triangles in Figure 1).This low velocity zone indicates the presence of fractures or
alteration in the basalt layer. The existence of this low-velocity zone in the middle of the
basalt layer is consistent with the model proposed for the water-well induced seismicity
proposed by Assumpção et al., 2010.
The Basin basement (Figure 2) was estimated between 2.5 – 3.0 km depth (Vs= 3.0 –
3.2 km/s), consistent with expected values from regional bore-hole data in the Paraná Basin.
This results show that RFs are powerful tools to study the structure of sedimentary basins.
References
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Assumpção, M., Yamabe, T. H., Barbosa, J. R., Hamza, V., Lopes, A. E. V., Balancin, L., Bianchi, M. B, 2010.
Seismic activity triggered by water wells in the Paraná Basin, Brazil. Water Resources Research, v. 46,
p. W07527.
Milani, E.J.; Melo, J. H. G., Souza, P. A., Fernandes, L. A. e França, A. B., 2007 Bacia do Paraná. In Cartas
Estratigráficas. Boletim de Geociências da Petrobrás v.15. n.2.
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SEG Expanded Abstracts 1.
Figure 3: S wave velocity profiles obtained using RF and surface wave joint inversion.