marees terrestres bulletin d'informations - UniversitÃ© de la PolynÃ©sie ...

More documents

Recommendations

Info

BLANK PAGE
Hydrological Signals due to the Seasonal Variation of Lake Nasser and its Effect to the Surrounding Crust as Deduced from the Tidal Gravity Observations Hassan, R. M., E.M. Abdelrahman, A. Tealeb, K.H. Zahran and G. Jentzsch Abstract Impounding of Lake Nasser started in 1964 and reached the highest water level so far in 1978 with a capacity of 133.8 km 3 . It is extending 500 km in southern Egypt and northern Sudan, thus forming the second largest man-made lake in the world. The water level fluctuates between 168m and 178m, the cycle being divided into four different periods of inflow, stability, discharge, and stability again. The variation of the mass of the water of the reservoir changes the potential field either by loading or by change of the ground water level. To detect these signals two years of continuous gravity measurements (from the year 2002 to the year 2004) have been used from a tidal station installed very close to the Lake. A combined plot of water level variation and residual gravity shows that the residual gravity follows water level variations with a time delay. Two methods have been used in the current study to evaluate the gravity variations in the tidal records due to variations of water level at Lake Nasser. In the first method, the data were divided into blocks, each block represents an epoch of a certain water level in the lake. These blocks reflect the situation in the lake during the decrease, stability, and increase of water level. The tidal parameters at different blocks with different water levels show some changes. These changes reach up to 4% in amplitude factors, and 0.5° in phase shifts. The variations in the elastic parameters follow to some extent the variations of water level in the lake. In the second method, cross-correlation was applied between residual gravity and change of water level in the lake. Generally, a moderate correlation coefficient (0.556) between residual gravity variation and water level variation, and a weak correlation coefficient (0.480) between residual gravity and ground water variation were recognized. Correlation coefficients of selected blocks follow to some extent the values of water level variation on one side and ground water variation on the other side. The study shows that variations of the Lake level affect the surrounding crust significantly. Variation of the lake has to be modelled and considered in the geodetic and gravity observations in this region if higher accuracy is to be achieved. 1. Introduction The filling of large reservoirs changes the stress regime, either by increasing vertical stress (compression) by loading or increasing pore pressure through the decrease of effective normal stress (Snow, 1972; Bell and Nur, 1978; Simpson, 1986; Roeloffs, 1988). At Lake Nasser, water level fluctuates four times during the year, according to the cycle of charge (inflow) and discharge (outflow), followed each by a period of stabilisation. The variations of water level in the lake affect the dynamic stability of the area, either by variable induced loading or variation in the underground water level. On the other hand, the analysis of tidal gravity observations at Aswan tidal station, which is very close to Lake Nasser, shows discrepancies between the observed and synthetic tidal parameters. These discrepancies may be due to variations in water lake level. In the region of Lake Nasser, the Nubian sandstones cover a large area, with a porosity of about 25%. So, any change in water level in the lake changes the underground water level. Seepage of water of the lake into the underground depends on the types of rocks in the underground, their state of weathering and tectonic influences (formation of clefts and cavities of different sizes). The run-off depends on the properties of superficial material, the evaporation on meteorological parameters (air temperature, humidity of air, and wind) and plant cover, etc. In hydrologic modelling, generalized input data and parameters are commonly used, which are representative for a certain area or a certain period of 11807
Page 1:
MAREES TERRESTRES BULLETIN D'INFORM
Page 6 and 7:
BIM 146 December 15, 2010 New Chall
Page 8 and 9:
1994), and TPXO.6 (Egbert et al., 1
Page 10 and 11:
4. An attempt to improve the region
Page 12 and 13: From Fig. 2, we also see that, the
Page 14 and 15: GPS network called GEONET (GPS Eart
Page 16 and 17: References Bos, M. S., Baker, T. F.
Page 18 and 19: Planets Space, 59, 307-311. Sato, T
Page 20 and 21: Fig. 1. Locations of the observatio
Page 22 and 23: BLANK PAGE
Page 24 and 25: the dense seismic observation netwo
Page 26 and 27: difference of loading effects of oc
Page 28 and 29: is done with an optical lever using
Page 30 and 31: 6. Application of laser interferome
Page 32 and 33: Figure 11. Strain seismograms of th
Page 34 and 35: Higashi, T., 1996, A study on chara
Page 38 and 39: 2. Fundamentals and techniques At t
Page 40 and 41: 100 low frequencies earth tide freq
Page 42 and 43: Tab. 4: Averages of noise levels at
Page 44 and 45: References Asch, G., Elstner, C., J
Page 46 and 47: The 1 / s part is also worth mentio
Page 48 and 49: As it was mentioned before the resu
Page 50 and 51: Fig. 9. Morlet Wavelet Spectrum, su
Page 52 and 53: 9. CONCLUSIONS This investigation w
Page 54 and 55: 2. Objectives of Aswan Tidal Gravit
Page 56 and 57: 5. Recording of Data The data logge
Page 58 and 59: Figure 5: Pre-processed hourly data
Page 60 and 61: Figure 6: Residuals of gravity tide
Page 64 and 65: time. In addition, models for gravi
Page 66 and 67: Fig. 3 (Zahran, 2005) shows gravity
Page 68 and 69: during the years 2003 and 2004. It
Page 70 and 71: Figure 7: Variation of phase shift,
Page 72 and 73: Figure 8: Induced gravity load and
Page 76 and 77: The instrumentation of the tidal st
Page 78 and 79: 4. Tidal analysis The tidal analysi
Page 80 and 81: References Banka, D., Crossley, D.
Page 82 and 83: 1. Introduction The estimation of a
Page 84 and 85: Figure 1 Air density distributions
Page 86 and 87: temperature effect is considered, t
Page 88 and 89: Figure 4 Attraction effect from the
Page 90 and 91: a ) up to 0.5° , b) 0.5°- 1.5° ,
Page 92 and 93: Figure 7 Atmospheric reductions for
Page 94 and 95: Figure 9 Amplitude spectra of diffe
Page 96 and 97: Figure 10 SG observation and gravit
Page 98 and 99: eduction in the long-period tides r
Page 100 and 101: oceans. The effect has a peak-to-pe
Page 104 and 105: Figure 1. Local earthquakes recorde
Page 106 and 107: 4. Recording of Data The recorded d
Page 108 and 109: Table 1: Adjusted tidal parameters
Page 110 and 111: Acknowledgements: The authors are i
Page 112:
BLANK PAGE
show all

marees terrestres bulletin d'informations - UniversitÃ© de la PolynÃ©sie ...

Create successful ePaper yourself

Delete template?

Save as template?