Study On Effective Excitation Of Explosion Source With Near-surface Consideration

Explosive source is the most commonly used source in land seismic exploration. It is animportant factor influencing the quality of seismic acquisition. The selection of the explosivedosage and motivate lithology directly affects the excitation effect. The excitation ofexplosive source is equivalent to dig a spherical cavity in the medium and then the radiationof pressure pulse is homogeneously dispersed inside the wall. Although different functionshave been used to express the pressure size of spherical cavity, there are still few theoreticalresearches carried out on the pressure function. And for the conventional explosive excitationmode, under the same external conditions, the explosive dosage must be increased to promotethe depth of exploration. But the increase of explosive dosage can intensify the loss ofenergies of seismic waves at high frequencies, which decreases the exploration resolution. Asa result, seismic energies at high frequencies must be remained while the depth of thepropogation of seismic waves has been ensured during source excitation process. At present,the following four aspects are mainly focused on pertaining to the study of source excitationmode:①Delay explosion source excitation;②Long dynamite column excitation;③Pointsource array excitation; and④the phrased source excitation. But the phased source is mainlyused in vibroseis, and the study of this aspect is still remained in theoretical research as for thecommonly used dynamite source in land exploration.Preceded from the physical process of explosion, this paper firstly makes an overview ofthe partition of near source wave field and the formation of the seismic wavelet. And with theknowledge of mechanics of explosion, the relationship of cavity pressure varing with timeduring the process of cavity extension is obtained by a quantitative study on the shock wavesgenerated by explosive effect in rocks. This relationship can be generalized to soil media. Andthen the wave phenomenon in viscoelastic media caused by cavity pressure is discussed, andthe analytical solution of seismic wavelet displacement is also derived. Several numerical examples are given to analyze the factors influencing the formation of seismic wavelet. Anddefects caused by the commonly used unit impulse are modified. It has significant meaning tonear-source wave field study and the improvement of excitation effect.And then based on the method of finite difference forward modeling, this paper hasconducted stimulations on some typically used excitation methods. Under the circumstance ofsame dynamite charge, the wave field characteristics and energy illumination figures ofdifferent excitation methods are theoretically studied and analyzed. With the study on themarginal energy density, energy lighting, and wave field snapshot of different sourceexcitation mode, the energy characteristics and wavelet frequency characteristics of differentexcitation methods are given. According to the numerical simulations of different geologicalmodels, the most advantageous methods under different geological conditions are alsodiscussed and presented by the comparison of the relative excitation efficiency, wavelet stackenergy, seismic record and anti-noise ability, etc. And beneficial advices have also been givento deal with problems emerging from the practical operation of different excitation methods.Finally, the different excitation modes of3D sources are simulated and the wave fieldcharacteristics are also given for better guidance to field operation.