www.elsevier.com/locate/jappgeo

Journal of Applied Geophysics 55 (2004) 239–248

GPR exploration for groundwater in a crystalline rock terrain

Jandyr de Menezes Travassosa,1, Paulo de Tarso Luiz Menezesb,*

aCNPq-Observatorio Nacional, Rua General Jose Cristino 77, 20921-400, Rio de Janeiro, BrazilbGEFEX/FGEL/UERJ, Rua Sao Francisco Xavier 524-4006A, 20550-013, Rio de Janeiro, Brazil

Received 21 October 2002; accepted 19 January 2004

Abstract

The Ground Penetrating Radar (GPR) method has been extensively used to map shallow subsurface features. Such kind of

information is very important for different types of studies, ranging from archaeological to groundwater search. Almost all GPR

surveys for groundwater exploration are usually conducted in sedimentary terrains. In this paper, we demonstrate the

applicability of the GPR method in the exploration of underground water in a crystalline terrain. An example of fixed offset data

collected at one known spring in the district of Petropolis (Brazil) is given. Common Mid Point (CMP) data were also collected

to estimate the velocity of the radar waves. The saturated region is clearly outlined in the GPR section as a zone of attenuation

and inversion of the wavelet phase polarity.

D 2004 Elsevier B.V. All rights reserved.

Keywords: GPR; Groundwater; Crystalline rocks; Geophysical prospecting

1. Introduction The available data indicates that the majority of the

Fresh water is a valuable resource for human

needs, specially in Brazil, which has a large portion

of its territory affected by a semiarid climate, and

where droughts are frequent events. To supply the

great demand, the Brazilian market for mineral water

has increased in the last few years at a rate of 3% per

year (DNPM, 1997). The main producing regions are

located in the southeastern part of the country (Fig.

1a) in the States of Sao Paulo, Minas Gerais and Rio

de Janeiro (Martins et al., 1997).

0926-9851/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.jappgeo.2004.01.001

* Corresponding author. Tel./fax: +55-21-587-7598.

E-mail addresses: jandyr@on.br (J. de Menezes Travassos),

ptarso@uerj.br (P. de Tarso Luiz Menezes).1 Tel.: +55-21-5807081; fax: +55-21-5853782.

Brazilian groundwater resources are located in faults

and fractures in crystalline rock (DNPM, 1997). It is

important to mention here that about 60% of the

Brazilian territory, or about 4,600,000 km2, consists

of crystalline rocks. This indicates the great potential

for mineral water exploration in this type of terrain.

Traditionally, electrical and electromagnetic meth-

ods are the most popular geophysical tools for

groundwater exploration in crystalline rocks (Meju,

2002, and references therein). The Ground Penetrating

Radar (GPR) method has been used extensively in

hydrogeological exploration (Van Overmeeren, 1994;

Beres and Haeni, 1991) and soil studies (Aranha et al.,

2002). In particular, it has demonstrated its effective-

ness in mapping aquifers in sedimentary rocks (Car-

dimona et al., 1998). As far as we are aware, however,

the literature provides no example of the GPR method

Fig. 1. (a) Map of Brazil with the main producing States (Minas Gerais (MG), Sao Paulo (SP) and Rio de Janeiro (RJ) in the figure). (b)

Location of the studied spring (black square in the center portion of the map) superimposed on a geologic map of the region (DNPM, 1998).

Sr—Serra dos Orgaos Batholith. Prn—Rio Negro Complex.

J. de Menezes Travassos, P. de Tarso Luiz Menezes / Journal of Applied Geophysics 55 (2004) 239–248240

being used for groundwater exploration in crystalline

rocks. The imaging of 3D subsurface structures, as

fractured reservoirs in crystalline environment, pres-

ent a higher complexity than the interpretation of the

traditional horizontal/sub-horizontal reflector of the

water bearing level at sedimentary terrains. A similar

situation occurs with the seismic method, that is

widely used for exploring sedimentary basins, but

still not for mineral and groundwater exploration in

complex terrains. Recent advances in seismic imaging

are overcoming these difficulties (Berrer et al., 2000;

Drumond et al., 2000).

In the present work, we demonstrate that the GPR

method can be highly effective in the groundwater

exploration in crystalline rocks. A case study at a

known spring is presented.

2. Geological setting

The studied area is located within the district of

Petropolis in the State of Rio de Janeiro (Fig. 1b). The

topography is uneven being on the scarp of Serra do

Mar chain of mountains, reaching an altitude of 2000

J. de Menezes Travassos, P. de Tarso Luiz Menezes / Journal of Applied Geophysics 55 (2004) 239–248 241

m. The formation of the relief is related to Gondwana

break-up, which began in Middle Cretaceous, with a

strong increment at the Early Tertiary (Tupinamba et

al., 2000).

The main tectonic unit of the region is the Serra dos

Orgaos batholith (see Fig. 1b). That batholith is made

up of granite and gneiss with a granitic and granodio-

ritic composition. The surrounding rocks belong to the

Rio Negro Complex, an assemblage of migmatites,

gneisses, gneiss-schist with interleaved quartzite layers

(paragneiss). The main rock types in the area are the

migmatites and high grade metamorphosed (Amphib-

olite facies) gneiss-schist, both classified as the Santo

Aleixo Unit (DNPM, 1998). Themigmatites frequently

exhibit estromatic, flebitic and schollen structures and

are mainly composed of biotite–hornblende gneiss/

amphibolite with interleaved pegmatite layers. The

gneiss frequently exhibit structures and foliations in

anti-form and syn-form with two independent folding

phases. Due to the high metamorphism and intense

folding it is very difficult to observe S0 (original sedi-

mentary bedding). Low-angle dipping metamorphic

foliation and several NW shear zones are also charac-

teristic in the studied region (Tupinamba et al., 2000).

Fig. 2 shows a migmatite outcrop of Santo Aleixo

Unit in a nearby quarry. The hammer is positioned

over a small leucossome boudin structure within a

mafic layer. At the left corner of the picture a

stromatic migmatitic structure, characterized by inter-

calation of felsic and mafic layers, is dominant. These

Fig. 2. Banded leucossome boudin (indicated by the hammer

position) in a migmatite of Santo Aleixo Unit. A stromatic migmatite

structure dominate in the left corner of the photo. These structures are

typical of Santo Aleixo Unit (courtesy of Miguel Tupinamba).

structures are very common in the studied region and

occurs at all size scales, from centimeters to tens of

meters. The occurrence of mineral water in the area is

usually associated with zones of fractures in the

banded and folded migmatite and paragneiss of the

Santo Aleixo Unit (Fig. 2). The reader can find several

examples of structures and rocks of Rio Negro Com-

plex and Serra dos Orgaos batholith elsewhere in the

literature (Tupinamba et al., 2000).

3. Hydrogeological setting

The hydrogeological knowledge of Rio de Janeiro

State is still not well established. At the whole state the

government has only 527 producing wells registered.

In a first attempt to characterize the hydrogeological

potential, CPRM (Brazilian Geological Survey) recent-

ly published the hydrogeological map of Rio de Janeiro

State in the 1:500,000 scale (Barreto et al., 2000). This

regional study was based on a multi-criteria analysis

integrated in a Geographic Information System (GIS).

The main parameters analyzed, in descending impor-

tance order, were: slope, fracture density (interpreted

from LANDSAT-TM images), soil type, soil manage-

ment, lithology and drainage density. The employed

methodology was partially based on the work devel-

oped by Lanvegin et al. (1991) when studying a

fractured granite in France.

Two mains aquifers systems, sedimentary and

crystalline, were identified by Barreto et al. (2000).

In the crystalline system, our target, groundwater

circulate through fractures and fissures. The storage

capacity of a crystalline aquifer is related to the

number and connectivity of fractures. Therefore, one

of the most important parameter to analyze is the

fracture density. This parameter is defined as the total

length (L) of the existing fractures at an area (L2)

divided by this area (L/L2). Five density ranges were

defined by Barreto et al. (2000), from the lowest to

highest, they are (in km/km2): 0–0.36; 0.36–0.72;

0.72–1.08; 1.08–1.44; >1.44.

The district of Petropolis is located in an area of

high fracture density range (1.08–1.44), therefore

being a high favorably area for groundwater occur-

rence. Indeed, just one mineral water company in-

stalled in that district, is responsible for about 27% of

the production of mineral water of Rio de Janeiro

J. de Menezes Travassos, P. de Tarso Luiz Menezes / Journal of Applied Geophysics 55 (2004) 239–248242

State (Martins et al., 1997). The production zone,

composed by four springs, is located at 1000 m of

altitude, by the scarp of Serra do Mar mountain chain.

This occurrence has been explored since 1953, with

average annual production of 28,000,000 l (Martins et

al., 1997). Physical and chemical characteristics of the

mineral water can be found in the literature (Martins et

al., op. cit.). The water is classified as radioactive at

the spring, probably due to the radionuclide present in

the percolated rocks. The water has a pH of 6.1 at 25

jC, a conductivity of 3.5 mS/m and 34.8 mg/l of

dissolved salts in its composition.

4. Data acquisition

Virtually all the data were collected keeping the

transmitter and the receiver in a fixed offset configura-

tion. AGPR section ismade up ofmany traces collected

along a profile thus allowing the observer to locate

targets. Each new trace is obtained while dragging the

two antennas together along the profile. The wavelets

reflected from two targets appear on the corresponding

traces plotted as a function of known as two-way time

(TWT). This is the time it takes the pulse to be emitted,

to bounce back, and to be recorded at the receiver. The

TWT can be converted to depth when velocity of radar

waves in the subsurface is known. An estimate of

velocity can be achieved through velocity analysis of

Common Mid Point (CMP) sections. In the CMP field

configuration, the transmitter and receiver are moved

away each other up to a maximum distance. This

distance is a compromise between achieving greater

investigation depths and consequent increased absorp-

tion of the electromagnetic waves in the medium with

increased transmitter–receiver distance.

The data were collected with a Pulse Ekko IV GPR

(Davies and Annan, 1989) with a 1000-V pulser. The

transmitter antenna radiates a broadband wavelet that

has an amplitude spectrum as wide as its center

frequency: 100 MHz. The transmitting antenna has a

broad radiation pattern reaching apertures between

90j and 180j. The bi-static antenna configuration

allows a great flexibility of data collection strategies.

Antennas were dragged along the profiles with a

constant step of 0.20 and 0.25 m, keeping their mutual

distance constant (fixed offset) or increasing that

distance continuously (CMP).

The total time window used in the fixed-offset

profiles was 250 ns, enough to reach depths of 10–12

m. That time window, however, was increased to 350

ns for the CMP profiles.

The whole survey amounted to more than 500 m of

fixed-offset reflection profiles mostly deployed on

uneven terrain. The spatial density of traces was

0.25 m/trace throughout. The antennae were kept 1

m apart. A section of a 120-m-long profile on a

relatively flat terrain and crossing the main one of

the four known springs in the area is used here to

illustrate the effectiveness of the GPR to locate water

in fractures. A producing well was drilled at the spring

till 145 m depth. The hydrostatic level is confined

between 4.9 (static) and 62 (dynamic) m depth (Bar-

reto et al., 2000).

Two 25-m CMP profiles were done at two locations

in the survey area in order to estimate an average

velocity to be used in the fixed-offset sections. Anten-

nae separation was increased stepwise in increments of

0.1 m from an initial separation of 1 m. The two CMP

profiles yielded similar results. We show here the CMP

profile done at the well location (Fig. 3a).

5. Velocity analysis

The propagation velocity of radar waves, Vr, is a

function of the dielectric constant of the subsurface.

The dielectric constant, in turn, is affected by its water

content. The propagation velocity can be determined

by CMP measurements. From a CMP section, it is

possible to infer the velocity of the direct waves (in air

as well as the ground wave), the velocity of the

reflected waves, and of the refracted waves in some

special circumstances. Here, we concentrate on the

velocity of reflected waves.

The reflections from the interfaces between layers

with different dielectric constants appear as hyperbolas

in CMP sections (Fig. 3a). This characteristic shape is

based on the assumption that the arrival time for signals

from reflectors varies hyperbolically with the separa-

tion between the transmitter and the receiver. This

assumption is valid as long as the reflectors have small

dip. The curvature of a given hyperbola depends on the

velocity of the radar waves. Therefore, the velocity

analysis of a given hyperbola will give an average

velocity to the depth of the reflector.

Fig. 3. (a) CMP profile at the studied spring. The airwave is represented by the first straight line in time. The groundwave is represented by the

second straight line, note that beyond 10 m, this wave is highly attenuated. Subsurface reflectors are represented by hyperbolas. (b) Velocity

analysis for the CMP profile. Velocity is incremented in 0.001 m/ns steps from 0.01 to 0.15 m/ns.

J. de Menezes Travassos, P. de Tarso Luiz Menezes / Journal of Applied Geophysics 55 (2004) 239–248 243

The velocity analysis used here uses the concept of

velocity stack in a constant velocity earth (Yilmaz,

1987). CMP traces are compensated for normal move-

out assuming a constant velocity hyperbolic equa-

tion. Traces are then stacked. A range of velocity

from 0.05 to 0.15 m/ns was covered with increments of

0.001 m/ns. When a given velocity in that range

matches the normal moveout velocity, a reflector will

stack coherently. This will result in larger amplitude in

the stack. On the other hand, when a given velocity

does not match that of the reflector, traces add together

incoherently. This will result in smaller amplitudes.

J. de Menezes Travassos, P. de Tarso Luiz Menezes / Journal of Applied Geophysics 55 (2004) 239–248244

The velocity analysis using the CMP profile yields

a three-layered velocity model of 0.065, 0.076 and

0.072 m/ns (Fig. 3b). We adopt an average velocity of

Vr = 0.07 m/ns for the whole section, which is close to

the estimate given by the first important reflector at 60

ns, arrowed in Fig. 3b. That reflector is probably the

interface between the saprolite and the granite-gneiss

below. As a matter of fact, the shallower part of the

saprolite has a higher velocity (0.07 m/ns) than the

remainder of the section, as estimated directly from

the groundwave signature, a straight line in Fig. 3a.

The adopted velocity is 54% less than it is usually

tabulated for granite-gneiss (0.13 m/ns). This can be

attributed to a higher water content in soil and rock.

There is one continuous reflector at 150 ns, i.e., below 5

m deep, yielding the almost the same velocity as it can

be seen in Fig. 3b. Note that the static hydrostatic level

measured at the well is 4.9 m, corroborating our

interpretation.

6. Groundwater signature

The water in the area appears in fractures indicating

that our target is not a long and continuous reflector as

would be the case of a water table. Due to the fractures,

Fig. 4. (a) Raw GPR time section on crystalline rock, note the several diff

and 50 in the profile. (b) Same GPR section f–k migrated and converted to

Compare with (a), migration untied the bowties on the section and turn th

level at 4.9 m depth is located at trace 32 m.

we expect that reflection sections will be cluttered with

diffractions. Diffractions are hyperbolic events caused

by the discontinuities in the rock that may interfere with

each other obscuring the sections. Diffractions and

interference patterns can be dealt with by using migra-

tion, an imaging technique that focuses the energy

along hyperbolas to the true spatial position fromwhich

the energy originated. Migration moves dipping reflec-

tions to their true subsurface position and collapses

diffractions, thus increasing spatial resolution. The goal

of migration is to make the fixed-offset section appear

similar to the geologic cross-section in depth along a

GPR profile. The section presented in this paper (Fig.

4) is migrated assuming that the rocks below the

saprolitic layer are more or less homogeneous.

In this work, we use the f–k migration approach of

Stolt (Yilmaz, 1987). It is assumed that the GPR data

can be considered as zero offset and that the average

velocity estimated through the velocity analysis above

can be taken as a constant background velocity along

the profile: Vr = 0.07 m/ns. We have also chosen a

relatively flat profile to apply the migration. The data

is dewowed, clipped, resampled and tapered before

migration.

A limitation in this procedure lies in the fact that

our data is 2D and out-of-plane diffractions will be not

raction patterns (bowties) below 100 ns, specially between traces 40

depth, an uniform velocity of Vr = 0.07 m/ns is assumed throughout.

em into dipping reflectors. A producing well, with static hydrostatic

J. de Menezes Travassos, P. de Tarso Luiz Menezes / Journal of Applied Geophysics 55 (2004) 239–248 245

correctly dealt with the migration process. That will

harm our section smearing out the data at the fracture

zone. Such effect can only be dealt with in a true 3D

survey, where the inline and cross-line spatial sam-

pling are compatible. In that case, one could possibly

perform a two-step migration that is satisfactory for a

simple subsurface. For a more involved structure, one

would require a full 3D migration (Jakubowicz and

Levin, 1983).

Fig. 4a shows a 70-m raw data section clipped

from the 120-m-long profile crossing the studied

spring (trace 32 m). A spreading and exponential

compensation gain assuming an attenuation of 0.1

dB/m was applied to the section. Data were low-pass

filtered with a cutoff at 250 MHz to reduce high-

frequency noise and clipped in time to chop off values

earlier than the first break. The section was migrated

assuming a constant velocity throughout (0.07 m/ns),

the final result is shown in Fig. 4b. Water-filled

fractured rock appears in the section as a region of

lower amplitude reflections beyond trace 25 m and

below 5 m depth. That region gets deeper towards the

end of the section, extending to trace 60 m. Fracture

induced diffraction were virtually eliminated by the

energy focusing provided by the migration.

The wavelet phase polarity can also be used to

determine the presence of water. The phase of the

wavelet is defined in this paper as the sequence of

phase polarities as seen along a given trace. Fig. 5

shows the end portion of the trace obtained at 35 m

Fig. 5. End section of trace 35 m. The arrow shows the first of a

series of phase inversions, occurring at the top section of the water-

filled fractured rock. Amplitudes are clipped at a maximum of 3.2.

The vertical scale is arbitrary.

along the section shown in Fig. 4. The trace was AGC

gained and appears clipped at an arbitrary maximum.

The AGC attempts to equalize the signals by applying a

gain which is inversely proportional to signal strength,

and therefore does not preserve relative amplitude

information. The first of a series of phase polarity

inversions occurs at 178 ns as seen in Fig. 5. This gives

the top of the water-filled fractured rock at 6.23 m, less

than 1 m deeper than it appears to be in Fig. 4b. This

discrepancy may be due to the uncertainty in recogniz-

ing the phase polarity sequence properly, because the

leading edge of the wavelet cannot be ascertained due

to closed spaced reflections, as may be the case on the

top of the fractured rock.

7. Interpretation

A great deal of effort in interpreting radar profiles

goes into not only understanding reflections and

diffractions but also into deciphering of interference

patterns. The focusing of energy provided by migra-

tion make that task easier. The expected product is to

recognize changes in reflector characteristics such as

configuration, continuity, frequency and amplitude so

to characterize radar facies. Radar facies are 3D

regions representing particular combinations of phys-

ical properties like lithology, stratification, fracturing

and fluid contents. Recovering the full geometry of

such regions is not an issue here as we are dealing

with a 2D profile, but with the data we can have an

idea of the layering of the gneiss and recover the

contact between less and more fractured rock.

The section in Fig. 4 gives a good image of the

subsurface, revealing features such as the top portion

of the water-filled fractured rock and the structure of

the gneiss. The interpreted section is shown in Fig. 6.

The most prominent feature is the top of the fractured

rock delimiting an extensive saturated zone (35 m

wide), which is responsible for the high drainage rate

of the spring. This interpretation is corroborated by

the existence of the producing well at trace 32 m.

In the radar section of Fig. 4b, it is possible to

identify a series of semi-parallel folded reflectors all

long the profile and small structures such as a lens

shape between 0 and 30 m in the profile that is very

similar to a boudin structure (B in Fig. 6). As said

before these structures (Fig. 2) are very common in

Fig. 6. Interpretation of the radar section shown in Fig. 4. The F’s are interpreted as fractures and B is interpreted as a boudin structure, very

common in the region, refer to Fig. 2.

J. de Menezes Travassos, P. de Tarso Luiz Menezes / Journal of Applied Geophysics 55 (2004) 239–248246

the region. The overall pattern of the reflectors along

the profile resembles a migmatitic texture, with alter-

nated veined/folded mafic (biotite–hornblende) and

felsic layers (quartz). Another important feature iden-

tified are the sub-vertical fractures (F in Fig. 6) that

occur all long the section. These fractures are associ-

ated to the main conduits to water ascension in the

area.

Independent geophysical data is in accordance

with the GPR data. 1D Interpretation of one VES

positioned at trace 42 m along the profile indicate

a four-layer earth (S. Berrino, personal communi-

cation, 1997). Schlumberger soundings, due to its

low costs and simplicity of use, are still tradition-

ally employed in many regions to enhance the

odds of finding groundwater in crystalline terrains

(Carrasquilla et al., 1997). The main idea behind

these surveys is, after identifying a favorable

fracture in aerial photo-interpretation, to try to

find lower resistivity zones within high resistive

rocks. That lowering in the resistivity pattern

should be associated to groundwater. The success

rate of that type of approach reaches 85–90% of

the drilled boreholes.

To further reduce the ambiguity of the geophys-

ical interpretation of the shallow subsurface we

incorporated the geological knowledge of the weath-

ering profile in the region (Barreto et al., 2000).

Descriptions of the typical weathering profiles of

tropical and subtropical basement complexes are

found elsewhere in the literature (refer to Fig. 4, p.

141 of Meju, 2002).

The 1D model at the spring starts from top to

bottom with a 2-m saprolite layer of 1500 V m,

followed by a 5-m fractured layer of 3000 V m and

then to a fractured saturated zone of 300 V m that may

extend down to 50 m, well beyond the penetration

depth of the GPR data. This 10 times lowering in the

resistivity values indicates a high degree of fracturing

and connectivity. A half-space of highly resistive fresh

rock terminates the model. The resistivity of the

saturated zone is within the range of common frac-

tured aquifers at igneous and metamorphic rocks

(refer to Fig. 5, p. 142 of Meju, 2002). It is interesting

to note that this simple 1D interpretation presented a

reasonable estimate of the top of the saturated zone (7

m) when compared to the borehole log and the radar

section (5.7 m). The main discrepancy, about 20%

error, is associated to the bottom of the saturated zone,

that was estimated in 50 m by the 1D inversion and is

62 m at the borehole. This is comprehensive as we are

interpreting with a 1D method a complex 3D earth.

But as stated before, for exploration purposes, the

main interest is just identify a zone of low resistivity

within crystalline rocks that could associated to

groundwater, so enhancing the chances of a successful

drilling.

8. Conclusions

This work presents the results from a GPR survey

done for water exploration on crystalline terrain

(migmatized gneiss). Reflection data were collected

with the fixed-offset configuration along a profile 70

m long. The velocity was estimated doing velocity

analysis on CMP data. The estimated velocity value

was 0.07 m/ns. This value is 54% less than the

tabulated velocity for granite-gneiss and is attributed

to high water content in rock.

J. de Menezes Travassos, P. de Tarso Luiz Menezes / Journal of Applied Geophysics 55 (2004) 239–248 247

The raw GPR section was cluttered with diffrac-

tions. A migration scheme was applied assuming the

constant value of 0.07 m/ns throughout. The energy

focusing provided by migration can be used to re-

move/attenuate interference patterns and make the

GPR section appear similar to the geologic cross-

section in depth. As expected, the fracture induced

diffractions were virtually eliminated in the migrated

section, enhancing the visualization of the geologic

structures.

In the migrated radar profile the water filled

fractured zone is clearly outlined by a wide (35 m)

region of lower amplitude values. Those are result of

the scattering of the incident energy as well as due to

the smearing out resultant from the process of migrat-

ing out-of-plane diffractions. The depth to the top of

this saturated zone is variable along the profile (4–10

m). The presence of water in the fractured zone can

also be mapped from inversion on the polarity of the

wavelet phase at the radar traces.

To further reduce the inherent ambiguity of the

geophysical interpretation a priori geological knowl-

edge of a borehole located at trace 32 m was

incorporated. In this borehole the water producing

zone is confined within 4.9 and 62 m depth. There is

a very good agreement between the GPR data and

the available borehole information. A priori knowl-

edge of the weathering profile and migmatite struc-

tures of Santo Aleixo Unit were also incorporated to

help in the geological interpretation of the radar

profile.

The main limitation of the method is the relative

shallow depth of investigation. The GPR data provid-

ed a detailed characterization of shape and extension

of the shallow saturated zone. Depth of investigation

was limited to 10 m, insufficient to image the whole

saturated zone (till 62 m depth). The imaging of

greater depths can be achieved by integrating the

GPR data with another geophysical method. In our

case, independent geophysical data from a VES

Schlumberger sounding are in accordance with the

GPR results and the available geological knowledge.

Acknowledgements

We would like to thank the editor-in-chief A.

Hoerdt, S. Hautot and an anonymous reviewer for

their constructive comments that helped to improve

the text. We also thank Miguel Tupinamba for the

photo of Santo Aleixo migmatite. PTLM was

supported by an UERJ-Prociencia scholarship.

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