利用微震数据对震源下方构造直接成像

现有的微震记录直接成像方法是将微震记录既当作入射记录，也当作散射记录，从而实现偏移成像.但此方法并不能突出透射波所携带的来自震源下方的深层散射波信息.本文在假设已知微震位置与子波的前提下，提出了对微震下方构造进行逆时偏移的成像方法.该方法类似于常规的逆时偏移，只是震源位置在地下.这使得在成像时，地下更深部的入射波场相比震源在地表时会更为精确，因此能够获得更加准确的成像结果.该方法会给成像结果带来一种尾波高频干扰：地下的震源发出的上行波与上方介质作用后，所产生的多级散射波会干扰反传波场.对此，在成像过程中，对入射场和散射场都进行左右行波分离，以压制该噪声.而在子波信息未知，无法重构入射场时，使用了激发时间成像条件，也能够实现同等效果的偏移成像，且不会出现尾波高频干扰.利用数值实验验证了本文方法的有效性.

Direct imaging structure beneath the source using microseismic data

In the existing method for imaging structure using passive seismic data (microseismic data or ambient noise data), active shot gathers are reconstructed first, then implementing the imaging process using the reconstructed gathers. The reconstructed active shot gather is actually extracted from the free surface multiples. In this case, the deep information carried by the transmitted wavefield is ignored and the problem of passive imaging is transformed into the imaging of active data. Hence the imaging effect of the deep subsurface is poorer than the shallow. The direct imaging method of microseismic data regards the microseismic record as the incident wavefield as well as the scattered wavefield, which avoids the reconstruction of the active gather. However, this method cannot highlight the information carried by the wavefield from the deep source. In this paper, assuming that the position and the wavelet of the microseismic event are known, we propose a Reverse Time Migration (RTM) method to image the structure below the source. This method is similar to the conventional RTM, except the source is underground. During imaging, the deeper wavefield will be more precise than that in the case when regarding the microseismic record as the incident wavefield. Hence we can acquire better imaging results. However, the upgoing wavefield from the source underground will bring internal multiples, which could disturb the receiver wavefield. We decompose both the source wavefield and the receiver wavefield into the leftgoing and rightgoing parts to suppress this noise. Numerical tests have demonstrated the effectiveness of the proposed method.