Seismic wave propagation through surface basalts – implications for coal seismic surveys
Weijia Sun1,2,4Binzhong Zhou1Peter Hatherly3Li-Yun Fu2
1CSIRO Exploration and Mining, Kenmore PO Box 883, QLD 4069, Australia.
2Institute of Geology and Geophysics, Chinese Academy of Sciences, PO Box 9825, Beijing 100029, China.
3Coalbed Geoscience Pty Ltd, 10 Waiwera Street, McMahons Point, NSW 2060, Australia.
4Corresponding author. Email: [email protected]
Abstract. Seismic reflection surveying is one of the most widely used and effective techniques for coal seam structure delineation and risk mitigation for underground longwall mining. However, the ability of the method can be compromised by the presence of volcanic cover. This problem arises within parts of the Bowen and Sydney Basins of Australia and seismic surveying can be unsuccessful. As a consequence, such areas are less attractive for coal mining. Techniques to improve the success of seismic surveying over basaltflows are needed.
In this paper, we use elastic wave-equation-based forward modelling techniques to investigate the effects and characteristics of seismic wave propagation under different settings involving changes in basalt properties, its thickness, lateral extent, relative position to the shot position and various forms of inhomogeneity. The modelling results suggests that:
1) basalts with high impedance contrasts and multipleflows generate strong multiples and weak reflectors; 2) thin basalts have less effect than thick basalts; 3) partial basalt cover has less effect than full basalt cover; 4) low frequency seismic waves (especially at large offsets) have better penetration through the basalt than high frequency waves; and 5) the deeper the coal seams are below basalts of limited extent, the less influence the basalts will have on the wave propagation. In addition to providing insights into the issues that arise when seismic surveying under basalts, these observations suggest that careful management of seismic noise and the acquisition of long-offset seismic data with low-frequency geophones have the potential to improve the seismic results.
Key words: coal seam, finite difference, forward modelling, inhomogeneity, long-offset recording, low frequencies, sub-basalt imaging.
Introduction
Tertiary volcanic basalts (high velocity layers) exist in both the Bowen and Sydney Basins of Eastern Australia. The use of conventional seismic reflection surveys for coal mine planning is often compromised and produces sub-surface images of variable quality because the wave propagation through these heterogeneous basalt layers becomes complex. The difficulty of exploring beneath these basalts makes these areas less attractive for coal mine exploration. Problems with sub-basalt imaging are not unique to the coal mining industry. In the petroleum industry, the imaging difficulty in the areas of basalt cover around the world is well known (Papworth, 1985; Samson et al., 1995; Fruehn et al., 2001).
The subject of sub-basalt imaging is a very popular topic in the research and exploration communities. Two special issues on sub-basalt imaging have been published by Geophysical Prospecting (Williamson, 2003; Christie and White, 2008).
These two issues resulted from a workshop entitled ‘Sub- basalt imaging: exploiting the whole wavefield’ held at Cambridge University in 2002 and a dedicated session to sub- basalt imaging at the 2005 EAGE annual conference in Madrid.
Workers involved in sub-basalt imaging in the petroleum sector, mainly give the following reasons for the difficulties in imaging beneath high-speed surface layers:
1. The strong impedance contrast between the basalts and the underlying sedimentary rocks prevents the penetration of
seismic energy into target zones (Fruehn et al., 1998;
Behera, 2006).
2. The roughness of the basalt boundaries causes significant disruption and scattering of the transmitted wavefield (Behera, 2006).
3. The scattering due to heterogeneity of the basalt (Ziolkowski et al., 2001; Hobbs, 2002).
To improve seismic imaging in these cases, modifications to data acquisition and data processing procedures which improve data quality and increase signal-to-noise ratio (S/N) have been proposed (Fliedner and White, 1999; Hu et al., 2003). Commonly used techniques involve long offset, low frequency and converted wave data acquisition (Ryu, 1997; Wombell et al., 1999;
Hanssen et al., 2003; Lau et al., 2007; Spitzer et al., 2008). In data processing, special procedures such as pre-stack depth migration (PSDM) have improved the quality of sub-basalt images (Fruehn et al., 1999; Reshef et al., 2003; Gallagher and Dromgoole, 2008). However, there is no single method or processing approach which is robust in all geological settings.
Successful imaging techniques have yet to be developed, despite the numerous efforts that have been made.
Wave-equation modelling techniques are often used to study wave propagation in different situations and have been used to investigate the influence of high speed layers on seismic wave propagation (Battig and Hearn, 2001; Hanssen et al., 2003). These studies mainly concern petroleum exploration. In this paper, we
Ó ASEG/SEGJ/KSEG 2010 10.1071/EG09015 0812-3985/10/010001
investigate coal mining contexts and use a 2Dfinite-difference- based full elastic wave-equation forward modelling algorithm to further study the effects and characteristics of seismic wave propagation under different basalt settings.
Elastic wavefield modelling
To investigate the characteristics of seismic wave propagation beneath basalts, a 2D 4th-order staggered-gridfinite-difference (FD) modelling scheme based on Virieux (1986) has been implemented to simulate elastic P-SV wave propagation in 2D heterogeneous media. The advantage of the staggered-grid scheme is its superior computation accuracy compared to the conventional central FD operators. A perfectly-matched-layer absorbing boundary condition (Collino and Tsogka, 2001), which is the most effective algorithm, is incorporated into our FD scheme to reduce spurious boundary reflections. These boundary reflections are caused by a lack of information on the wavefield outside the calculation area when the spatial differentiators operate on grid points towards the edge of the model. In addition, an accurate and stable implementation of the elastic-free surface condition, which requires the stress at the surface to vanish, is applied to the staggered-grid modelling scheme (Mittet, 2002).
In our implementation, the modelling algorithm does not consider wave absorption. However, this should not affect our results as we are mainly concerned with the relative illumination of the target zones. For the source, a Ricker wavelet with a central frequency of 100 Hz is used. Both the horizontal and vertical space sampling intervals are 1 m. The origin of the horizontal coordinate x is at the left edge of the FD grid model. The source is located at the ground surface, 100 m from the left of the model.
Results are mainly analysed in the form of (i) vertical component synthetic seismograms for receivers at the ground surface and (ii) as the maximum amplitude of thefirst arrivals (direct P-wave) at receivers located beneath the basalt (z = 180 m).
The latter were measured to provide an indication of the amount of seismic energy transmitted through the basalt for the various situations we chose to investigate. Energy in this context is given by the maximum peak amplitude of the direct arrival.
To illustrate the nature of the synthetic seismograms, a simple model in Figure 1 with a layer of basalt at 40 m depth and with a thickness of 40 m is used as a base model in the paper. Figure 2 presents vertical component seismograms from both surface and vertical seismic profile (VSP) records for this model. The seismograms show the expected direct waves, refracted waves and reflections (P- and SV-waves) from the coal seam.
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Fig. 2. Vertical component synthetic seismogram for the model shown in Figure 1, which has a 40 m thick layer of basalt with its top at a depth of 40 m:
(a) The surface reflection seismic record; (b) VSP seismograms recorded from the borehole receivers located 100 m away from the shot point. These seismic records display the expected direct and reflected events with reflections from the top and bottom of the basalt and a P-wave reflection from the coal seam at 0.16 s (zero offset) followed by multiples.
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Fig. 1. A model with a uniform basalt layer 40 m thick and a line of receivers at 180 m depth. The top of the basalt layer is at 40 m depth.
Reflections from the top and bottom of the basalt are also evident but these are mainly masked by the direct arrivals. The shot noise that often dominates the near offset traces from explosive sources is absent and the ground roll (P-SV surface waves) is not particularly strong, presumably due to the thickness of the surface layer. It is also interesting to note the cycle skipping (shingling) evident in the refractedfirst arrivals. Such behaviour is to be expected when velocity inversions occur.
Effect of basalt velocity
Thefirst set of models involves the same basalt layer as shown in Figure 1 and with the P- and S-wave velocities of the basalt varying between those of the surface layer and a layer of twice that velocity. The transmitted direct waves are measured using the receivers at a depth of 180 m. The maximum peak amplitudes of the direct arrivals from these receivers are plotted in Figure 3, where it can be seen that when velocity contrasts between the basalt and the surrounding rocks are moderate, for example the velocity ratio is less than 1.6 (or for the case of weathered basalts), the amplitude variations for these cases are small. However, when the basalt velocities are significantly larger than the surrounding rocks (or for the case of fresh basalts), the transmitted energy is significantly reduced, especially at long offsets.
These results suggest that weathered basalts will have no significant effect on wave propagation but the strong impedance contrasts associated with fresh basalts will reduce the amplitude of seismic waves below the basalt. Problems may therefore arise with seismic reflection surveying if the S/N becomes too small.
Effect of basalt thickness
The second set of models involves the same layering as shown in Figure 1 but with the basalt thickness varying between 0 and 80 m and with its top boundaryfixed at a depth of 40 m. The receivers remain at 180 m depth.
The amplitudes at the depth of 180 m are shown in Figure 4.
At near offsets, there is little variation and similarly for cases involving thin (less than 8 m) and thick (greater than 40 m) basalts, there is little variation in the amplitudes at larger offsets. However, at offsets greater than 200 m (geophone position 300 m), variations in amplitude are evident for the basalts with thicknesses between 8 and 40 m. At these intermediate thicknesses, it appears that interference and tuning effects are influencing the energy levels. The amplitudes drop off most rapidly for the case of the 20 m thick basalt– i.e. for the case when the dominant wavelength of the
P-wave in the basalt (48 m) is about twice the thickness of the basalt. In addition, it can also be observed that basalts that are thin compared to the wavelength (~40 m) do not affect the amplitudes, even for large offsets. This observation is consistent with the relative successful imaging experiences with thin basalt covered area (Gallagher and Dromgoole, 2008).
Effect of lateral extent
In many coal mining areas, the basalts are of limited areal extent and do not cover the whole survey area. We use the model in Figure 5 to illustrate the effect of a discontinuous layer of basalt.
Figure 6 shows amplitudes of thefirst event for sensors at a depth of 180 m when the basalt layer is 40 m thick and extends 200 m from the left-hand edge of the model. For comparison purposes, results are also shown for cases involving no basalt and 40 m thick continuous basalt. As expected, the down-going wave shows little influence from the basalt, once it follows a path that does not intersect the basalt. Beneath the basalt, the amplitudes are similar for those involving a continuous layer of basalt. This means that the amplitude reduction caused by the partly-coved basalt is appreciably smaller than the fully-covered basalt, especially for the far offsets. These results demonstrate that long-offset recording can help to improve the quality of seismic data if the basalt layer is not continuous. Provided normal moveout stretch does not become an issue, long-offset recording would also increase the fold and thus further improve the seismic results.
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Fig. 3. Direct wave amplitudes measured under basalt for the model shown in Figure 1 and with different basalt velocities varying between those of the surface layer and a layer of twice that velocity. The horizontal axis indicates the lateral position of the geophones as shown in Figure 1.
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Fig. 4. The amplitudes of the direct wave measured under the basalt for the basalt models with different the thicknesses.
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Fig. 5. Geological model involving partial basalt cover. The elastic parameters for the model are the same as shown in Figure 1.
Effect of distance from basalt
Intuitively, the influence of a basalt layer with limited extent should decrease as the distance between the basalt and the target layers increases. This is assessed using the model shown in Figure 5 and with lines of receivers at depths of 0 m to 100 m beneath the basalt. The results in Figure 7 clearly show the diminished influence of the basalt as the depth to the receivers increases. This demonstrates that the depth of the coal seams below basalts of limited extent is important and that coal seams immediately below basalt cover are more difficult to image.
Effect of boundary roughness and heterogeneities
As previously mentioned, it has also been proposed that the roughness of the boundaries of the basalt layer and the heterogeneities within it influence the transmitted wavefields.
Figure 8a shows a velocity model with irregular basalt boundaries of a Gaussian type (Ikelle et al., 1993), and Figure 8b with two internal incursions 10 m thick and 200 m length with the properties of layer 1. Figure 8c shows a model with an inhomogeneous basalt layer. The vertical component seismograms at the ground surface generated for these models are shown in Figure 9.
The results display the scattering expected in such situations but the main direct waves and reflection events evident in the results for plane boundaries (Figure 2) are still present and there is not a great deal of difference in the results. Presumably the
irregularities are too small with respect to the dominant wavelength to significantly affect the wave propagation. These heterogeneities have also affected the generation of multiples. No longer are these coherent events as in Figure 2. They now appear to be expressed as the pervasive random noise that is present in the seismograms.
Effect of multi-layered basalt
Another form of heterogeneity is that due to multiple basaltflows, which frequently have unconsolidated sediments between them.
We use the model in Figure 10 to illustrate this situation. In this model, there are alternating layers of high-velocity basalts (P- wave velocity of 4800 m/s) and low-velocity layers (P-wave velocity of 1600 m/s). Each layer is 2 m thick and the total package is 40 m thick. The corresponding vertical component seismograms of both surface and VSP records generated from this model are shown in Figure 11. In general, the seismic records are very similar to those in Figure 2. However, the coal seam reflections from this model are weaker and the multiple reflections are stronger. A similar observation can be made from the seismic responses for the incursion model in Figure 9b. Therefore, the S/N ratio for the multi-layer basalts is reduced and the imaging of layers below the basalts becomes more difficult.
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Fig. 6. Comparison of the amplitudes of the direct wave for a limited basalt layer (Figure 5) and the amplitudes for the model with a continuous basalt layer (Figure 1) and for a model with no basalt.
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Fig. 7. The declining influence of partial basalt cover with increasing distance from the basalt as shown by the amplitudes of the direct wave.
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Fig. 8. Basalt models used for seismic simulation: (a) rough boundaries;
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Fig. 9. Synthetic seismograms for the models in Figure 8: (a) rough surfaces; (b) rough surface containing incursions; (c) rough surfaces with heterogeneous basalt layer.
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Fig. 11. Vertical component synthetic seismograms for thefinely-layered basalt model shown in Figure 10. (a) The surface reflection seismic record; (b) VSP seismograms recorded from the borehole receivers located directly below the shot point as shown by the vertical red line in (a).
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Effect of the source frequency
Low frequency sources have been successfully applied in the petroleum industry to image below basalts. The two main reasons for the success are that low frequencies provide better penetration, and they are less sensitive to small-scale heterogeneities. However, coal seismic exploration uses higher frequencies than petroleum seismic exploration. It is necessary to investigate the behaviour at these higher frequencies.
We investigate the effect of source frequency using the model shown in Figure 8b. The results in Figure 12 show that the lower the dominant frequency, the greater the amplitude, especially at the greater offsets. The basalt layer can therefore be viewed as becoming a thinner layer at the lower frequencies, as shown with Figure 4. However, for thicker basalts, there might also be a counter argument for moving to higher frequencies if the basalt layer can be regarded as a thick layer. With higher frequencies, imaging problems caused by coal seams being thin with respect to the dominant wavelength are also less likely to occur than with lower frequencies.
Conclusions
We have used full elastic wave equation modelling to investigate the effects of near-surface basalt layering on the propagation of seismic waves. In general terms, the layer thickness and roughness need to be considered in the context of the dominant wavelength of the seismic waves. Our key conclusions are: (i) once the thickness of the basalt layers is less than about half the dominant wavelength, thin layers have less effect than thicker layers; (ii) basalts of limited lateral extent have less effect than continuous basalt layers;
(iii) the further the coal seams are away from the basalt of limited lateral extent, the less the influence of the basalt on the wave propagation; and (iv) the propagation of the seismic waves need not be adversely affected by surface roughness and random inhomogeneity within the basalt. However, basalts involving multiple flows cause reductions in the strength of reflections from below the basalts and can generate strong multiples.
Overall, improved seismic results might be possible through judicious choice of spread length, source frequency and management of noise. As part of our on-going research, we are undertaking an experimental seismic program which will allow us to investigate thesefindings.
Acknowledgements
This work was supported by the Australian Coal Association Research Program (ACARP), BHPB Illawarra Coal, Xstrata Coal, BMA Coal and Anglo Coal Australia. Comments by the two anonymous reviewers were appreciated and led to the investigation of the models involving basalts with different velocities and multipleflows.
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Manuscript received 28 February 2009; accepted 14 December 2009.
http://www.publish.csiro.au/journals/eg
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Weijia Sun1,2Binzhong Zhou1Peter Hatherly3Li-Yun Fu2
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ޔ⇣ߥࠆ᧦ઙਅߢߩ㔡ᵄવߩലᨐߣ․ᕈࠍ⺞ߴࠆߚߦޔᒢᕈᵄേᣇ⒟ᑼߦၮߠߚࡈࠜࡢ࠼ࡕ࠺ࡦࠣᛛⴚࠍ↪
ߚޕࡕ࠺ࡦࠣߩ⚿ᨐߪᰴߩߎߣࠍ␜ߒߡࠆޕ1) 㜞ࠗࡦࡇ࠳ࡦࠬߩࠦࡦ࠻ࠬ࠻ࠍߒޔⶄᢙߩࡈࡠ߆ࠄߥࠆ₵ᱞ ጤߪޔᒝᄙ㊀ߣᒙ㕙ࠍ߽ߚࠄߔޔ2) ⭯₵ᱞጤጀߪෘ₵ᱞጤጀࠃࠅᓇ㗀߇ዊߐޔ3) ৻ㇱ߇₵ᱞጤߢⷒࠊࠇ ߡࠆ⁁ᘒߢߪޔో߇ⷒࠊࠇߡࠆ⁁ᘒߦߊࠄߴߡᓇ㗀߇ዊߐޔ4) 㜞ᵄᢙߩ㔡ᵄߦߊࠄߴޔૐᵄᢙߩ㔡ᵄߪ㧔․
ߦᄢ߈ߥࠝࡈ࠶࠻ߦ߅ߡ㧕₵ᱞጤਛࠍࠃߊㅘㆊߔࠆޔ5) 㒢ࠄࠇߚ▸࿐ߩ₵ᱞጤጀߦኻߒߡߪޔ⍹ጀ߇ࠃࠅᷓ⟎ߦ
ࠆ߶ߤޔ₵ᱞጤ߇㔡ᵄߩવߦਈ߃ࠆᓇ㗀ߪዊߐޕߎࠇࠄߩⷰኤߦࠃࠅޔ₵ᱞጤጀਅࠍኻ⽎ߣߒߚ㔡តᩏࠍታᣉߔࠆߣ ߈ߦ↢ߓࠆ㗴ߦኻߔࠆᵢኤ߇ᓧࠄࠇࠆߣߣ߽ߦޔ㔡ᵄࡁࠗ࠭ߩᵈᗧᷓ▤ℂ߿ޔૐᵄᢙ․ᕈߩࠫࠝࡈࠜࡦࠍ↪ߚ㐳ᄢ ዷ㐿㔡តᩏ࠺࠲ߩขᓧߪޔ㔡តᩏߩ⚿ᨐࠍะߐߖࠆน⢻ᕈ߇ࠆߎߣ߇ℂ⸃ߐࠇࠆޕ
ࠠࡢ࠼㧦⍹ጀ㒢Ꮕಽᴺࡈࠜࡢ࠼ࡕ࠺ࡦࠣਇဋ⾰ᕈ㐳ᄢዷ᷹✢ૐᵄᢙᏪၞ₵ᱞጤጀਅࠗࡔࠫࡦࠣ
滆祢͑ 笊怺橚汊͑ 皻空͑ 洊砒穞垚͑ 痊昷砒汞͑ 其壟͑ ͑ 昣痊͑ 痊昷砒痖斲櫖͑ 洇殯͑
Weijia Sun1,2, Binzhong Zhou1, Peter Hatherly3, Li-Yun Fu2
1 䢎㭒ὒ䞯㌆㠛㡆ῂ₆ῂG 䌦㌂G G ὧ㌆SG 㡺㓺䔎⩞㧒Ⰲ㞚G 2 㭧ῃὒ䞯㤦 㰖㰞㰖ῂⶒⰂ㡆ῂ㏢
3 䢎㭒䌚ὧ㰖㰞㰖㭒䣢㌂, 㡺㓺䔎⩞㧒Ⰲ㞚
殚͑ 檃G 㕞○㞦 ₲╆ℯ 㕪╆ᜮ ▷㕞 ሆⴊḖ ᾲ╆㧲ዊ ⮞㨎 ኒṆᅺ ▷㕞 マሎℯ ⶫ ⰿ⅗❷ マ㕞ℯ(longwall mining)⯲
⮞㩲 ᅗႪ⯞ ⮞㨎 ႚⰿ ᖪṆ Ⰾ⭃ᢲἊ ႚⰿ 㭂ᆖⲛⰒ ℯ ⶫ⯲ 㧲ᔲⰎ. ኒᲆᔲ Ⰾ 㕞○㞦 㕪╆ℯ⯲ ∞㨎⯚
㫮╊⧮⯲ ⴎⱆ⩪ ⯲㨎 ⪛㨿⯞ ₵ᜮ. 㫒ⶖ⯲ ⮆∞⹚(Bowen Basin)⫚ ❶ᝢ ∞⹚(Sydney Basin) Ⱆ√⩪ Ⰾᲆ㧶 㫮╊⧮Ⰾ
∞㢆㧲ᅺ Ⱒ⯖Ἂ, Ⰾᲆ㧶 ⹚⪇⩪▶ 㕞○㞦 㕪╆Ḗ Ⰾ⭃㧲⪆ ⹚㧲⹚ሆⴊḖ ⲯ㫯㰢 ₷㪚ᕎᜮ ᠊ᜮ ⩎Ჾ⭚Ⰾ Ḗ ⚲ Ⱒ. ᅊᆖⲛ⯖ᳶ ኒᲆ㧶 ⹚⪇⯚ ▷㕞ᆫ 㭞⹚ᳶ ᆚ➆⯞ ᓦ⹚ ὕ㧲ᄦ ᢶ. ᰖ▶, Ⰾᲆ㧶 㪞῎⧮ ⹚⪇⩪▶ ⚲㨣ᢲᜮ 㕞○㞦 㕪╆⯲ ○ᆏ᷺⯞ ᘬⰎዊ ⮞㧶 ዊℯⰎ 㧞⬮㧲.
Ⰾ ⪊ሆ⩪▶ᜮ 㕞○㞦ᡳ ⲯ❷⩪ ዊㆢ㧶 ὂ᠒ṛ ዊℯ⯞ Ⰾ⭃㧲⪆ 㪞῎⧮⯲ ῖ○ヂ, ᣪ፲, 㬻㨿 ⪊ⰿ○, ☻❺⭪⩪ រ㧶
╛រⲛⰒ ⮞㋲, ⨫㧶 㪯㕶⯲ ∢ቺ○Ⰾ 㕞○㞦 Ⲟ㞦⩪ ⩎ᨺ㧶 ⪛㨿⯞ ⶖᜮႚḖ ╎㡎→. 㕞○㞦 ὂ᠒ṛ ᅊᆖᳶ√㗊
⯦ᆖ Ⴓ⯚ ᅊᳺ⯞ ⩕⩢. 1) Ⱎ㧖៲✾⯲ ヂႚ 㔊 㪞῎⧮ᆖ ⶫ 㯪Ḟ(multiple flow)Ⰾ Ⱒᜮ ᅗ⭊ ⶫ₲╆㞦ႚ Ⴏ㧲ᄦ ᔲ㕚ᔲἊ Ⱆヂ₲╆㞦ႚ ⨗㧲ᄦ ᔲ㕚ᔶ. 2) 㪞῎⧮㋏Ⰾ ⨡⯞ ᅗ⭊ 㪞῎⧮⯲ ⴎⱆႚ 㞦⯲ Ⲟ㞦⩪ ₒ㋲ᜮ 㭂ᆖႚ ᣪፖ⭎
㪞῎⧮㋏⩪ ⯲㧶 㭂ᆖ ⲛ⯚ ᄝ⯖ᳶ ᔲ㕚ᕆ⯖Ἂ, 3) 㪞῎⧮Ⰾ √∞ⲛ⯖ᳶ ᠈⪆ Ⱒ⯞ ᅗ⭊ 㣶ἎⲞㅎႚ 㪞῎⧮⯖ᳶ ᠈⪆
Ⱒᜮ ᅗ⭊⩪ ⋞㨎 ኒ 㭂ᆖႚ ₒ⨗㧲ᄦ ᔲ㕚ᕆ. 4)Ⲛⶖ㞦⯲ 㕞○㞦(㝓㰢 ⭪ᄊṆ ⪾㦞◥⩪▶)ႚ ᅺⶖ㞦⯲ 㕞○㞦⩪ ⋞㨎 㪞῎⧮㋏⯞ ⰲ 㙏ᆖ㨂⯞ ⧦ ⚲ Ⱒ⩢. ᪪㧶 5) ▷㕞㋏Ⰾ 㪞῎⧮㋏⯖ᳶ√㗊 ỚṆ ᩂ⩎Ⳓ ዤ⯚ ᆍ⩪ ⴎⱆ㧺⚲᳷
㪞῎⧮㋏Ⰾ 㞦⯲ Ⲟ㞦⩪ ₒ㋲ᜮ ⪛㨿Ⰾ ⲛ⯦⯞ ⧦ ⚲ Ⱒ⩢. Ⰾᲆ㧶 ⪊ሆᅊᆖ⯚ 㪞῎⧮㋏ 㧲√⩪ រ㧲⪆ 㕞○㞦 㕪╆Ḗ ⚲㨣㧺 ᧦ ₶╷㧺 ⚲ Ⱒᜮ ῒⲶ⩪ រ㧶 㙏ナᲿ⯞ Ⲷᆏ㧺 ⑪ ⧞ᝢᰖ 㕞○㞦 ⰻ⯦⯞ ⲛⲢ㰢 ㄲṆ㧲ᅺ Ⲛⶖ㞦⚲
រ⪇⯲ ⹚⪾㢊⯖ᳶ ⭪ᄊṆ ⪾㦞◥⩪▶ 㕞○㞦 㕪╆ⰪᵦḖ 㬧㨂⯖ᳶ⠂ 㕞○㞦 㕪╆ ᅊᆖḖ Ⴖ►❶㕆 ⚲ Ⱒᜮ ᄝ⯞
⪆.
渂殚檺㍳䌚SG 㥶䞲㹾⻫SG 㩫⳾◎ⰗSG ‶㰞㎇SG 㤦ỆⰂG 㡺䝚㎡G ₆⪳SG 㩖㭒䕢㑮SG 㞪㡒☪G 䞮G 㡗㌗䢪G
