Hunter, J.A.Pilon, B.J.Todd.V.S.Allen and P.J.B arnett, Shallow geophysics in a hydrogeological investigation o f the Oak Ridges Moraine, Ontario; in Proceedings of the Symposi[r]
Trang 1C A P A C I T I V E C O U P L E D A R R A Y - T H E N E W T Y P E O F S E N S O R F O R
R E C E I V I N G R E S I S T I V I T Y V A L U E S I N G E O P H Y S I C T E C H N I Q U E
N g u y e n D u e T a n
D epartm ent o f Physics, College o f Science, V N U
Abstract: G ro u n d resistivity conditions in a variety of geological environments
w e re m e a su re d u sin g a te chn iq ue th a t d o es not rely on g a lv a n ic c o n ta c t
be tw ee n th e s e n so rs an d th e terrain T h e c a p a c itive -co u p lin g m e th o d c o n v e rts a
c u rre n t ap p lie d th ro u g h o n e pa ir o f a n te nn a an d a volta g e p o te n tia l rece ive d
co rre sp o n d in g to e v e ry p a rticu la r c o n s ta n t K w h ich is a lso d e p e n d e n t u p o n th e
allo w s ta k in g a p p a re n t resistivity valu es o f a surve y B e cau se c o n ta c t re s ista n ce
p ro b le m s a re o b via te d , surve ys in ve ry resistive c o n d itio n s (> 1 0 0 0 0 o h m -m ) can
b e co n d u c te d [1 ] H ow e ver, it is g o od to u s e th e m eth o d in th e d iffic u lt a re a (for
e x a m p le , a b o g g y g ro u n d area, a froze n gro u n d are a o r c o n c re te d are a )
Be ca u se o f th a t, to fin d a good fo rm fo r th e a n te n n a a rra y s u sin g in th is m eth o d
is ne ed fu l T h is p a p e r w a n t to in tro d u ce th is m eth o d (CC R : ca p a c itiv e -c o u p lin g
a p p a re n t re s is tiv ity m e th o d ) and s o m e o u r e x p e rim e n ta l re su lts w ith th e new
fo rm s o f c a p a c itiv e co u p lin g arrays
1 E l e c tr o m a g n e t i c f ie ld s o f c u r r e n t s
The electrom agnetic field associated w ith th e c u rre n t in th e ground is not so easily understood intuitively If th e ground is m ade up of uniform layers th e re su ltin g fields
m easured on th e surface a r e independent of th e ground conductivity a n d so depend on the
m agnitude of th e c u rre n t an d th e geom etry of th e source c u rre n t line an d th e location
of th e sensor used If th e ground is inhom ogeneous th e sym m etry of the re s u lt is broken and anom alous fields ap p e ar which depend on th e varia tio n s in th e conductivity The
ap p a ren t resistivity is a function of th e frequency of th e applied cu rre n t T he way, the
sw itched DC c u rre n t an d th e altern atin g voltage is applied can re p re s e n t by th e equivalent circuit across the sam ple or betw een th e field electrodes w hich h a s th e form a s figure 1 At
DC the capacitor is a n open circuit an d all th e c u rre n t passes th rough th e sum of III and R2, developing a voltage Vp = I (R l + R2) As the frequency increases, c u r re n t can flow through the capacitor A t high frequency the cu rre n t is effectively s h o rt circuited by the capacitor and th e voltage developed is simply IR l (R l »
R2) B etw een th e DC an d high frequency lim its th e voltage
across the capacitor is p h ase shifted from th e c u rre n t an d so
adds an im aginary com ponent to the to tal voltage m easured
across the circuit T hus we see that th is circuit faith fu lly
reproduces the key features observed in measurem ents It is
im portant to note th a t th e interface im pedance acts like a
gate or valve for th e cu rren t B oth zw (total im pedance) and
the im pedance of c (= 1/coC) become large a t low frequency
M id
R
r % \
HI t
F ig l The equivalent circuit for alternating current
Trang 22 T h e a p p a r e n t r e s is t i v i ty v a lu e s
We can e stim ate th e resistivity of th e gound in one geological environm ent by
m ea su rem en t as figure 2 The electrode a rray for th is experim ent is four electrodes The
distances betw een th e s e electrodes are given by rIt r- 2 , r3 and r., T he a p p a re n t resistivity value (p) of this m ea su rem en t is p = K (AV/I) with K depend on the geom etry (r„ r- 2 , r3, r4)
of the tra n s m itte dipole (c u rre n t line) and the receive dipole (voltage line)
In th is m ethod we have given a microscopic model of the interface (fig.3) The model
w ith m any m odifications to accurately reflects some of th e substance reactions a t the interface o f the c u rre n t from CCR dipoles The app a ren t resistivity value (p) of the
m easurem ent w ith th e capacitive-coupling a rra y is given by th e sam e form ula p = K (AV/I)
H ere K also depend on th e geom etry of CCR dipoles
3 E x p e r i m e n t s a n d r e s u lt s
Fig.4a Flat capacitor dipole-dipole
Some form s of capacitors used for tran sm itte
dipole and receive dipole in o u r experim ents are
shown in Fig.4 In Fig 4a, a n angle of flats of the
capacitor is nam ed A The dependence of sensitive
of m easurem ent on th e values A is estim ated The
curve 5.a show s th e dépendance on A of flats of the
receive dipole T he cu rve 5.b shows th e dépendance
on A of flats of th e tra n s m itte dipole A selective
m icrovolm eter SE90V LF (m ade by SCINTREX Co.)
and an alte rn a tin g voltage g enerator (20 kHz) are
Fig.4b Cylinder capacitor dipole-dipole
Fig.5 The dependence of sensitive of measurement on the values A
Trang 3T aking th e m easurem ents with
dipole-dipole a rra y (L= 60 cm, see Fig
6) we obtain th e effect o f soil to V/1
value U sing m easurem ents w ith box
50cm) th e values V/I depend on the
distance r a s follow:
w ithout the box: V/I = 5.54 mV/mA
These m easurem ents a r e in the
sam e su rrounding an d th e n th e effect
of soil to V/1 value is very clearly
T he experim ent re s u lts on some
resistivity conditions in re a l geological
environm ents a re show in Fig.7 an d Fig.8
In Fig 7 the object b u ried underground is
a concreted tu n n el w ith a ir an d w ater In
Fig.8 two objects buried u nderground are a
(dimensions: 0.8m X 1.2m X 0.8m) and a
long tu n n el of a ir (dim ensions 0.4mx0.4m)
transmittadlpol*
Fig.6 The model of the measurements with dipole-dipole array
Fig.7 The experiment on Fig.8 The experiment on
An anom alous fields a p p e a r as th e dielectric changes strongly from th e soil en v ironm ent to the a ir environm ent T hese figures show th a t th e values V/1 depend on th e effect of a connect th e values V/I of th e m esurem ents to th e ground re sistiv ity conditions in a variety
of geological environm ent
C o n c lu s io n s
The re su lts of o ur experim ents show th a t th e effect of a dielectrically change of geological environm ent is very clearly T h a t is conditions for OCR m ethod T he obtainable
m esure in CCR m ethod N ext tim e our study will be continued on th ese subject an d on researching th e m ethod constructing geophysical im age from CCR data
Acknowledgements We would like to th an k V ietnam N ational P rogram of F u n dam ental
Research for financial support
R e fe re n c e s
1 S.E.Pullan, A.Pugin, L.D.Dyke, J.A Hunter, J.A.Pilon, B.J.Todd.V.S.Allen and P.J.B arnett,
Shallow geophysics in a hydrogeological investigation o f the Oak Ridges Moraine, Ontario;
in Proceedings of the Symposium on the Application of Geophysics to Engineering and Environmental Problems, (1994)
2 V.M Timofeev, A.w Rogozinski, J.A Hunter, and M Douma, A new ground resistivity