Rock bolt condition monitoring using ultrasonic guided waves

Transcription

1 33-44:ml Jul 4/3/09 8:18 AM g 95 Sy Rk bl d mg ug ul gudd wv by B.J. Buy*,. S. Hy* d.w. Lvdy Rk bl gy l u f h mg duy bu f flu h fy f mg. Gudd ul wv g f h df d wh hd k bl w vgd. Axymml d hdml f lm mdl w bul, f lly uld bl d h h f bl wh muld ll k. Exml bl w h lld g blk d yl w md f lm mdl f dff df. Eugg ul w bd f h mll xymml d h-dml f lm mdl, wll dug h xml vg. I mmdd h fw wh gy-bbg lm huld b ulzd d hgh fqu d lg bl. O h gy f mdl uh h h b blhd, h mdl uld l b ud ul wk f u mml qum dm h gy f h lld bl. Idu Sfy l u f h mg duy. I Suh Af udgud m gud fll d w d 2006 wh 85 l klld h d1. Muh hf fud h gy f h f f mg ul. O h blz h f wh hd k bl. h bl lly l d h fxd h f wh v h f fm llg. Sug h f wh k bl ly x d h quly d dy f h u dm h lf d lbly f h ul2. M h 16 mll k bl lld h y Suh Af m. Of h, 45 -hd bl3. h, hwv, vu d d wh h -hd k bl h mgh mm h fy. Exml lud wk k d d, whh my u ufy bdg bw h k d h k bl. ll du uh vg, ml mxg d lly uld bl l ud4. F, fm xmly hlf f h fully gud bl w d b ffv hy huld hv b5,6. h quly f ll d gug f -hd bl hf gdd u blm by h mg duy. Dmg k bl gy h b ddd by vu gu ud h wld. O f h vl h h u h ull-u. Hwv, h m-umg d duv. A d hf dvl duv h uld b ud dm h d f h bl u. h GRANI ym dvld Sld dv, whh lld l xl mul h md h bl. h vb gl g fm h mul mud by lm d h m dv. h l gl g fm h mul d by ul wk dm h d f h bl7. h ddvg f h GRANI ym h wk ly h k bl h hv b hzd ll. Rh ud ugg h ul g hld h m m duv hqu5. h Blm, whh w dvld Swd, bd h l d b ud gud bl wll ly d m h gu. h Blm u wh zl yl d m ml d flxul wv h bl. h * Dym Sym Gu, Dm f Mhl d Aul Egg, Uvy f. S S d hlgy, Ml S d Mufug, CSIR. h Suh Af Iu f Mg d Mllugy, SA ISSN X/ vd Ob 2008; vd vd Fbuy h Jul f h Suh Af Iu f Mg d Mllugy VOLUME 108 REFEREED AER FEBRUARY

2 33-44:ml Jul 4/3/09 8:18 AM g 96 Rk bl d mg ug ul gudd wv wv g hugh h bl, fl h bl d d h h vd by h. I l h l h gd gug wll bb m ll f h wv gy h k, lvg ly mll h h h, wh uff gug wll ul d h. h Blm d bd gug, bu f h md f h gu d uudg k h m, wv gy mgh d h k bf uld h mj df, gllg gd gug8. M ly h -duv g f k bl ug gudd ul wv h b d fbl h dg df9. Gudd wv g f h ul wv h g ld md wh bud. h ul wv x fl d f wh h budy f h ld, whh u md v bw m d h wv10. hf, dff gudd wv md u yldl ld. Eh f h md h ul wv uu. h wv uu db h dbu f l m h yldl ld. Sm md hv lg l m mlud h uf, wh h fu m m h mddl f h yld. h wv uu dm h vy f h ul md ul flw y. Smm my b bl fm ug ly gl md. O mhd f gg mly gl md mly Gu-wdwd, bu x. h bdwdh f h gl hu dud d wll u md h d f h dm fquy h fquy um11. h h d by Bd d Lw9 l h fllwg: h du Gu wdwd bu ud x gudd wv h bl fm h f d. h wv h fld fm h h d d fm y mj df. Fm h fl vl m d kwldg f h wv vly d uv, h f h df h bl lgh b luld. h mxmum g lmd by h x f h u h h wv x g. h mj u f u f h mbddd k bl h ul gy d lk fm h bl h uudg k. Wh gudd wv, h quly f h gug d h l f h df b dmd. Sf md b ld, whh l v dff md h h Blm. Bd d Lw9 vgd h ff f f guddwv md, fqu d x d dfy h ml gudd wv md f dg df. Subl md d fqu w ld by m f lylly bd d uv, luld f ml yld. h uh blhd h mbddd bl huld b d by xg h f xlly ymml lgudl md kw h L(0,1) md, lw-fquy g (30 70 khz), d h f -xlly ymml lgudl md kw h L(1,1) md, hghfquy g (2 5 MHz). h lw-fquy b ud dfy df uh l bl ul d bly h h bl uf. By, h hgh-fquy v uf df, bu gv lbl d f h bl lgh9. h udy by Bd d Lw9 fud mly h df f ubl md, whh gfly flud by h mbddg k mdulu, h xy hk d mdulu, d h bl-xy bd quly. Owg h lm f h lyl mdl, w bl mul mlx df. dl wh m mld df d gm, h f lm (FE) mhd mmly ud mdl wv g. h my dvg f h FE mhd h h umu mml FE d vlbl, hu lmg y d dvl lzd d. Fuhm h f lm mhd h h dvg h h dfful d wh mlx gm d df ml hdl. I vlbly, u-fdl d h hd - d -g, ugg h f lm mhd vl mdllg h f k bl df mul. Dff df uh ll kg d lly uld bl b mdlld. h udy xd h wk by Bd d Lw9 vg dmg m l mbddd bl, whh dv fm u yld, ug h gudd wv md d fqu uggd by Bd d Lw. h h b d by ug h FE mdl f vu df, whh muld d md xml mum bl. m d wh h dff FE df uld b md xml m, whh l dguh h udy fm h l lyl h. h udy w lmd lw fqu bu f mu u lm. B l d mdllg u uld, hwv, b ddd d my b xd h h l uld fuu b xdd hgh fqu by ug w g f mu. O h gy f mdl uh h hv b blhd, hy uld l b ud ul wk f u mml qum. Exly xml vg dvl uddg f h b hy d h g mhd vlvd, h g f gudd wv bl w f vgd xmlly ubudd bl d fly mbddd bl. h w fmd ug 18-mm dm zl m du mufud by Ml S d Mufug, CSIR (Suh Af), md h f d f h bl. h du w 1-3 zm d, whh d f gully d qu ll f zl m ml ud xy mx. h fl uf f h d w vd wh lv lg d w w ldd h uf. A Nl Ium CI 6110 d qu d w ud l h du. h mxmum ml h u hl w 5 MHz. ylly l 20 ml d dd f gd f h gl mlud qud. hf, h mxmum fquy f w lmd 250 khz. h mxmum uu vlg f h d w lmd ± 10 V, whh d mlf. h A-303 z Amlf fm AA Lb Sym w ud. h mlf hgh-vlg, hgh-d l z dv. h mlf bl f 96 FEBRUARY 2009 VOLUME 108 REFEREED AER h Jul f h Suh Af Iu f Mg d Mllugy

3 33-44:ml Jul 4/3/09 8:18 AM g 97 Rk bl d mg ug ul gudd wv dvg u ± 200 V 200 ma fqu fm DC 450 khz. h d qu d w dv by Mlb D Aqu lbx. Mlb gm w w g dff uu fquy ul d mu h m f vl. h L(0,1) md h lw fquy g (30 70 khz) w xd by lyg Gu wdwd bu vlg h du. Ulzg umb f yl h bu du h bdwdh d ul l fl bu h dff fquy m f h wv vl dff d. h kw d. h ff bm wh xmg h d uv (gu vly v. fquy l). h gu vly h d whh k f wv vl hugh ld. h gu vly uv fml uv f gudd wv wvgud, d h g h lyl wv g qu. A d-ff udy w dud dm h m umb f yl. I w fud h fw h x yl ud h flg gl bm dd d m h x yl mgh l ly fl. I w l h h ml umb f yl f h 1.5-m bl wh g fquy g f khz, wuld b 6-yl ul. h uld mxmum x m f xmly 0.2 m ddg h fquy. Aumg h d f h wv k l b f h d f m/, h df l h f d f h mbddd bl h b dd 0.5 m. h du w d l u dgd ulz h du mululy m d v. h uu f h d qu d w d h mlf, whh u w d h u f h u. h l f fllw: wh h vlg gl by h d qu d, mlfd by h mlf bf xg h du. h uu d f h u d h u f h d qu d. A h m wh h du xd, h uu d f h u l h hgh vlg gl bu dd l h gl ± 0.2 V h u f h d qu d. h fl fm h mbddd d u f fw mlld d x h du h du vlg gl. h vlg f h fl mll h 0.2 V d h gl b h uu d f h u. h d dd w mzd du h d xd h vy m f h u f x. Wh mg h du, w md lg h d x wh h bl x. h glu w vly dbud v h uf f h du. h w y v -hmgu bdg h uf f h bl, d v h du fm bdg d flxul md fm bg xd. h mu f glu l m: h m glu h uf h g h gy l. Ay gy l uld b ffdd f h mbddd bl. I dfful fd h lh bw vlg d dlm bu h x mlud f h f md h xml bl b ld h x vlg, h quly f h glu bd bw h du d h bl ff. Fuh h h du d ulg h bl uld b xd mv h bly f h xml ul. Ubudd bl A 1.5-m lg mld l bl wh dm f 20 mm w udd h g ubudd fgu. h mud vlg m f 40-kHz Gu wdwd x hw Fgu 1. h mlud f h x gl muh hgh h h mlud f h flg gl. h mlud f h x gl w dlbly ld bu h vg d f h u l h mlfd uu gl fm h mlf bf ld h du. h ud by h mlfd x gl wh h um flu h d f h df l h f d f h bl, whh b dd ug lw-fquy gudd wv. h fl fm h mbddd d f h bl b lly. h gu vl f h ubudd k bl mdl b md h fml hmm-ch fquy qu12 whh w flly dvd f umfl wvgud vuum. h d f gu f wv 40 khz, dmd fm h hmm-ch fquy qu, ud m/, h h f L (0,1) d fl huld u 0.6 m. h h f h 40 khz m Fgu 1. Ah fl b bw h f L(0,1) d fl d h d L(0,1) fl. h d f h f flxul md (l kw h F(1,1) md) fquy f 40 khz m/. h f d fl f h F(1,1) md huld u 0.97 m whh mh h fl Fgu 1. Mlgm f h du -hmgu bdg f h du h bl my b h h h F(1,1) md. h hl u f h F(1,1) md xmly 50 db/m m 40 khz h f h L(0,1) md. h d d fl wll hf b vbl h m f h u ff m f 1.5 m hw Fgu 1. I vd fm h Fgu 1 h h vl m mud m wll h hl vlu f h vl f h hhmm-ch fquy qu gu. h v h h xml u d FE h ulzd h udy huld b dqu xl m mlx df. Embddd bl Fllwg h xml f Bd d Lw9 blk w mul h ff f f k m h mbddd xml bl. h dd Fgu 2. h d f h blk f m mul h d h l d lum m Suh Af. h u g f ggg mx w dd h lkg wv v hm fm fg wh h wv gg h bl. h z f h m blk w h u h dg fl f h blk wuld f wh h d fl f h bl. l h du f fl bdg df uly d xm wh dff df, flud y f w ugh, wh lw m h h ud ul m. Fbgl w h Jul f h Suh Af Iu f Mg d Mllugy VOLUME 108 REFEREED AER FEBRUARY

4 33-44:ml Jul 4/3/09 8:18 AM g 98 Rk bl d mg ug ul gudd wv Fgu 1 Mud m f 40 khz Gu wdwd bu x f h 1.5-m ubudd bl dud bdg df. M f h -hd k bl lld h Suh Af l d lum m bw 0.9 d 1.5 m lgh3. fly mbddd bl, 0.9 m lg mld l bl wh dm f 20 mm w lld h g blk. h g w bw 4 d 5 mm. h L(0,1) md w g xd h bl ug Gu wdwd bu. h f d' fl f h L(0,1) md ud 0.4 m hw Fgu 3. I m h ubudd bl dbd gh 2.1, h mlud f h flg gl f h mbddd bl muh mll h h flg gl f h ubudd bl. h du h lkg f h wv h uudg k. Hwv, h d fl uld b lly mud f h ml ubudd d mbddd bl fgu dd h. I vd h h xml -u dqu xl h ff f m mlx df. Fgu 2 Exml g blk vully ld d f ml k bl, bu h blly h m u h k bl ud h mg duy. h vm h f m f ml k bl d llwd uff m f ly llg h flly F lm mdllg f gudd wv Gudd wv g k bl uld b mdlld by ug full h-dml mdl xymml mdl. Aumg h xl ymmy f h gmy, ld, budy d d ml, h xymm mdl llw ly h h-dml bl d uudg k m wh w-dml mdl. Fgu 3 Mud m f 40-kHz Gu-wdwd bu x f h 0.9 m mbddd bl 98 FEBRUARY 2009 VOLUME 108 REFEREED AER h Jul f h Suh Af Iu f Mg d Mllugy

5 33-44:ml Jul 4/3/09 8:18 AM g 99 Rk bl d mg ug ul gudd wv Hwv, df u whh ymm ud h x. I uh h-dml mdl f h k bl wll b y. Chg g m d lm z l h u f h FE lu. I gl, h uy f h mdl b d by gly mll g m. Wh h m lg, h hgh fquy m dquly lvd. By, m whh mll w f lul m. h z f h lm h uh wy h h gg wv lly lvd. Suggd m vl d lm z f ml g hm fllw:13 m z Elm z [1] [2] wh f mx h mxmum fquy f, d L m h mmum wvlgh vlvd. h MSC.Sfw kg, mg MSC., MSC.N d MSC.Dy, w ud wh 3.2 GHz um 4 mu d 3 GB f RAM. h llw xl, wll ml lv. h ff f mh dy d m z f h ml d xl lv ud w vgd. Dff mdl f h ubudd bl w bul wh dff mh d d m z. h gu vly f h f h mdl w dmd d md. I uld b ludd fm h vg h f h ml lv, h m z m l h h lm z, wh f h xl lv, h lm z m l h h m z. Dmg m f wh dg ly. I d, hwv, ml h qu f m d dlly h m qud lv h ym f qu. S Bd d Lw9 h h x f h mlud du ud by dmg muh mll h h mlud du ud by lkg h mbddd bl, h udy xludd dmg llw f h d f lg mdl. Hwv, xludg dmg h FE mdl hb fm dyg m g ( wll l b wh mg FE h mud ul). Ivg f h ff f dmg h f lm mdl mmdd f fuu h. Ah mdllg u whh hd b ddd w h budy f h FE mdl. h f budy f h FE mdl f h k m u h lkg wv b fld d umd h gg wv h bl. h blm b vm by ug gybbg bud. h y f lm, hwv, ly vlbl lzd wv g FE kg. h h ud h udy w hf mv h budy lg d wy fm h bl h h budy wuld flu h ul14. h, hwv, u h mdl bm vy lg d quly m umg lv, lmg h udy h lwfquy whh uld mll blm h h h hgh-fquy. Bu, d bf, h b l d mdllg u b udd by dg h lw-fquy. I my b xd h w g f mu wll mk lly fbl mdl h hgh-fquy. Ubudd bl vfy h FE mdl d udd h l f gudd wv mdllg, h f udy udk w mdl g ubudd k bl. h k bl mdl w xymm mdl wh dm f 10 mm mm g h 1.5-m lg, 20-mm dm k bl dd l. h mdl hw Fgu 4. I lw-fquy bd h L(0,1) md h xl dlm fl whh mk bl x h dg f h mdl. A m-vyg u (Gu wdwd bu) w ld d f h mdl. h lm z d m z w luld dg Equ [1] d [2]. F xml, h vly f k f wv 40 khz ud m/ d h mdl huld hv 5 000/( ) = 6.25 mm xymm RIA 6 lm d h m z huld b 1/( ) = 1.25 µ. h f d' fl wll u 3/5000=0.6 m. h f d d d fl lly, h l m huld b 1.5 m. hf wh m z f 1.25 µ l umb f m huld b ud. h ml ud f h mld l hughu h udy w: Yug mdulu f 206 G, f 0.3 d dy f kg/m³. Fgu 5 d h dlm m f h xymm mdl f h ubudd bl. I h h flg gl h FE mdl hv dubl h mlud f h x gl, bu h dg d h gg gl wll h ugg gl. h F(1,1) md b mdlld wh xymm mdl. h k f h wv k b ud lul h vly f k, whh l kw h gu vly. h x du h wy h h gu vly uv f h ubudd k bl xymm mdl w luld d md h ul fm h xm h ubudd bl. Cm f gu vly uv f ubudd bl h vly f gg wv f h m m m ul g. I bulk wv Fgu 4 R f xymm mdl f ubudd bl h Jul f h Suh Af Iu f Mg d Mllugy VOLUME 108 REFEREED AER FEBRUARY

6 33-44:ml Jul 4/3/09 8:18 AM g 100 Rk bl d mg ug ul gudd wv Fgu 5 Cmud d mud m f 40-kHz Gu wdwd bu x f 1.5-m bl, bu gudd wv hg h fquy d hk hg d b d d uv, wh h gu vly ld g h fquy15. dm h gu vly fm m hw Fgu 5, h m f vl f fl huld b mud fm h k f h f wv k h k f h x wv k. h w d f Gu bu v g f fqu xg h ubudd f lm k bl mdl. Dff f lm mh d m z w ud mmdd by Equ [1] d [2] f h dff fquy bu. h ul bd fm h ubudd xml bl dud bv w ld wh h ul fm h FE mdl d umd h hmm-ch fquy qu. I l fm Fgu 6 h bh h xml d FE ul g wll wh h hmm- Ch fquy qu uv. h mxmum dff 1.2, dmg h l d uy f h f lm mhd wll h xml -u f mulg m mlx df. Df w m u f hg f k bl h l ul ud by ll d h k fmd by ll. h mlx, whh dv fm u yld d ml gm, b llly mdlld ly by f lm. Axymm mdl d h-dml mdl w bul d md h xml bl lld h g blk. A fly lld bl w l muld f u m d f. Smulg fully uld k bl Fgu 6 Gu vly uv f h FE mdl, xml bl d h hhmm-ch fquy qu A xymm mdl w bul f 0.9-m lg 20 mm dm bl, 5-mm bdg d 1-m hk m blk. h dm f h mdl hw Fgu 7. h z f h m blk h FE mdl w h u h dg fl f h m blk wuld f wh h d fl f h bl. h khz m-vyg u ul w ld h d f h bl. Axymm RIA 6 lm f 5 mm z w ud. I w luld h flg wv wuld u m. Ug h mmd 100 FEBRUARY 2009 VOLUME 108 REFEREED AER h Jul f h Suh Af Iu f Mg d Mllugy

7 33-44:ml Jul 4/3/09 8:18 AM g 101 Rk bl d mg ug ul gudd wv Fgu 7 Dl f h xymml mdl f h fully uld k bl dud bv, h m w 1 µ ulg 650 m. h ml ud f h, d m hughu h udy w: Yug mdulu f 10 G d 20 G, f 0.3 d 0.25 d dy f 1200 d 2500 kg/m³ vly. h ul bd f h FE mdl wll h xml bl f 40-, 50- d 60-kHz ul hw Fgu 8 10 vly. F 40-kHz x (Fgu 8) h f d fl ud 0.4 m. I bl h h xml bl d h FE mdl hw gd gm m f dfyg h m f h f d' fl. Fuhm why h h FE h h xml ul. h du gld dmg, dud l. Gd gm w l fud f 50 khz (Fgu 9). h u f h gl w lw h h 40-kHz. F h 60-kHz ul (Fgu 10) h flg mlud w v lg h h 40 d 50-kHz ul. Au du lkg h m blk hf lw h hgh fqu, h fqu h gy vl m h f h bl. F 60-kHz ul h xml bl hw gf vb f x. h du h gg f h du h du ly lghly dmd. h gg w l fud by Bd d Lw h udy9. I m hugh 50-kHz ul ml h bw h ff f gy lkg d du gg. lly uld k bl l ul f bl hd l b ud by m mxg f h d ly, wh h qud m bvd. Udg ul dqu mxg wh vg dub h lly ud. Im mxg l u wh lg bl wh h f h hl h hd l m mx bf h bm. Im hl h d, h, lg mh wll l ul lly uld bl. h vg dd m hl wh ly h d f h bl w hd by h. h FE mdl f h lly uld bl hw Fgu 11. A xymm mdl wll hdml mdl w bul. h h-dml mdl w bul vg h bly f mdllg xymm df. F h xymm mdl, h bl, bdg d m blk w mhd wh 5-mm RIA 6 lm. h 40-kHz 60-kHz u ul w ld h f d f h bl. h m z w 1 µ d h l umb f m 800. I k MSC.N xmly hlf hu lv h blm. h h-dml mdl f h bl w bul wh 5- mm HEX 8 lm. h w l mdlld wh 5-mm HEX 8 lm. h m blk w mdlld wh 10-mm HEX 8 lm. Wh h fgu, h mdl hd xmly 1.2 mll lm. h lg lm w y du h mdl mk lvbl. h m z w 0.1 µ d h l ly m 0.75 m. h blm w lvd wh MSC.Dy, xl lv. I k MSC.Dy xmly 4 hu lv h blm. h ul f h xml bl, xymm d h-dml mdl d Fgu Fgu 8 Mud d mud m f 40-kHz Gu wdwd bu x f h 0.9 m fully uld bl h Jul f h Suh Af Iu f Mg d Mllugy VOLUME 108 REFEREED AER FEBRUARY

8 33-44:ml Jul 4/3/09 8:18 AM g 102 Rk bl d mg ug ul gudd wv Fgu 9 Mud d mud m f 50-kHz Gu wdwd bu x f h 0.9 m fully uld bl Fgu 10 Mud d mud m f 60-kHz Gu wdwd bu x f h 0.9 m fully uld bl h fl fm h f h ul b lly f h xymm wll h hdml mdl f 40-, 50- d 60-kHz x ul. Hwv, h h-dml mdl h fld gl bm hd u m f h hgh fqu. A hgh fqu h vly f h ul h u l fl fm h budy f h m blk. h fl f wh h fl fm h f h ul, ug h hd gl. F h hgh fquy x, h fl fm h f h ul mll h h f h bl d. h llu h m gy g h f h bl hgh fqu d m gy g h bl uf lw Fgu 11 Dl f h mdl f lly uld k bl 102 FEBRUARY 2009 VOLUME 108 REFEREED AER h Jul f h Suh Af Iu f Mg d Mllugy

9 33-44:ml Jul 4/3/09 8:18 AM g 103 Rk bl d mg ug ul gudd wv fqu. h fl fm h f h ul f h xml bl l dm h gy vl m h f h bl hgh fqu. h dff bw h f lm mdl d h xml bl h h fl fm h f h ul muh mll f h xml bl. A xl h m gy dd h xml bl wg lkg, whh uld b xly mdlld by h FE mdl bu f h u f h ml. Fm h bv ul, ludd h h gl gfly flud by df wh h fquy f h gl hgh, bu h gy g m h f h bl. Ll kg Ll kg fm ggv ml w g hugh mll k h k d, dug ml l fd f h xd bl uf. Ul gudd wv bu w ud vg dff ful dm h vy f k. h k w vgd: A kd bl A k f h d k A k h bl, d k. h dm f h mdl hw Fgu 15. Oly h xymm mdl w ud, h full hdml mdl w lg lv h mu vlbl. h ul f 50-kHz mdl hw Fgu 16. h fl fm h k f kd bl b lly Fgu 16(). h d-fl f h bl l lghly vbl. M f h gy fld bk fm Fgu 12 Mud d mud m f 40-kHz Gu wdwd bu x f h lly uld bl Fgu 13 Mud d mud m f 50-kHz Gu wdwd bu x f h lly uld bl h Jul f h Suh Af Iu f Mg d Mllugy VOLUME 108 REFEREED AER FEBRUARY

10 33-44:ml Jul 4/3/09 12:44 M g 104 Rk bl d mg ug ul gudd wv Fgu 14 Mud d mud m f 60-kHz Gu wdwd bu x f h lly uld bl Fgu 15 F lm mdl: dm f h k h k, whh b h ul md bv, mly h h gy vl m h f h bl f h L(0,1) md. F h k h d m hw Fgu 16(b), h mlud f h f fl mll h h fl f h kd bl. h hw h h flu f h vy lg f h lw fquy L(0,1) md. I h w, hw Fgu 16() wh h bl, d m hv k, h mlud f h fl fm h k h bl lg h h fm h k h m d. h g fm h h gy vl m h f h bl h fqu. I lly bl d muld ll k wh h f lm mdl. Cl fl b fm h k h bl. If h bl, d k kd, dff fl bvd. h dff fl ml h f h ul. Ovlg f ug gl l fuh ml h f h ul. I hf mmdd h m hd gl g hqu huld b ulzd f fuu ud. F, kwg h h f h xg gl uld hl w vlg fl. Fgu 16 Cmud m f 50-kHz Gu wdwd bu x f k (), (b) d () 104 FEBRUARY 2009 VOLUME 108 REFEREED AER h Jul f h Suh Af Iu f Mg d Mllugy

11 33-44:ml Jul 4/3/09 8:18 AM g 105 Rk bl d mg ug ul gudd wv Clu d mmd h u f gudd ul wv f h u df f k bl d m hld m g mhd, mbl ul uld b bd fm h xml d f lm mdl dd h udy. h L(0,1) md lw fquy g w ulzd d ld, d by Bd d Lw9, vg h dff ful h u -hd k bl. g fudml uddg f h bv g mhd, ubudd bl d fly mbddd bl w ud h b f. h ul bd fm xymml mdl f ubudd bl m wll wh h fm h ubudd xml bl. h gu vly uv f h mdl m wll wh h gu vly uv f h hmm-ch fquy qu f d, hwg h l f h mhd. w df w h dd. h f w lly uld bl. h FE mdl f h bl m wll h xml ul. F h f lm mdl, h mlud f h fl fm h f h ul mll f h hgh fquy gl h f h lw fquy gl. I my b ludd h hgh fqu, h gy g m lg h f h bl; quly h hgh fquy md wll b l v l ul. I mmdd h lw fqu huld b ud wh df uh h hv b dfd. I h l h h fquy f mzd fl. Wh g hugh h fqu ul fl fm h d f h bl uld b bd, uld bbly l h b fquy f dg lly uld bl. Hwv, h huld b fuh vgd. h d y f df vgd w muld ll kg. I bl d h muld ll k wh h f lm mdl. Cl fl b fm h k h bl. If h bl, d k ll kd, dff fl bvd, mlg h f ul. Lkw, vlg f ug gl l bu mlg h f ul. I hf mmdd h du b gv gl g u fuu ud f h u. F, kwg h h f h x gl uld hl w vlg fl. Fuhm, h wh h ll df w muld, ly vy ml k w dd. Fuh vg huld d h, uh ld d mull k. h udy dd ly h m f vl m f lg h muld df. Hwv, h mlud f h flg gl my l fm bu h u d l f df. h huld b dd fuu ud, gh wh h ff f dmg wll h ff f du mug h bvd. B l d mdllg u w ddd d b xd h h b dvld h uld b xdd hgh fqu wh w g f mu by lyg gy-bbg bud. h bf f ug hgh fqu h h hgh fquy md l v ml, xy hk d uf df. Fuhm h hgh fquy md b ud b lbl d f h bl' lgh. If h bl' lgh kw, h d fl b d fm h h fl, mlfyg h f h mld fl f h lw fquy. O h gy f mdl uh h h b blhd, h mdl uld l b ud ul wk f u mml qum. Akwldgm h uh gfully kwldg h fl u f h Suh Af M Hlh d Sfy Cul h xu f h h. Rf 1. SOUH AFRICAN DEARMEN OF MINERALS AND ENERGY M Hlh d Sfy I. Aul R h:// Ad ROBERS, D.. g f mg ul u lm d ym f hd k m. M' d. Nl: Uvy f Nl BORNMAN, H. f Du, Suh Af k bl ul. Suh Af k bl um. Uublhd -ml Buy B.J. (25 Al 2005). 4. MARK, C., COMON, C.S., OYLER, D.C. d DOLINAR, D.R. Ahg ull g f fully gud f bl. dg, 21 Il Gud Cl Mg Cf, Mgw, WV, Augu 6-8, 2002, KELLY, A.M. d JAGER, A.J. Clly vlu hqu f h u g f l d gug ffv b f dug fll f gud ju d fl. R ubmd SIMRAC, R N. GA205, Ob HARER, G.S. d BASSON, E. Dvl w mhdlgy f dmg h ffv f gud u. R ubmd SIMRAC, R N. GA412, Juy SARKEY, A., IVANOVIC, A., NEILSON, R. d RODGER, A. Ug lumd m dym mdl f k bl du g d f ul wk f dg f l d. M, vl. 38, 2003, HURNER, H.F. Rk blg Rf bk, AMFO Fl R. ld fm h Swdh by BC Bu d A, Cuy f CSIR. Jhbug (Adx f Klly A.M., Jg A.J.), Clly vlu hqu f h u g f l d gug ffv b f dug fll f gud ju d fl, BEARD, M.D. d LOWE, M.J.S. N-duv g f k bl ug gudd ul wv. Il Jul f Rk Mh & Mg S, vl. 40, 2003, ROSE, J.L. Ul wv ld md. Cmbdg. Cmbdg Uvy, CLIFFORD, B. Ul f gy g f kbl m. C Rh R N. 3792/R d by Rk Mh hlgy Ld f h Hlh d Sfy Exuv, SECO, F., MARÍN, J.M., JIMÉNEZ, A., ONS, J.L., CALDERÓN, L. d CERES, R. Cd: A l f h mul f wv g yldl wvgud. 9h Il Cg Sud d Vb. Old, Fld, MOSER, F., JACOBS, L.J. d QU, J. Mdlg l wv g wvgud wh h f lm mhd. ND&E Il, 1999, vl. 32, ROSS, M. Mdlg mhd f l bud f md. Fluduu gg. Uvy f Cld Buld, SASZEWSKI, W.J., BOLLER, C. d OMLINSON, G.R. Hlh mg f uu. Jh Wly & S, Chh, UK, h Jul f h Suh Af Iu f Mg d Mllugy VOLUME 108 REFEREED AER FEBRUARY