Seismo-Microplasticity phenomenon in the rocks

doi:10.4236/ns.2010.23025

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Vol.2, No.3, 155-159 (2010) Natural Science

http://dx.doi.org/10.4236/ns.2010.23025

Seismo-Microplasticity phenomenon in the rocks*

Eduard Innokentevich Mashinskii

Institute of Petroleum Geology and Geophysics, Siberian Branch of the RAS, Novosibirsk, Russia; [email protected]

Received 26 November 2009; revised 8 January 2010; accepted 30 January 2010.

ABSTRACT

The seismic records of borehole-to-borehole me-

asurements on frequency of 200 Hz in the mi-

crostrain range have been analysed. Microplas-

ticity manifestations caused by seismic wave

are detected on seismic records. It is the lad-

der-like stepwise change in amplitude course in

some parts of the seismic trace. The step dura-

tion (time plateau) presents the amplitude-

dependent time delay that shifts the arrival time

and protracts pulse front. The microplastic

process occurs owing to the anomalous re-

alignment of the internal stresses on the micro-

structural defects in “elastic” domain. Result is

the useful contribution for improvement of the

theory of wave attenuation in the rocks. It can

also be used in solving the applied problems in

material science, seismic prospecting, diagnos-

tics etc.

Keywords: Anelasticity; Deformation And Time

Delay; Anelastic Seismic Parameters; Amplitude

Dependence Of Wave Velocity And Attenuation

1. INTRODUCTION

Perfection of the wave attenuation mechanisms is up to

now one of main tasks in Earth’s sciences. For under-

standing attenuation mechanisms, the new knowledge is

necessary about the rock anelasticity. Viscoelastic model

of standard linear solid well describes the dispersion and

relaxation but insufficiently correctly explains, for ex-

ample, the amplitude-dependent effects. So, there are

some contradictions relative to character of amplitude

dependences of wave velocity and attenuation. There are

data about decrease of wave velocity and increase in

attenuation with increasing amplitude [1-4]. However,

there are also facts when increase in the strain amplitude

leads on the contrary to the increase in wave velocity

and decrease in attenuation [5-9].

Detection of the quasi-static rock microplasticity has

strengthened supposition about some unknown factor in

charge of the amplitude dependence of seismic parame-

ters [10-12]. It is necessary to mark that in Earth’s sci-

ences the microplastic anelasticity is not yet the gener-

ally accepted fact as against solid-state physics. In con-

trast to the viscoelasticity, microplasticity appears only

when stress reaches critical value. The microplastic strain

can increase and decrease, appear and vanish during

increasing stress. The amplitude-dependent effects in

rocks and quartz crystals were interpreted as indirect

attributes of microplasticity that is possible even on the

small strain amplitudes [13,14]. Assumption about mi-

croplasticity processes during seismic wave propagation

was also made in works [15-16].

The direct testimony of seismic microplasticity was

received in borehole-to-borehole measurements during

the amplitude effect study [17]. This paper describes mi-

croplasticity manifestations detected on the seismic traces

in result of the detailed analysis of the field materials.

2. EXPERIMENTS AND DATA ANALYSIS

The propagation of seismic pulse with diverse amplitudes

in the area between two boreholes was studied. The ex-

periments were performed in the Bystrovka research area.

The measuring instruments were mounted in two bore-

holes 110 mm in diameter and 12 m depth spaced 7 m. A

source was located in one of the boreholes and a receiver,

in the other. The source in borehole 1 and receiver in

borehole 2 were successively installed at depth of 2, 6,

and 10 m. Detailed description is in the work [17]. The

upper part of the section is comparatively homogeneous

and is composed of loams as far as a depth of a few tens

of meters. Rock is partially water-saturated as far as 8.5 m

with compressional wave velocity Vp = 240 – 300 m/sec.

After the depth – 9 m the wave travels in completely wa-

ter-saturated loams with Vp = 1500 m/sec.

The measurements were made in accordance with the

following procedure. The source and the receiver were

successively located at depths of 2, 6, and 10 m in the

diverse combinations. Such source-receiver configura-

tion enables to study the pulse propagation in the differ-

ent direction. Basic seismic records were made on the

location of source–receiver in the lateral (horizontal)

*This work was performed with the support of the Russian Fund of Fun-

damental Researches, grant N 05-09-00405.

E. I. Mashinskii / Natural Science 2 (2010) 155-159

156

-0.004

0.001

0.020.021 0.022 0.023 0.024 0.025

Time, sec

Amplitude, V

-0.04

-0.02

0.02

00.005 0.01 0.015 0.02 0.025

(

)

-0.0028

-0.0018

0.02184

(c)



(b)

Figure 1. Seismic traces recorded in the 6 m–6 m “source-receiver” location on four amplitude values.

direction: 2–2, 6–6, and 10–10 m. On the short directions,

the seismic pulse propagated in the partially saturated

rock (6–6 m) and completely saturated rock (10–10 m).

The radiator of seismic signals consists of a set of pie-

zoelectric disks. The signal radiates through a liquid

spacer and hermetic elastic jacket contacting the borehole

wall. Predominant frequency of P- pulse is about 200 Hz.

The pressure receiver has the sensor of the piezoelectric

type (PDS-21) and, therefore, records compression ex-

tension waves. The receiver contacts the borehole wall

via an elastic spacer with a liquid. There is a preamplifier

with the amplification coefficients K = 100. The signals

were recorded in the digital form high-resolution during

of time (Bordo-B-421 system), and were processed on a

computer. The digitization time is 8 microseconds and 40

microseconds, the amplitude range is approximately (4 –

50) × 10-8. The discrete amplitude change was fulfiled in

a closed cycle, from the minimum to the maximum value

and back (Amin… → Amax… → Amin, 4 values upward and

3 values downward).

Some parts of seismic trace have the form of ladder

with the horizontal steps or plateau. The steps are evi-

dence of interruption in the stress course and the pres-

ence of the time delay and deformation delay. The pre-

sumable reason of such effect is microplasticity caused

by seismic wave. The signs of microplasticity were de-

tected on many seismic records. The typical fragments

from seismic traces with seismic microplasticity mani-

festations (SMM) are presented on Figures 1 and 2.

SMM take place for signals with low and high intensity,

i.e. small and great deformation rate.

A SMM example with small strain rate is shown in

Figure 1. Here are presented four seismic traces recorded

on four amplitude values in the 6–6 m “source-receiver”

location. Change in amplitude value during defined time

interval determines the amplitude steepness RA/t. The

amplitude value Ai is determined by number n of the

amplitude quantization steps (Aqu): Ai = n  Aqu during

one time quantization step, tqu. The amplitude steepness

is calculated as RA/t = n  Aqu/ tqu, i.e. it is determined by

way of number n. Thus, steepness is n = 2 for ampli-

tudes A1, A2 and n = 5 for amplitudes A3, A4. The step

length determines the time delay duration. The time de-

lay in this case is from one tqu to more than ten of tqu. On

the recordswith the small amplitude steepness, tqu is

equal 40 microseconds.

A SMM example of amplitude change with high rate

(n = 50 and n = 100) is presented in Figure 2. This re-

cord concerns the case when both source and receiver

were located at the depth of 10 m. Here the traces are

presented for the upward and downward amplitudes. In

principle, there is close coincidence in the form of the

repeated traces although there are some nuances (see

inset b). The delay duration in this case is tens of micro-

seconds (tqu = 8 microseconds).

The time delay changes the arrival time (see inset c in

Figure 2) and can also influence on the wave front dura-

tion. In the same time, duration of the time delay depends

Eab

E. I. Mashinskii / Natural Science 2 (2010) 155-159

157

-2

-1.5

-1

-0.5

0.5

1.5

00.001 0.002 0.003 0.004 0.005 0.006

Time, sec

Amplitude, V

A1-up

A2-up

A3-up

A4-up

A3-down

A2-down

A1-down

-0.46

-0.36

-0.26

-0.16

-0.06

0.00568 0.00608 0.00648

A1-up

A2-up

A3-up

A4-up

A3-down

A2-down

A1-down

(a)

-0.27

-0.24

-0.21

0.006008 0.006208

A2-up

A3-up

A4-up

A3-down

A2-down

(b)

-0.1

-0.05

0.0043 0.0045

A1-up

A2-up

A3-up

A4-up

A3-down

A2-down

A1-down

(c)

Figure 2. Seismic traces recorded on 4 upward and downward amplitudes in the location of “source– receiver” on 10 m.

on strain amplitude. Therefore, the amplitude variations

can lead to the pulse parameters change. The experi-

ments show that the increase in strain amplitude causes

the displacement of an arrival time towards smaller time.

The protracting of the wave fronts caused by time delay

occurs in complex way. In the present time, could say

that the intensity increase in wave shows the greater ex-

pressiveness of SMM. For the study of these effects, the

conducting of the special experiments is needed.

3. DISCUSSIONS

Data about dynamic microplasticity in solid states and

quasi-static microplasticity in the rocks confirm possi-

bility of SMM conditioned by seismic wave. The strains

level in static test and in seismic wave in the moderate

amplitude range is approx selfsame (～10-6 – 10-5). The

difference in the strain rate is not obstacle for the mi-

croplasticity process as a physical mechanism in both

cases is, seemingly, the same. The point is that mi-

croplasticity is the frequency-independent (time-inde-

pendent) process at least on seismic frequencies. Nu-

merous data testify about prevalence of microplasticity

effects in metallic materials, alloys, ceramics, thin-film

materials and other solid states in the acoustical and

low-frequency range [18,19]. Avowed feature of medium

with microplasticity is the dependence of wave attenua-

tion on strain amplitude. As regards rock microplasticity,

here there are the obvious gaps in one's knowledge. In-

asmuch as the direct manifestations of rock microplas-

ticity have been established only during quasi-static

stress in lab conditions (measurements on the samples)

[10], any study of rock microplasticity under dynamic

force is the considerable advancement in these investiga-

tions. Especially importantly, when it concerns the dy-

namic research in natural conditions.

In itself the physical experiment in situ for the pur-

pose of microplasticity detection in the rocks triggered

by seismic wave is original. In this experiment, natural

medium in which propagation of seismic pulse occurs is

test subject. It is unlimited medium in contrast to labo-

ratory samples of small size. This medium consists of

the dry and water-saturated rocks. It determines the ex-

periment specificity. Microplasticity detection became

possible also thanks to the using of the high-resolution

seismic record in the time domain (microseconds) in

which there is no need for the usual record of seismic

signals. Besides, new information and results originality

have been obtained thanks to the sounding by the dif-

ferent-intensity pulses upward and downward. Mi-

croplasticity amount depends on the energy level applied.

One can surmise that with increasing wave intensity

occurs the switching on the sources of microplasticity

with the multi-level hierarchy.

SMM can be explained in the following way. Inas-

much as the piezoelectric receiver registers the change

in dynamic stress (strain) with time, the flat steps on

the trace mean the brief interruption in the course of

158 E. I. Mashinskii / Natural Science 2 (2010) 155-159

stress. Such stress stop supposes similar stop on the

stress-strain curve that can occur owing to the mi-

croplasticity process. Total strain  = e + µ + v-e in

the small-strain range consists mainly of elastic and

microplastic component (e + µ), as viscoelastic com-

ponent v-e is comparatively small [10,11]. During

loading (or unloading), both components have own

contribution to the total strain. The stop in the course of

stress occurs when the stress reaches some critical

value σcr (for example, in the point b see inset (b) in

Figure 1). At that moment, the redistribution of con-

tribution between components occurs thus that contri-

bution of the microplastic component become pre-

dominant. In the extreme case, the strain increment

occurs only thanks to microplasticity as the increment

due to the elastic deformation does not occur at all [10].

Therefore, the yield process is also possible. The re-

grouping in components occurs thus that effective

modulus Eab and accordingly the stress and strain re-

main invariable during interval b – c. It is possible ow-

ing to the distinctive feature of rock microplasticity that

can both increase and decrease or even vanish during

stress. In the point c after delay, the stress recom-

mences own course with the same modulus. There is

the hierarchical set of the diverse critical stresses that

switch on the microplasticity sources when amplitude

increases (decreases). Thus, seismic wave switches on

the process of the anomalous redistribution of stresses

and strains and respectively adequate to it process of

the structural realignment in the rock. It takes a definite

time that leads to deformation delay. In order to ex-

clude the instrumental factor in appraisal of SMM, the

amplitude error of analog-digital converter is checked.

Testing of this device shows that the own error in pla-

teau duration does not exceed 2 microseconds in the

broad range of the amplitude steepness.

Similar deformation stops caused by microplasticity

were detected in C60 single crystals [20]. It is shown that

magnetic field manipulations lead to a change in the

strain rate, the decrease in the rate being accompanied by

a brief interruption of deformation. The deformation

delay (incubation period) was observed also in the

high-temperature superconductors, possessing by mi-

croplasticity [21]. Delay appearance is bound with pres-

ence of opposite internal stresses for decrease of which

time is required. The wave attenuation mechanism in the

rock with microplasticity (referring to the experiments

[10]) is theoretically substantiated also in work [22]. As

regards the mechanism of wave attenuation, the combi-

nation of known mechanisms is possible, for instance, as

the hybrid relaxation-hysteresis mechanism [12]. It must

not be ruled out also the acoustoplastic effect in the

rocks as in metals and alloys [23]. The instance of stress

jumps marked on seismic record evokes the great cau-

tion in interpretation of this effect, and therefore for the

time being, we do not examine this question.

4. CONCLUSIONS

The result of this work is the new knowledge about na-

ture of propagation of the mechanical oscillations in the

Earth. The unnoticed heretofore presence in seismic re-

cord of microplasticity manifestations was detected

thanks to the high-resolution signals measurements. The

quasi-static microplasticity of the rocks, dynamic mi-

croplasticity of many solid states denoted the prospect of

our search. The dynamic microplasticity in the rocks

definitely is bound with critical amplitudes but its de-

pendence on strain rate is not yet established. One may

surmise data about rock microplasticity will enlarge the

comprehension of some known effects that had not for-

merly of the satisfactory physical explanation. We see it

now in the delay effect of the arrival time and the pulse

shortening-widening.

As stated above, the nature of microplasticity in the

rocks can coincide with known mechanisms in the solid

states (for instance, dislocation microplasticity) or to be

quite other (quasi-microplasticity). In the last case in

spite of the difference in mechanism, the microplasticity

manifestations can be the same as in usual solid states.

This question requires subsequent clarification. Microplas-

ticity affects the little-known anelastic processes of the

small-amplitude wave propagation. The new knowledge

about nonlinear-anelastic processes during wave propa-

gation will help in discovery of new diagnostic indica-

tions permissive to increase the efficiency of seismic

method for search of oil-gas deposits.

5. ACKNOWLEDGMENT

The author thanks G.V. Egorov for the help in the experimental work.

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