Geophysical Prospecting Volume 30 Issue 3 1982 [Doi 10.1111%2fj.1365-2478.1982.Tb01310.x] v. Iliceto; g. Santarato; s. Veronese -- An Approach to the Identification of Fine Sediments

8/10/2019 Geophysical Prospecting Volume 30 Issue 3 1982 [Doi 10.1111%2fj.1365-2478.1982.Tb01310.x] v. Iliceto; g. Sant

1/17

Geophysical Prospecting 30, 331-347, 1982.

A N A P PR O A C H T O T H E I D E N T I F I C A T I O N O F

F I N E S E D I M E N T S BY I N D U C E D P O L A R I Z A T IO N

L A B O R A T O R Y M E A S U R E M E N T S *

V. ILICETO,**

G.

SANTARATO** and

S .

VERONESE**

A B S T R A C T

ILICETO, ., SANTARATO,

. and

VERONESE,.

1982, An Approach to the Identification of Fine

Sediments by Induced Polarization Laboratory Measurements, Geophysical Prospecting 30,

Time-domain-induced polarization (IP) laboratory measurements were performed on

about 200 fine sediment samples with varying water content. The results permitted an analysis

of I P properties of clays, loams, silts, and sands.

Particular emphasis has been given to the analysis of the chargeability m as a function of

lithotype and the water content.

By analyzing decay curves, a new parameter was identified. It is a statistically specific

characteristic of the lithotype and is independent of the water content. Therefore, it provides a

diagnostic parameter for lithotype identification. In association with the values of chargeabi-

lity and electrical resistivity, this parameter permits a reliable evaluation of water content and

yields useful information about the porosity and permeability of the lithotype.

331-347.

1. I N T R O D U C T I O N

Several authors have studied chargeability of sedimentary rocks, both clastic and

compact, as a function of various parameters, notably porosity, metallic grain size,

and temperature (e.g. Vacquier, Holmes, Kitzinger and Lavergne 1957; Marshal1 and

Madden 1959).

The work of Ogilvy and Kuzmina (1972) contains an interesting and extensive

review of the dependence of chargeability m on the aforementioned variables, and

suggests also the possibility of m being a function of water content. The relative

contribution of various polarizing mechanisms, such as membrane and double-layer

effects, is known to change according to lithotype. This fact led us to the thought that

*

Paper read at the

42nd

meeting of the European Association of Exploration Geophysicists,

Istanbul, June 1980. Final version received October 1981.

**

Institute

of

Mineralogy of the University, 44100 Ferrara, Italy.

0016-8025/82/06004331 02.00 982 EAEG

331


2/17

332

V .

I L I C E T O

E T A L .

it should be possible to distinguish the nature of sediments on the basis of their IP

behavior, even in cases of variable water content.

The present work describes the results of a laboratory study on the water-

content-dependent I P (time domain) properties of fine sediments that originated

from various sites in the

PO

valley. Special attention was given to discovering a

parameter which would permit differentiation of lithotypes apart from water content.

Such a parameter would be of importance in hydrology, particularly in studying

layers consisting of sediments, which are water-saturated to varying degrees and

situated within an alluvial mattress. Such differentiation would be extremely difficult

using standard resistivity methods.

2 . L A B O R A T O R Y E A S U R E M E N T S

The basic parameter measured by time-domain

IP

techniques is chargeability

m.

The

literature provides several definitions according to the technique of measurement

employed. We define chargeability as

where

V M Ns

the potential differences

of

energization measured at the electrodes M N

and

QP(t)

is the transient potential difference measured at

M N

during discharge.

According to this definition,

m

is dimensionless.

Figure 1

shows a block diagram

of

the instrumentation employed for time-

domain

IP

measurements in our laboratory. The instrument is suitable not only for

laboratory work, but also for field measurements of small AB-MN separations. It

consists of three parts:

(a) A commutator generates a square-pulse electric current that is fed to the

current electrodes AB. The response at the potential electrodes M N from a polariz-

able target is shown magnified in fig. 2(a). A selector permits the choice of four

different charging times TAB

=

2,

4,

8,

16

s. A

time-reference cable connects the

commutator to the digital timer and to the receiver.

(b) The improved version of the digital timer (Iliceto 1979) permits a choice

between seven intervals of integration t

-

l = 20,40, 80, 120, 160,240, and

300

ms.

During the integration interval, an integrator circuit is closed which furnishes the

value of the integral directly to the receiver voltameter (right-hand side in fig. 1).The

start of the integration interval can be increased in 10-ms steps from 10 ms to 9.990

s

after opening of the circuit A B .

(c) The receiver contains the integrator circuit that is driven by the digital timer,

a voltameter, a potentiometer for zeroing the spontaneous potentials (SP), a three-

way function selector and a relay driven by the commutator through the time-

reference cable. The function selector either permits the zeroing of the SP or the

measurement of the charging potential difference V M N .t can also connect the inte-

grator circuit to the voltameter, thus directly providing the integral value of (1).


3/17

I D E N T IF I C A T IO N O F S E D I M E N T S

333

0 - 1 o o v

Themeasurement ofthechargeability often involves extremely weak signals. There-

fore, readings

of

the single energizations of the target can be completely spurious in

the presence of even a slight drift of SP. Under the assumption of a linear drift, m is

determined as the mean of the sum of a series

of

measurements. Figure 2(a) illustrates

a cycle of measurements corrected for the error resulting from a linear drift. m is

measured by five charging cycles of alternating polarity; in the third of these the

integrator is carefully short-circuited. If

p

s the value of the integral in (1)read on

the voltameter in the ith cycle, m is thus given as

m =

1oqPl +

P 2

+

P4

+

P5)/[4hm(h - l ) l .

(14

Patella, Schiavone and La Penna (1977) proposed a computation method for the

elimination of both constant and variable SP from single measurements of m.

Instead, the procedure expressed by (la) provides reliable results in real time and

thus avoids cumbersome calculations that would have to be performed later.

With sufficiently small integration intervals t 2

-

l , (1) results in a satisfactorily

reproducible decay curve. Figure 2(b) shows the decay curve obtained for a sand

sample with different integration intervals. The data points lie sufficiently close to the

decay curve, independent of the interval. This makes it possible to use intervals for

chargeability measurements which are in inverse ratio to the signal intensity.

Several sediment samples, each weighing several kilograms, were placed in a

rectangular container measuring 30 x 15 x 4 cm n its upper surface was placed a

Wenner-type array consisting of four non-polarizable electrodes.

The water content was determined by weighing dry and wet samples. However, in

practice, we preferred to establish the weight loss in a water-saturated sample after

heating in an oven. Such a procedure assured a better distribution of the water

contained in the sediment.

The chargeability measurements were performed with varying water content.

Linearity was assured in all experiments.

P O W R

5 v

Fig. 1

Block diagram

of

instrumentation.


4/17

334

V . ILICETO E T

A L

m Xt

sand T=4sec

o

AT=0.24rec

. T=O.l2sec

v AT=O.Ol)Slc

x AT=O.O4SOC

Fig.

2.

a) IP response

of

a polarizable target a t potential electrodes

M N

arbitrary scales);p

is defined in Equ ation la ). b) Decay curve

of

a sandy sample, obtained using different

integration intervals

At.

3. DESCRIPTION

F T H E

S A M P L E S

The sediment samples studied originated from various areas of the

PO

alley. The

samples were selected in view of various fluvial and fluvio-glacial sedimentation

products that reflect the source basin of the POconfluents. The sediments deposited

by the

PO

tself reflect the mineralogic variety of the Appeiinine and Alpine source

area. The lithologically varied Alpine contribution prevails in Adige and Piave

sediment .


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IDENTIFICATION

O F

SEDIMENTS 335

The samples have been subdivided into three classes according to granulometric

classification

(1) Sands, with a grain size bigger than 74 pm,

(2) Silts and loams, with average grain size between 74 pm and 2-3 pm,

(3)

Clays, with particles smaller than 2-3 pm.

This classification has direct application in engineering studies and hydrology. The

granulometric curve also provides information on porosity and water content of the

sediments.

The IP study of samples have been developed in successive steps. Initially, a

comparative examination was made of chargeability of different samples. Sub-

sequently, the exponentials of individual decay curves were analyzed in order to find

parameters related

to

granulometry and water content.

4 . A N A L Y S I SF DECAY U R V E S

Roussel

(1962, 1967)

conducted a penetrating study into the shape of the decay

curves. He based his work on the following model:

k

m = 1 j exp (

- t / z j ) .

j = l

The constants

A j

and z j have been obtained from graphical decomposition.

A

similar

method has been used later by others, notably Phillips and Richards

(1974)

and

Bertin and Loeb

(1974),

who limited the number of exponentials to 2 or 3.

Methods suitable to computer data processing have been proposed by Bertin and

Loeb (1974, 1976) and Patella, Schiavone and La Penna (1977).

We determined the constants

A j

and z j by least-squares non-linear regression

(Draper and Smith1966).Wechose Marquardt's version ofthis algorithm (1963),which

was adapted for computer use by Robinson (1979).The estimates are based on standard

statistical parameters of the correspondence between model (2) and the experimental

curve (for example, matrix of variance and covariance and confidence limits of the

parameters).

The initial estimates A ') and

z y )

of the parameters in 2) have been deduced from

a mean of values graphically determined from some decay curves. This was possible

because of the relatively restricted range into which decay curves of a single lithotype

fall.

5 .

C H A R G E A B I L I T Y

N D

W A T E R O N T E N T

Decay curves have been obtained for all samples and various moisture values at two

charging times:

TAB

= 4 and

TAB

= 16

s.

The decay curves for TA,= 4 s in fig. 3 are

examples of those obtained for sediments under extreme conditions (water-saturated

or completely dry) and for intermediate water content. The decay curves obtained at


6/17

336

V .

I L I C E T O E T A L .

S i l t S T I )

I I I I

I 2

3

4

C l o y - ( C 2 )

H 2 0

lOO~/O

I I I 1

I

2 3 4

sec

Fig. 3. Examples of decay curves, obtained from lithotype sample at varying water content

(charging time

TAB 4 s, t

=

1.28 s).

16

s

charging time were omitted, because they did not contain additional informa-

tion, even when analyzed by the method of exponentials (see below).

More than 200 decay curves obtained in our laboratory have been reconstructed

on

the basis of at least

20

chargeability values, measured at suitably spaced times.

Such values have then been used for exponential analysis.


7/17

I D E N T I F I C A T IO N O F S E D I M E N T S 337

For a comparative test, it was necessary to single out the value of rn most suitable

for the demonstration of interrelationship between chargeability, lithotype, and

water content. This value was obtained at 1.28 s after the opening of the A B circuit

(TAB

4

).

At this delay time the induced electromagnetic phenomena of the in-

strumentation were negligible, but the chargeability values were still measurable. In

addition, this delay time was close to one of the three used for the measurement of rn

in our previous work (Iliceto, Santarato and Veronese

1979),

which contains preli-

minary results from laboratory experimentation of fine sediments, including those

exposed to a 30 ,aqueous NaCl solution. The presence of aqueous salt solutions

invariably reduces the value of rn, often below the sensitivity of the instruments, and

independently

of

water solution and granulometry. Therefore, no attempt has been

made to study chargeability of samples immersed in salt solutions.

1.0

0.9

0 8

0 7

0 6

E

0 5

0.4

0 3

0 2

0

0

TAB=4sec

t

=

1 28

ec

f

ST,

OST, Si l t

X ST,

ST4

v

STs

*

LI

L3

A L2

Loam

I I I

I

I I

I I

10 20

30 40 5 60 70 00

90 100

H20

Fig. 4. Chargeability rn versus water content of clay, loam, and silt samples

( T A ,=

4

s,

t = 1.28 s .


8/17

338

V .

ILICETO ET AL.

5.1

Clays, loams, and s lts

Figure4 hows the results of measurements done on clays, loams, and silts. The water

content strongly depends upon the lithotype. For the clays, the possible water con-

tent ranged from 40% to more than 100 of the dry sample weight, and for the silts

from

10%

to

60 .

The chargeability values obtained for clays are very low rn < 0.15). The charge-

ability of loams and silts ranges from about 0.05 to 1.00. A unified graphical

presentation

of

chargeability as a function of water content (in )has been suggested

by some continuity of a functional relationship between m,water content, and the

variation of the lithotype. There is a range passing from extremely low chargeability

values which are essentially independent of the water content (clays) to high charge-

ability water content dependence in silts. Even though such a relationship cannot be

seen easily in samples

L1

nd L 2 , it is clear in other samples of silt.

Table

1.

Percentages ofsand

4>

74 pm) and clay

4 74 pm). Sample L 3 ,which showed higher

chargeability with increasing water content, had the same clay fraction but a higher

sand content (20 ).In all silt samples the clay fraction was less than 13 , whereas

the sand fraction reached 30 or more. The chargeability rose markedly with an

increase in water content in all samples of this type.

5.2

Sands

Various sand samples have been analyzed with a moisture content ranging from

2

to 30 of the dry sample weight. The results are shown in fig. 5 .

All

of these samples

show distinct characteristics. The variation of chargeability

as

a function of water

content shows a typical bell shape. The maximum does not correspond to any

particular moisture range, although it is present in each sample. With the exception


9/17

3.0

2.5

2 0

8

E

1

I S

O.

TAB= sec

t=1.28sec

I I

I I 1 1

5

10 15 20

25 30

Fig. 5 . Chargeability

rn

versus water content of sand samples

( T A ~

4 ,

t

=

1.28

s .


10/17

340

V . IL ICETO ET

A L .

of one sample that has a maximum around 15 water content, the maxima occur

for water contents between

5

and 10 . The exceptional sample has a significantly

higher chargeability than other samples, increasing the range of

m

variation 10-fold.

Without it,

m

would vary only between

0.3

and 1.5, unlike silts in which

m

varies by a

factor exceeding 10.

In search of a hypothesis capable of explaining the range of variation and the

bell-shaped distribution of chargeability, we have attempted to establish exper-

imentally the incidence of some lithological parameters specific in sands for the

determination of their general chargeability. We started by taking into consideration

the granulometric composition of sands and dividing them into smaller classes by

TAB=

4 ec

t = 1 . 2 8 s e c

m

0

I I

I

3 5

10 15

20 25 30

' H 2 0

Fig. 6 . Chargeability m versus water content of the granulometric classes obtained from the

sand samples

S3

and

S7

(TAB=

4

s,

t = 1.28 s .


11/17

ID E N T IFICA T IO N

O F

SEDIMENTS

1

p

\

\

\

\

9

341

100

200 300

400 5

D

Fig. 7. Chargeability rn versus average grain diameter in pm ) at 10 of water content

-

sand

samples

S 3 , S 7 ( T A B

4 s,

= 1.28 s).

sieving in a dry state. Sample S 3 has been subdivided into six granulometric classes

4

>

250 pm, 250 pm

> 4

> 180 pm, 180 pm > 4 > 125 pm, 125 pm >

4

> 90pm,

90pm >

4

> 74

pm, 4

250 pm) is absent in sample S3 and the finest fraction

(4

Geophysical Prospecting Volume 30 Issue 3 1982 [Doi 10.1111%2fj.1365-2478.1982.Tb01310.x] v. Iliceto; g. Santarato; s. Veronese -- An Approach to the Identification of Fine Sediments

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