Study On Scientific Drilling On Wenchuan Earthquake Fault Zone By MT

May12,2008Wenchuan earthquake shocked the world, to people’s lives andproperty it was enormous disaster. Passed and two years ago more than512earthquakes, aftershocks are continuing, the unique event of an earthquake, thedisaster also brought to people the opportunity of science. Earthquake occurred in theLongmen mountain fault zone, in the fault zone on the implementation of scientificdrilling, it is very important for the study of earthquake mechanisms, continue tomonitor the aftershocks and the study of Longmen mountain.Scientific Drilling is based on multi-disciplinary observation and test’s research,to reveal the deep earthquake fault zone composition, structure and structuralproperties, remodeling fault zone physical and chemical processes, in order to provideuseful information on earthquake early warning. Core drilling can be more intuitivewith the underground rock composition, but because of the difficulty and high cost ofdrilling, well drilling as a result is elusive, so single well for the study of complexgeological profile is slightly thin. In the dramatic mountain terrain, seismic wave can’tbe received easily; DC and time domain sounding are limited by the above reason.Magnetotelluric (MT) can overcome many above factors, to solve certain well sitenear underground electrical structure within the depth of the fault in the groundcharacteristics such as the extension and angle. Drilling is the "point" data,Magnetotelluric is the "side" information,"point" can guide the "side" and "side" canserve the "points". They are combined to solve not only the construction of theLongmen mountain deep fault and remodeling the physical process, but also canpromote the development of geophysics.To fault (crack) distribution due to differences in physical properties of thepremise, the lines should be vertical to the main fault as far as possible, and study thetendency towards deep changes. Three basic Magnetotelluric sounding profiles identified four boreholes near site features underground electrical structure.Qualitative analysis of the frequency of apparent resistivity, phase characteristics,provides the basis for the inversion and interpretation. This also added a new test ofthe apparent resistivity (Basokur) inversion imaging effects, Carney explained ATVresistivity inversion to provide a reference.1st and2nd drilling (WFSD-1, WFSD-2)inversion profile of interpreting, cross-reference verification, precise electricalconstruction. Comprehensive geological and geophysical interpretation of a keysection in Hongkou Township in Dujiangyan central Longmen mountain faultsituation the structural characteristics, for further research to provide geophysicalbasis for the central fault, and the results are more consistent with the actual drilling.For the special geological features (octagonal temple to be at the central Longmenmountain fault zone, the area is brittle-ductile zone, on the plate for the Peng-guancomplex on the lower plate for the clastic rock), on the Magnetotelluric inversionprofile explain the proposed new point of view: high and low resistance contact withthe fault zone, the division of fault plane should be close to the high impedance side.Respectively, we can demonstrate that the view is correct from the numericalsimulation and geology. WFSD-3,3P are on the Longmen mountain fault zone beforemiddle faults, the frequency diagram and inversion results were complicatedgeological activities in the region early, it is more associated with secondary fracture,showing the characteristics of the Longmen Mountain imbricate thrust faults.For the signal distortion,resulted by horizonta galvanic heterogeneity inelectromagnetic field,and combining with the structure features of the workingregion,this paper uses the Mohr circle decoposition method to rotate the electric fieldE and the magnetic field H,and gets the useful informations of the regional structureand the local structures. The results show that, after decomposing the impedancetensors, the Mohr circle decoposition method can elimiate the galvanic distortionseffectively,and invert the subsurface structures moe factually and clearly. This paperalso shows that, in practical works,we should according to the structure features of theworking region,and choose the applicable decomposition method of the impedancetensors. Then a desirable result can be achieved.Integrated Scientific Drilling, Shi-Fang and Han-Wang Magnetotelluric soundingprofiles, inferred on the Longmen Shan fault zone exposed in the section angle arelarger, the increase of dip with depth decreases, and in the ground1-2Km the anglechanges faster with the angle reducing about15°-25°, with the extension of thedeeper depth changes will slow down gradually. In fact, this fault dip with depth smaller surface phenomenon, they are also important features for Longmen Mountainimbricate thrust faults.