Correlation of near-surface stratigraphy and physical...

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Journal of Applied Geophysics 53 (2003) 121–136

Correlation of near-surface stratigraphy and physical properties of

clayey sediments from Chalco Basin, Mexico, using Ground

Penetrating Radar

Dora Carreon-Freyre*, Mariano Cerca, Martın Hernandez-Marın

Centro de Geociencias, Universidad Nacional Autonoma de Mexico, Campus Juriquilla, Queretaro, Apartado Postal 1-742,

Queretaro 76001, Mexico

Received 21 September 2001; accepted 19 May 2003

Abstract

Detailed measurements of water content, liquid and plastic limits, electric conductivity, grain-size distribution, specific

gravity, and compressibility were performed on the upper 7 m of the lacustrine sequence from the Chalco Basin, Valley of

Mexico. Eight stratigraphic units consisting of alternating layers of clay, silt, sand, and gravel of volcanic origin are described

for this sequence. The analysis of contrasts in the physical properties permitted to identify potential reflectors of radar waves: (i)

change in the electrical conductivity at 0.4 m depth; (ii) increment in the clay and water content at 0.8 m depth; (iii) bimodal

behavior of the water content at 1.3 m depth; (iv) increment in the sand content and decrease in water content at 2.6 m depth;

and (v) the presence of a pyroclastic unit at 3.7 m depth. Radargrams with frequencies of 900 and 300 MHz were collected on a

grid of profiles covering the study area. Correlation of radargrams with the reference section permitted the spatial interpolation

of variations in the physical properties and the near-surface stratigraphy. Contrary to the expected in these clayey sediments,

electric contrast enhanced by variations in water content and grain size permitted the recording of the near-surface sedimentary

structures. Distinctive radar signatures were identified between reflectors. Furthermore, lateral discontinuities of the reflectors

and their vertical displacements permitted the identification of deformational features within the sequence.

D 2003 Elsevier B.V. All rights reserved.

Keywords: Clayey sediments; Near-surface stratigraphy; Physical properties; Radargram; Radar signature; Fractures

1. Introduction

Sedimentary sequences filling the Valley of Mex-

ico, and other basins in Central Mexico, consist

0926-9851/03/$ - see front matter D 2003 Elsevier B.V. All rights reserve

doi:10.1016/S0926-9851(03)00042-9

* Corresponding author. Tel.: +52-442-238-11-04x112; fax:

+52-442-238-11-05.

E-mail addresses: [email protected]

(D. Carreon-Freyre), [email protected] (M. Cerca),

[email protected] (M. Hernandez-Marın).

generally of various types of fluvial and lacustrine

deposits with particle sizes varying from gravel, sand,

and silt to clays, often interbedded with pyroclastic

rocks and lava flows. These materials present a high

variability in their properties such as strength, grain-

size distribution, mineralogical composition, and wa-

ter content. Major heterogeneities can be present also

at near-surface sequences of soils or single layers. The

mineralogical composition of the shallower layers,

generally bearing clay-size particles, can vary lateral-

d.

D. Carreon-Freyre et al. / Journal of Applied Geophysics 53 (2003) 121–136122

ly to different kinds of crystallized and amorphous

clay minerals (Warren and Rudolph, 1997). Under-

standing the mechanical behavior and precise charac-

terization of the near-surface stratigraphy of these

sequences may be critical to geological engineering

and geotechnical studies for the development of urban

infrastructure.

It has been demonstrated that electrical and geo-

technical properties of clayey sediments are closely

related (Saarenketo, 1998; Fam and Santamarina,

1997; Ohtsubo et al., 1983). Since the mechanical

behavior of fine-grained sediments also depends on

water content, Ground Penetrating Radar (GPR) pro-

vides a useful nondestructive method for geotechnical

characterization of these sequences. In the last 25

years, GPR applications for the stratigraphic prospec-

ting of sediments and for engineering geology and

civil engineering studies have been widely presented

in literature (Kruse et al., 2000; Saarenketo and

Fig. 1. Location of the studied area. The Mexico Valley is located in the ce

lacustrine basins. The Chalco Basin is located in the southern part of the M

Chichinautzin ranges. Location of the studied sequence and previous stud

Scuillon, 2000; Van Overmeeren, 1998; Doolittle

and Collins, 1995; Lorenzo, 1994; Goodman, 1994;

Davis and Annan, 1989; Ulriksen, 1982; Morey,

1974). Nevertheless, a detailed correlation of the

reflected patterns of radar wave with physical and

mechanical properties of clayey sediments has not

been presented yet. The aim of this paper is to

correlate the spatial variation of water content,

grain-size distribution, and compressibility within a

sedimentary sequence of lacustrine-volcanic origin

with the recorded reflectors in GPR sections.

2. GPR prospecting of sedimentary sequences

The principles of the GPR method have been

explained extensively in literature (Davis and Annan,

1989; Ulriksen, 1982; Morey, 1974). GPR transmits

high-frequency electromagnetic pulses in which

ntral part of the Trans-Mexican Volcanic Belt and comprises several

exico Valley. It is bounded by the Santa Catarina, Las Cruces, and

ies are indicated.

D. Carreon-Freyre et al. / Journal of Applied Geophysics 53 (2003) 121–136 123

ranges the dielectric permittivity of materials predom-

inates over electrical conductivity (Ulriksen, 1982).

For simplicity, in this text we use the term ‘‘permittiv-

ity’’ to refer to relative permittivity (ratio between

permittivity of the prospected media and free space).

This parameter indicates the polarization of the ele-

ments forming geologic materials, caused by the

application of an electrical field. At radar frequencies

polarization affects mainly the bipolar molecules of

water (Topp et al., 1980) in saturated material such as

clayey materials. According to Fukue et al. (1999), the

physical characteristics of sediments such as micro-

structure, grain size, water content, and mechanical

characteristics in clay-bearing materials are also relat-

ed to other electric properties such as resistivity.

Furthermore, Saarenketo (1998) showed that varia-

tions of dielectric properties in clay and silty particles

are related with their Cation Exchange Capacity

Fig. 2. Correlation of the near-surface stratigraphy (7 m depth) in the Cha

relative positions from southwest to northeast: (a) Core B (Caballero and O

and Site 2 excavation presented in this work; and (d) Site 1 excavation (Za

size data presented were previously reported by Zawadski (1996).

(CEC). This author concluded that the lower the

CEC the more orderly is the arrangement of water

molecules around the soil particles.

In particular, Warren and Rudolph (1997) reported a

direct relation between the clay-mineral phase (allo-

phane, smectite) with its CEC, porosity, and water

content for the basin of Mexico. Thus, radar-wave

behavior is influenced by: water content, physico-

chemical characteristics of pore water, solid–liquid–

air proportion, structure, and voids ratio of the solid

phase. The contrasts in these properties can be

recorded as reflectors in radar sections or radargrams.

In addition, these parameters are directly related with

different geological and mechanical aspects of the

sedimentary materials (Fam and Santamarina, 1997).

Several authors have shown that variations in the water

content and water retention capacity of soil particles

are related with its geological depositional conditions

lco lacustrine plain. The stratigraphic columns are presented in their

rtega-Guerrero, 1998); (b) BH4 Core (Zawadski, 1996); (c) CH core

wadski, 1996). Note variations in depth of pyroclastics layer. Grain-


(see Chandler, 2000, for clay sediments; Van Over-

meeren et al., 1997; Freeland et al., 1998, for sandy

deposits). These studies validate GPR surveying as a

useful tool for physical and geotechnical prospecting

of clayey sequences.

Fig. 3. Diagram of the grid of GPR profiles collected in the studied

zone: a, b, c, and d collected in N 20j E direction; e, f, g and h

collected in N 70j W direction. Most of the profiles are 30 m in

length except for profile g where the length is 40 m. CH core and

Site 2 excavation are described in this study.

3. Near-surface stratigraphy of the Chalco site

The Valley of Mexico is a volcanic-bounded basin

located in the central part of Mexico within the Trans-

Mexican Volcanic Belt (Fig. 1). Volcanic activity has

been intense in this area from Pliocene to recent times.

A lacustrine system characterized the Valley of Mex-

ico until pre-Hispanic times. Later, the urbanization of

the valley has forced the withdrawal of groundwater

level leaving only lake remnants and large extensions

of plains. The near-surface clayey deposits of the

Mexico Basin are mainly composed by saturated

smectitic clay and amorphous material derived from

weathering of volcanic ash (Warren and Rudolph,

1997). The complex mechanical behavior of the

‘‘Clays of Mexico City’’, especially brittle failure

associated with high water contents on highly com-

pressible materials, has been extensively studied and

different mineralogical and grain-size distributions

have been reported (Peralta-y-Fabi, 1989; Mesri et

al., 1975; Marsal and Mazari, 1969; Lo, 1962).

The Chalco Basin is located at the southernmost end

of the Valley of Mexico (Fig. 1). It is a horizontal plain

(formerly a lake) filled with lacustrine sedimentary

sequences overlying a regional granular aquifer.

According to Urrutia-Fucugauchi et al. (1994), drain-

age was restricted about 780,000 years ago by the

formation of the Chichinautzin volcanic field to the

south. Important variations in salinity or water level

related to changes in weather have occurred through the

time in the Quaternary volcanosedimentary sequence

of the Chalco Basin (Urrutia-Fucugauchi et al., 1994;

Lozano et al., 1993). We have selected a study site in

the north margin of the Chalco Basin near the Santa

Catarina village. Access to the area is provided by a

north–south paved road and it is important to mention

that an electricity line runs adjacent to it (Fig. 3).

Urbanization in the Chalco area began in the early

1980s and has not covered the total extent of the plain;

in this part, the land is still used for agriculture.

Regional extraction of groundwater is made by a line

of 14 wells named Ramal Mixquic of the Santa Cata-

rina well field.

Detailed stratigraphic sections of the upper part of

the sequence are available for the marginal and central

parts of the Chalco lake. The upper 7 m record a time

span between ca. 17,450–14,500 years to the present

according to 14C dating made in a core from the

central part of the basin (Caballero and Ortega-Guer-

rero, 1998). Zawadski (1996) described the upper 6 m

of the sequence from the margin of the lake that

consists of 11 layers of alternating clay, organic silt,

sands and gravel of volcanic origin, with thicknesses

ranging from 5 to 60 cm. Caballero and Ortega-

Guerrero (1998) assigned four stratigraphic units

(brown silt with lapilli, brown silt, gray diatomite,

and black-brown clay from top to bottom) and seven

tephra layers for the first 7 m from the central part of

the lake sequence. The physical correlation of these

sequences is based on the reconnaissance of the

Tlapacoya 2 unit (Ortega-Guerrero and Newton,

1998) in the studied CH core (see Fig. 2).


The study area is affected by a north–northeast

trending regional fracture of several tens of meters

long, with uplifted borders, and a surface opening of

more than 1.0 m that closes with increasing depth

(Figs. 3 and 4a). The fracture is filled with clayey

Fig. 4. (a) Panoramic view of the Chalco Plain to the southwest, the regi

indicated; (b) excavation in Site 2 permitted to detail the stratigraphy of

observed.

sediments but opens seasonally. This fracture is the

most evident structural feature observed in the sur-

face; however, it has been shown that numerous

subsurface fractures and subsidence affect the lacus-

trine sequences (Ortega et al., 1993).

onal fracture with uplifted borders and the location of CH core are

the upper 2 m, and small fractures within the clay sequence were


4. Characteristics of the GPR used

The GPR Zond 12c, manufactured in Latvia by

Radar Systems, was used for the soundings. A porta-

ble computer connected to the control unit permits the

direct recording of raw data collected by the radiat-

ing–receiving antennae. With this equipment, a single

operator moves the antennae along the surveyed

surfaces in monostatic mode (the antenna operates

both as radiating and receiver). Table 1 summarizes

the measurement parameters of the survey.

The alternate application of two transmission fre-

quencies in the same profile, using shielded 900- and

unshielded 300-MHz antennae, permitted the recog-

nition of the particular types of structural features

recorded by each frequency. The analysis of a nor-

malized frequency spectrum indicates that the effec-

tive frequencies through the studied media were ca.

500 and 100-MHz for the 900- and 300-MHz anten-

nae, respectively.

Since the available antennae of the GPR were

monostatic, the analysis of subsurface velocities pro-

vided by the common midpoint (CMP) method could

not be performed. However, the detailed near-surface

stratigraphy observations permitted a satisfactory

evaluation of investigation depths (see Figs. 3 and

4b). By matching the position in depth of a specific

Table 1

GPR measurement parameters and characteristics of the radar wave

in the Chalco site

Center frequency

of antennae (MHz)

300 900

Impulse power 400 400

Length of antennae (cm) 102 52

Resolution (cm) 100 20

Recording time

window (ns)

240 100

Battery power supply (V) 12 12

Characteristics of the geological media

Estimated frequency

through the media (MHz)

100 500

Permittivity 40 40 (55)

Velocity (cm/ns) 4.7 4.7 (4.0)

Maximum depth of

penetration (m)

f 4.5–5 f 1.4–1.9

Recorded feature stratigraphy and

deformation

change in physical

property and

stratigraphy

Thickness (cm) 10–20 40–100

reflector (a stratigraphic boundary associated with

contrast in physical properties in most of the cases)

with the reflectors on the radargrams, we obtained an

accurate evaluation of the bulk velocity of propaga-

tion of electromagnetic waves through the media. This

methodology allowed the time/depth conversion using

a bulk velocity of propagation for the sequence of 4.7

cm/ns. Vertical variations in the bulk velocity of

propagation could be taken into account only for the

900-MHz sections, in which we obtained a lower

velocity (4.0 cm/ns) by adjusting the depths of well-

known reflectors such as sand lenses.

Eight profiles were obtained in two perpendicular

directions (N 20j E and N 70j W) in order to identify

the spatial variations of the recorded reflectors (Fig. 3).

North–south profiles a, b, c, and d were obtained

parallel to the regional fracture observed in the area,

whereas the east–west profiles e, f, g, and h were

perpendicular to this structure. The length of each line

profile was 30 m, except profile g that had a total

length of 40 m.

5. Methodology of laboratory analysis

Assuming that physical and mechanical variations

can produce sufficient electric contrast to be recorded

in a radargram, then the depth of specific reflectors

can be obtained from the detailed analysis of a

reference stratigraphic section. This method has been

used recently with success for correlation of radar data

with different properties of soils (Oleschko et al.,

2003). Representative samples of approximately 20

cm each from the Chalco core (CH) were analyzed at

the Geomechanical Laboratory of the Center of Geo-

sciences (National University of Mexico, UNAM).

For the first 2 m, the data were complemented with

the analysis of disturbed samples collected in the Site

2 excavation (Fig. 4b). The sequence below the water

table that occurs at about 0.8 m was considered to be

in saturated conditions.

The main physical characteristics that were mea-

sured in the sequence in order to identify physical

discontinuities that can be related to electrical con-

trasts were:

(a) Gravimetric (Wgrav%) water content is the weight

ratio between pore water and solid fraction

Fig. 5. Composite stratigraphic column of the Chalco soils using CH

core and Site 2 excavation. Ten stratigraphic units (A–J) were

described in the upper 7 m; the geometry and filling of the regional

fracture are also shown. The position of physical contrasts found in

the sequence (i –v) is described in the caption of Fig. 6. These

contrasts were identified as potential reflectors of electromagnetic

waves.


averaged from three measurements obtained by

the ovendrying method (American Society for

Testing and Materials, ASTM D2216-92, 1998a),

whereas volumetric water content (Wvol%) was

computed by the ratio between Wgrav and the

porosity (Santamarina et al., 2001); Wgrav% is a

parameter widely used for geotechnical character-

ization of soils (see Das, 1997), a value of 300%

frequently obtained for the clays of Mexico means

that for each gram of solid there are 3 g of water;

(b) Atterberg limits (ASTM D4318-95) were deter-

mined using the Casagrande coup for the liquid

limit (LL) and the Terzaghi method for the plastic

limit (PL);

(c) Electric conductivity (EC) was measured using a

Cole Parmer TDS Conductivity meter in a

suspension of soil–water (1:2.5) (saturated paste,

USDA, 1996);

(d) grain size was determined by hydrometer and sieve

analysis (ASTM D422-63, 1998b) of samples

representative of each 20 cm; clay and silt contents

were added and presented as fine-grained sedi-

ments in Fig. 6;

(e) specific gravity of particles was determined by the

pycnometer method (ASTM D854-92, 1998c),

and;

(f) compressibility index was computed as a variation

of the void ratio in a log pressure cycle of the

compressibility curve (Lambe and Whitman,

1969), determined by the one-dimensional con-

solidation test (ASTM 2435-96, 1998d).

6. Results

6.1. Description of the near-surface sequence of

Chalco

The description of the stratigraphic units defined in

the CH Core was based on a correlation with available

column descriptions. The BH4 core and Site 1 exca-

vation of Zawadski (1996) are located within the study

area, whereas Core B of Caballero and Ortega-Guer-

rero (1998) is located in the center of the lake (Figs. 1

and 2). Notably, greater thickness of the sequence

above the Tlapacoya 2 unit is observed in Core B.

Within the study area, smaller variations in thickness

can be related to local depositional changes and/or

deformation features. Eight stratigraphic units (Fig. 5)

are described for the first 7 m depth of the studied CH

core: (unit A) a layer of organic soil with abundant

vegetal roots of approximately 0.5 m with an upper

horizon of high-salinity soils; (unit B) a layer of pale-

brown silt with pyroclastic rock fragments that suggest

intense volcanic activity during the deposition of this

layer, with thickness ranging from 1 m in the center of

the lake (Core B) to 0.1 m in the margins; (unit C) a

brown clay of high plasticity; (unit D) underlying there

is a brown-reddish clay layer with carbonate concre-

tions reaching 2 m depth, vertical fractures were

observed in the lower part of this unit in Site 2; (unit

E) a brown clayey silt separated from the overlying

clay layer by a thin lens of sand (see Fig. 3b); (unit F) a

sandy layer of variable thickness (10–30 cm); (unit G)

black silty clay of approximately 1 m thick; (unit H) 25

cm of pyroclastic gravel that correlates with Tlapacoya

2 unit of Ortega-Guerrero and Newton (1998) reported

at f 5 m depth in Core B (Caballero and Ortega-


Guerrero, 1998); and (units I and J) comprises a

sequence of brownish to greenish gray clays charac-

teristic of redox lacustrine environment and a grey-

brown clay layer with pyroclastic rocks observed at 6

m depth. The results obtained in the characterization of

the upper 7.0 m are presented in Fig. 6.

The analysis of the physical properties permitted to

identify five potential reflectors of radar waves (Fig.

6): (i) a decrease from 15 to 3 mS/cm in the electrical

conductivity at 0.4 m depth, this contrast is slightly

above (f 10 cm) that can be added to the grain-size

contrast in the upper boundary of the pyroclastic silt

unit B; (ii) an increment in the clay content associated

with an abrupt increase in gravimetric water content

(from 100% to 300%) that is observed at the upper

boundary of unit C (0.8 m depth); (iii) a bimodal

water content in the brown-reddish clays associated

with the lower boundary of the unit C at 1.3 m depth

Fig. 6. Measured physical and mechanical properties of the upper 7 m of

water content, (c) Atterberg limits (L: liquid, P: plastic), (d) electrical c

compressibility (oedometer test). Contrasts in the sequence are be observed

change in grain size (from sand to clay) and increase of water content (from

increase of fractures below high-plasticity clays at 1.35 m depth; (iv) a var

depth related to, and 3.75 m depth (v) related to a lens of pyroclastic grav

methodology. Note that units C, D, and E are high compressible soils wi

and decrease in the clay content; it is important to

mention that fracturing of the clay layers increases

below this level; (iv) an abrupt increment in the sand

content (volcanic ash layer) and a decrease in water

content from 100% to 50% at 2.6 m depth; and (v) a

lens of pyroclastic gravel at 3.7 m depth.

Because of its heterogeneous composition and high

water content a ductile–brittle mechanical behavior

was expected in the Chalco clayey sequence. As

mentioned above, units C and E present water con-

tents higher than their liquid limit, which suggest that

these materials can behave as fluids when their

structure is disturbed. In particular, a strong contrast

in the physical properties is observed in unit C,

elevated liquid and plastic limits, high compressibility

index (Cc f 3), and decrease of specific gravity. We

emphasize that the water table occurs at about 0.8 m

coincident with the increment of the water content and

the Chalco sequence: (a) gravimetric water content, (b) volumetric

onductivity, (e) grain-size distribution, (f) specific gravity, and (g)

at: (i) a decrease of the electrical conductivity at 0.4 m depth; (ii) a

100 to 300) at 0.8 m depth; (iii) a decrease of water content and the

iation of grain size (sand unit) and a decrease in water content 2.6 m

el. See text for ASTM and USDA test numbers and explanation of

th natural water content bigger than their liquid limit.


electric conductivity slightly increases below unit C.

Finally, unit F represents a major contrast in grain

size, with greater sand content, low water content, and

greater specific gravity.

6.2. Spatial correlation of physical and mechanical

properties with GPR profiles

We present the first GPR records of stratigraphic

and structural features for the lacustrine sequence of

Chalco, Mexico. GPR surveying has permitted the

Fig. 7. Correlation of the stratigraphic column and reflectors recorded in 9

surface stratigraphy was based on projection (showed with an arrow) of th

and continuous reflectors along the radargram were associated to changes

stratigraphic units is presented in Fig. 5. The stratigraphy of the first

electromagnetic waves of 4.0 cm/ns (permittivity 55 indicated in italics) ins

the whole profile. Calculated depths of investigation for both velocities ar

identification of subtle stratigraphic heterogeneities

related to differences in water content and grain size

within the studied sediments, and allowed interpola-

tion of sedimentary characteristics such as the conti-

nuity of sand or clay units. Radar sections collected

along profile a, with 900- and 300-MHz antennae (Fig.

7a and b), were used to correlate radar signatures with

the reflectors identified previously. The projected

location of the core on profile a is portrayed in radar-

grams in Fig. 7. The above-defined potential reflectors

were then identified in the radargrams based on the

00- and 300-MHz radargrams of profile a. Correlation of the near-

e CH core located at a distance of 0.5 m from this profile. Coherent

in physical properties or stratigraphic boundaries. Nomenclature of

1.5 m is better adjusted assuming a velocity of propagation of

tead of the calculated bulk velocity of 4.7 cm/ns (permittivity 40) for

e presented in the right side of the 900-MHz profile.


changes in well-defined radar signatures and the pres-

ence of coherent radar wave reflectors. The reconnais-

sance of a reflector assumed to be the upper boundary

of the sand layer at ca. 100 ns permitted the estimation

of a value of 40 for the bulk permittivity of the

sequence. However, for the 900-MHz antenna, best

adjusting of the depth of reflectors was achieved with a

permittivity of 55 (velocity of propagation f 4.0 cm/

ns). The velocity of propagation in the media is

assessed using the simplified formula v = c/e1/2, wherec is the velocity of propagation of waves in the air and

e is the real part of the permittivity. We present

between parenthesis and italics the calculated depths

and velocity obtained with both values of permittivity

for the 900-MHz radar sections through the figures.

The five reflectors were observed in the 300-MHz

antenna; however, best definition of reflectors i, ii, and

iii was obtained using the 900-MHz antenna.

6.3. 900-MHz profiles

The records of radargrams in two perpendicular

profiles with the 900 MHz are portrayed in Fig. 8.

Fig. 8. Two perpendicular 900-MHz radargrams corresponding to profiles

profile a in Fig. 7, profile b shows coherent reflectors between saline s

reflectors were recorded in profile e for the first 15 m of horizontal distance

diffraction hyperbola (noise with a branch slope of 30 cm/ns) probably g

Lateral continuity of coherent reflectors at constant

depths along the total distance is observed in both

profiles. Small disturbances in reflectors are observed

only within the profile e perpendicular to the regional

fracture in particular, before 10 m and between 15 and

20 m in distance. It is important to note that a

diffraction hyperbola probably related to the electric-

ity line was recorded at the eastern part of all the

profiles perpendicular to the fracture.

The shaded area in the upper part of the radargram

corresponds to the direct wave (from 0 to 10 ns); the

high attenuation of energy is controlled by the high

electrical conductivity caused by a superficial high-

salinity layer of soils. The position of reflector i along

the profiles is defined by the decrease of electrical

conductivity that occurs within the underlying organic

soil layer. The response of radar waves can be added to

the reflector located at the highly irregular upper

boundary of unit B. Reflector ii is located at the in-

crease in water content coincident with the contact

between unit B and C. Notably, a distinctive and homo-

geneous radar signature is observed between reflectors

ii and iii corresponding to the relatively homogeneous

b and e, in north–south and west–east direction, respectively. As in

oils, pyroclastics, and high-plasticity and fractured clay. Similarly,

; however, from 15 to 30 m, the useful information was masked by a

enerated by the electricity line (see Fig. 2).

Fig. 9. North–south radargrams collected with the 300-MHz antenna along profiles b, c, and d. The CH core is projected from distances of 5, 9,

and 17 m to profiles b, c, and d, respectively. Calculated depth of investigation using a velocity of 4.7 cm/ns is presented in each profile. The

main reflectors are indicated, question marks indicate not well-recorded positions.



clayey soil mass with high gravimetric water content

(f 300%) and without structural features.

6.4. 300-MHz profiles

Three profiles with the 300 MHz, parallel to the

regional fracture, are compared in Fig. 9 in order to

show the record of the identified reflectors and their

in-depth variation and structure on both sides of the

fracture. The depth of the recorded reflectors was

fitted with the depth of defined contrasts in the

projected position of the CH core. Best definition of

structure patterns was obtained with this antenna;

thus, disturbances in the radar signature can be related

with deformational features. Clearly defined radar

signatures observed between reflectors i– ii and ii–

iii correspond roughly to pyroclastic silt unit B and to

the high-plasticity clay unit C, respectively. The

irregular patterns of reflection show sinuous forms

that suggest differential deformation of the soils in this

part of the sequence. A decrease in thickness of the

clayey sequence above unit F towards the east is

evident by the gradually shallower position of reflec-

tors iv and v.

Two examples of profiles perpendicular to the

regional fracture are shown in Fig. 10a and b. In this

direction, lateral continuity of reflectors is highly

disturbed and heterogeneous radar signatures recorded

suggest that deformation effects are more intense in

this direction. A satisfactory position of the reflectors

was obtained by correlating the depth of reflectors

identified in the north–south profiles. Differences in

depth of reflectors become evident below reflector ii,

and distortion of layers is best observed in the radar

signature obtained between reflectors iv and v that

correspond to the units F and G. These radar sections

are not corrected topographically because small uplift

(f 20 cm) of the horizontal surface is only observed

in a 40-cm-wide band on both sides of the regional

fracture (see Fig. 4a). The flower-like geometry of the

Fig. 10. West–east radargrams collected with the 300-MHz antenna along

identified in north–south radargrams (Fig. 9) are indicated under its corresp

line and question marks indicate a not well-recorded reflector. Some usefu

slope of 30 cm/ns. The depth of this reflector decreases gradually in prof

caused by the electricity line. (c) Interpretative section of profile g; vari

differential deformation of high-plastic layers with high water content (ir

regional fracture is indicated with a dotted line in a schematic way. Note stra

located in both sides of the regional fracture.

regional fracture and the inferred effect of uplifting on

reflectors are presented in the interpretative section

(Fig. 10c).

7. Discussion

The Chalco near-surface lacustrine sequence con-

sists of alternating layers of plastic clay, volcanic

ashes, pyroclastic sand and gravel. The abundance

of pumiceous concretions within clayey units indi-

cates that volcanic activity was coeval with deposition

of the sequence. The upper unit of the sequence is

characterized by a high-salinity clayey layer formed in

an arid climate that prevailed from the beginning of

the Holocene (Urrutia-Fucugauchi et al., 1994).

The use of GPR has been particularly useful for the

characterization of the Chalco sequence because: (a)

the presence of volcanic layers within the clayey

sequence increases the reflection coefficient by en-

hancing physical and chemical contrasts (i.e. the high

water content of unit C); (b) the presence of deforma-

tion structures with a particular radar signature; and (c)

the detailed stratigraphic studies available that have

served as a basis for the correlation of the physical

properties and stratigraphic units.

The GPR method was expected to have a low

detection capacity in these high water retention ca-

pacity and low electrical resistivity clayey materials.

However, water content variations and grain-size

contrasts improved the capacity of detection consid-

erably. For example, assuming a bulk permittivity of

30 and 40 for saturated sand and clay layers, respec-

tively, the coefficient of reflection in this contact is

approximately 0.01, which lies at the limit of detec-

tion capacity of a standard radar (Annan, 1992; Davis

and Annan, 1989). In more conductive clay, permit-

tivity can increase considerably because of the salt

content of pore water. An increment of f 30% in the

permittivity of the upper part of the sequence (from 40

the: (a) profile e and (b) profile g. The i, ii, iii, iv, and v reflectors

onding crossing profile. The fracture zone is indicated with a dotted

l information was masked by a diffraction hyperbola with a branch

iles approaching to the electrical pole, suggesting that this noise is

ations in depth of the reflectors were correlated and interpreted as

regular patterns between reflectors ii and v). The geometry of the

tigraphical and structural changes of the soil mass in profiles c and d


to 55) is sufficient to double the reflection coefficient

(to 0.02) and a high dielectric contrast between the

clay and sand layers is recorded.

Ground Penetrating Radar has proven to be an

effective instrument for prospecting near-surface

stratigraphic variations in saturated lacustrine sedi-

ments due to the close relation between the behavior

of electromagnetic waves in the medium and physical-

mechanical characteristics of soils (water content,

salinity, grain size, compressibility, and plasticity).

The analysis of radar profiles permitted the spatial

correlation of near-surface stratigraphic characteristics

as well as the identification of discontinuities of the

recorded reflectors associated to subsoil deformation.

The identification of coherent reflectors along the

profiles related to the electric contrast was also useful

for the record of deformational features observed in

the clayey layers. In this way, the single vertical

fractures or fractured zones could be identified in

several radargrams.

The different estimated values of the velocity of

propagation of electromagnetic waves in the pro-

spected media and the achieved detection depths are

summarized in Table 1. Notably, the highest resolution

in stratigraphy was obtained for the first 1.5 m allowing

a better calibration of the reflectors obtained with the

900-MHz shielded antenna. Using the 900-MHz an-

tenna, the recorded reflectors correspond to vertical

variations in physical properties coincident with strati-

graphic boundaries. Thus, reflectors ii and iii are clearly

associated with the change in water content at the upper

and lower boundaries of unit C. One exception is

reflector i that is better linked to a vertical change in

electric conductivity because the corresponding upper

boundary of unit B is highly irregular. As expected,

best adjusting of the depth to reflectors was obtained

using a lower velocity of propagation because shallow

layers have higher values of electrical conductivity,

water content, and plasticity.

A discontinuous radar signature observed between

reflectors ii and iii in profiles with the 300-MHz

antenna suggests deformation of unit C. Differential

deformation of this high-plasticity compressible clays

layers can occur by self-weight consolidation, or

because they can behave as a fluid under certain

externally forced conditions (e.g. produced by seismic

activity or depressurization of subsoil). Lateral heter-

ogeneities of the clayey sequences can also have an

important influence on its mechanical behavior, par-

ticularly in the differential consolidation.

This deformation can also be related to the origin

and propagation of the regional fracture observed in

the study area. This fracture opens and closes season-

ally likely due to shrinking and swelling of the clayey

sequence. During dry weather conditions, the fracture

opens as a consequence of the water content loss and

is partially filled by detritus material. During the rainy

season, the clayey sequence expands closing the

fracture and, as new material has been added, the

deformation is accommodated upwards.

Detailed GPR field studies relating detection ca-

pacity to variations of velocity of propagation in

geological materials are scarce albeit theoretical stud-

ies have been developed about propagation of elec-

tromagnetic waves. A quantitative analysis should

involve complex calculations considering among oth-

er parameters, the efficiency of the used radar, the

transmitted frequency, electrical contrasts, and the

attenuation of the signal. The radargrams presented

in this paper show that some aspects of the velocity of

propagation variations within a stratified geological

sequence can be qualitatively assessed. Interpolation

of near-surface physical properties of soil using GPR

is at an initial stage in the case of clayey Chalco.

Nevertheless, encouraging results were obtained by

the correlation of recorded reflectors in profiles with

changes in specific physical properties and relevant

applications can be envisaged both in geological and

geotechnical studies.

8. Conclusions

A comparative analysis of GPR profiles supported

on detailed punctual analysis of mechanical and

physical properties permitted the extrapolation of

lateral and vertical variations of the near-surface

stratigraphy of a lacustrine sequence in the Chalco

Basin. Contrary to the expected in these saturated

sequences, electric contrast enhanced by abrupt

changes in water content and grain size permitted

satisfactory recording of the sedimentary structures.

Distinctive radar signatures were obtained for each

layer present between coherent reflectors. The best

definition in stratigraphy and structure was obtained

with the 300-MHz antenna, whereas grain size and


physical variations were best recorded using the 900-

MHz antenna. An irregular radar signature with sin-

uous form recorded the presence and geometry of a

highly plastic layer. Furthermore, lateral discontinu-

ities of the reflectors and their vertical displacements

permitted the identification of fractures zones and

differential deformation within the sequence.

Acknowledgements

The authors thank Klavdia Oleschko for valuable

advice. We are grateful to Gabor Korvin and Barbara

Martiny for discussions and helpful comments on an

early version of the manuscript. Two anonymous

reviewers provided constructive criticisms that enor-

mously helped to improve this work. We thank

Ricardo Carrizosa for his help on the analysis of the

Chalco cores at the Geomechanics Laboratory of the

Center of Geosciences, National University of Mexico

(UNAM). Financial support for this research provided

by the Program of Support to Technologic Researches

and Innovations UNAM, PAPIIT 1N111398 is grate-

fully acknowledged.

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