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International Journal of Geosciences, 2017, 8, 693-710

http://www.scirp.org/journal/ijg

ISSN Online: 2156-8367

ISSN Print: 2156-8359

DOI: 10.4236/ijg.2017.85038

May 27, 2017

2D TEM Modeling for a Hydrogeological Study

in the Paraná Sedimentary Basin, Brazil

Emerson Rodrigo Almeida

, Jorge Luís Porsani

, Fernando Acácio Monteiro dos Santos

Cassiano Antonio Bortolozo

Universidade de São Paulo, Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Departamento de Geofísica. Rua do

Matão, Butantã, São Paulo, Brazil

Universidade de Lisboa-IDL, Lisboa, Portugal

Abstract

This work uses 2D TEM (Transient Electromagnetic) modeling for a hydr

geological study in the Paraná sedimentary basin. The study area is located at

the northern region of the state of São Paulo, Brazil, where groundwater is e

ploited from two aquifer systems: one sedimentary, shallow, and the other

crystalline, deep. The in

terest in applying the TEM method in this area owes

to the high exploitation rates of groundwater from the crystalline aquifer sy

tem for irrigation, which is triggering considerable seismic activity locally.

This aquifer system is composed of fractured ba

salt within the Serra Geral

Formation and is about 120 m deep. Eighty-

six TEM soundings were acquired

at this location, but in nine cases the data did not fit the modelled curve for

1D geoelectrical models due to the geological complexity of the area. This p

per shows 2D geoelectrical modeling results based on the FDTD (Finite Di

ferences in Time Domain) method to explain the lateral resistivity variation

within the geological setting. A 2D model was generated for each sounding

and compared with 1D inversio

n models as well as with direct information

from wells. The results show some vertical variations of about 10 to 30 meters

on the upper interface of the basalt layer from Serra Geral Formation. They

are located at approximately 60 meters from the center of

the soundings. The

existence of these 2D structures in the subsurface can be related to the dra

nage system in the study area. The presence of these structures may indicate a

connection between the shallow and deep aquifer systems, acting like a co

duit that may contribute to the seismic activity reported.

Keywords

Transient Electromagnetic (TEM), 2D Modeling, Hydrogeophysics, Paraná

Basin, Brazil

How to cite this paper:

Almeida, E.R.,

Porsani

, J.L., dos Santos, F.A.M. and Bor-

tolozo

, C.A. (2017) 2D TEM Modeling for a

Hydrogeological Study in the Paraná Sed

mentary Basin, Brazil

International Journal

Geosciences

, 693-710.

https://doi.org/10.4236/ijg.2017.85038

Received:

April 12, 2017

Accepted:

May 24, 2017

Published:

May 27, 2017

7 by authors and

Scientific

Research Publishing Inc.

This work is licensed under the Creative

Commons Attribution

International

License (CC BY

4.0).

http://creativecommons.org/licenses/by/4.0/

Open Access

E. R. Almeida et al.

694

1. Introduction

The Transient Electromagnetic (TEM) method was developed initially for min-

eral exploration as an alternative to the frequency-domain electromagnetic me-

thods and it was introduced for groundwater exploration in the mid-1980s. The

TEM method has the advantage as it can investigate hundreds of meters in depth

with great capability of detecting conductive layers. According to the results of

many researchers worldwide, the geoelectrical stratigraphy given by TEM mod-

els usually has a good correlation with geological information, which makes it a

good method for mapping subsurface conductive zones [1] [2]. For this reason,

the TEM method is being used for water table mapping at great depths,

representing a reliable alternative to vertical electrical soundings (VES) that may

require long aperture ranges depending on the investigation depth. Krivochieva

and Chouteau [3] used TEM soundings in a joint analysis with the Magnetotel-

luric (MT) method for aquifer investigation in the Mexico Basin. The TEM me-

thod was used in this study to identify conductive zones below the basalt layer of

the Santa Catarina volcano, in a stratigraphic sequence comparable to that ob-

served in the Paraná Basin. Ezersky

et al

. [4] evaluated the structure of an aquifer

in Israel with the TEM method as well as other geoelectrical analyses to evaluate

the salinity of the groundwater.

The high sensitivity of the TEM method for mapping interfaces between an

upper resistive layer followed by a conductive layer makes it suitable for the

mapping of highly conductive layers, even in coastal environments [5] [6]. On

the other hand, it is also very sensitive to man-made conductive structures, like

fences or grounded power lines. Depending on distance and on soil conductivi-

ty, these structures may interact with the transmitter loop through the geological

material, creating disturbances typically from an oscillating circuit. The oscilla-

tion in the data may lead to misinterpretation when there is galvanic coupling,

or make it impossible to interpret the data at all when there is a capacitive

coupling [2] [7].

In the framework of 2D and 3D TEM surveys for geological analysis Danielsen

et al

. [8] evaluated the application of conventional and high electromagnetic

moment systems for groundwater studies using laterally-constrained inversion

of 1D soundings for a 2D interpretation. Jørgensen

et al

. [9] used the TEM me-

thod for 3D imaging of the cross-section of buried Quaternary valleys in Den-

mark. Their work studied several scenarios representing buried valleys for

groundwater exploitation, and the 3D imaging was based also on constrained 1D

inversions. Santos and El-Kaliouby [10] developed an algorithm for laterally

constrained joint inversion of electro resistivity (ER) and TEM data to calculate

quasi-2D geological models. Teatini

et al

. [11], conducted surveys with airborne

TEM to analyze the hydrogeological setting of a transitional coastal environment

in Venice, Italy. The data acquired were inverted as laterally constrained 1D

sounding inversions providing a quasi-2D analysis. Martínez-Moreno

et al

. [12]

used TEM surveys for groundwater applications in two volcanic islands in the

Republic of Cape Verde. They could retrieve a 3D resistivity distribution in

E. R. Almeida et al.

695

depth, based on 1D inverted geoelectrical models.

In Brazil, Meju

et al

. [13] developed a study where the TEM method and Ver-

tical Electric Soundings (VES) were combined in a constrained 1D inversion,

analyzing the results in a comparison with 2D inversion of audio-magnetotel-

luric (AMT) data for structural mapping and groundwater evaluation at the

Parnaiba Basin. Carrasquilla and Ulugergerli [14] developed a study using 32

TEM soundings in the Campos Basin for structural and groundwater evaluation.

Santos and Flexor [15] used the 1D inversion of TEM soundings for hydrogeo-

logical studies at the Resende Basin, in a densely industrialized area and there-

fore with high levels of EM noise. A study about the effects of urban noise was

carried out by Porsani

et al

. [16], where TEM soundings with different electro-

magnetic moments were compared in order to evaluate the applicability of the

method in similar environments. Porsani

et al

. [17] performed 86 central-loop

TEM soundings over the Paraná Basin, close to the city of Bebedouro, in order

to identify fractured zones with groundwater inside the basalt layer of the Serra

Geral Formation using 1D geoelectrical models and correlated those fractured

zones to seismicity events registered at the area.

Bortolozo

et al

. [18] developed a program for 1D joint inversion of Vertical

Electrical Soundings and TEM soundings and some applied results of the joint

inversion methodology for Bebedouro region are presented in Bortolozo

et al

[19]. With the joint inversion, it was possible do detect a 2D effect in the TEM

soundings, but the nature of the 2D structure could not be defined since the

study was carried out as 1D data inversion. In Campaña

et al

. [20], the pseudo-

2D inversion of TEM soundings was carried in Ibirá region, also in São Paulo

State (Brazil). Using this methodology, it was possible to analyze the 2D struc-

tures in the area. With the full 2D TEM inversion, Bortolozo [21] better defined

the 2D structures in the study area of Ibirá. However, the problem with the 2D

acquisition lies in the time necessary to obtain field data, and the processing was

also very time-consuming. Additionally, the 2D acquisition has problems with

negative apparent resistivity data, discussed in [22].

In the present study, the TEM method was employed near the city of Bebe-

douro (northern São Paulo state, Brazil), over the Paraná basin rocks (

Figure 1).

Data were acquired using a Geonics PROTEM57-MK2 system, with a 19 A

transmitter current. This system allows data acquisition at 30 Hz, 7.5 Hz and 3

Hz frequencies and three data curves were acquired for each frequency, with an

integration time of 60 seconds for each curve in order to enhance the sig-

nal-to-noise ratio. A 100 m × 100 m square transmitter single-loop was used in

the central loop array. Data treatment and preliminary 1D analysis were done

using IX1D

v3 (Interpex Limited) commercial software for electric and electro-

magnetic data inversion and interpretation. The software employs the Fouri-

er-Hankel Transform to compute the response from the desired layered model,

which corresponds to the response from the half-space containing the layered

model subtracted from the full response. The inversion is computed using Ridge

Regression and non-linear least squares fitting [23].

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