利用海底地震仪(OBS)进行探测时, 一般采用自由下落的方式将OBS布设在海底。由于海流和海底地形的影响, OBS的位置一般都偏离设定的位置。OBS重新定位是OBS数据处理的基础, 不正确的位置信息将导致错误的观测系统信息, 从而影响后续的处理效果。直达水波包含了OBS的位置信息, 一般利用直达水波的走时, 采用最小二乘的方式来确定OBS的实际位置。炮点位置精度、放炮时间延迟、炮点分布方式、OBS时钟漂移、走时拾取误差以及海水速度变化等因素都会影响重新定位的精度。文章采用数值方式研究这些因素对重新定位精度的影响, 并对东沙群岛海域实际的OBS站位进行精细重新定位。数值结果表明, 直线排列的炮点不能很好地反演出OBS的位置, 应该采用十字或者井字分布的炮点; 在偏移距小于10km时, 海水速度结构的精度对重新定位的影响可以忽略; 对OBS重新定位影响最大的是由放炮时间延迟、时钟漂移以及走时误差共同构成的右端项误差, 5ms的右端项误差可引起40m左右的偏差。实际数据重新定位的结果较好, 符合数值研究的结果。

An ocean bottom seismometer (OBS) was placed on the seafloor by free fall from the sea surface. Due to the impact of current and bottom topography, the actual position of OBS is usually deviated from the designated position. OBS relocation is the key part of OBS data processing, since the inaccurate position will introduce errors to the geometry and to the following processing. The direct water wave contains the information of OBS position, and the OBS position is inverted from the travel time of direct water wave by the least-square method. The accuracy of shot position, the delay of shot time, the distribution of shots, OBS time drift, errors in picking of first break and the sea water velocity all have impacts on the accuracy of OBS relocation. In this paper, we show the impacts of these factors on OBS relocation, and use some actual OBS in the Dongsha sea area as an example. Numerical results show that to invert the OBS position accurately, the shots should be distributed along a non-straight line. When the offset is less than 10 km, the influence of sea water velocity structure can be ignored. The most influential factor comes from the right-hand error consisting the delay of shot time, OBS drift and picking error. And 5 ms error may lead to 40 m digression. The numerical result agrees with the actual OBS data.