积水采空区地面-钻孔瞬变电磁探测技术

小煤窑积水采空区威胁煤矿安全生产,为有效识别钻孔旁侧遗漏的积水采空区并确定其空间位置,采用地面发射阶跃磁场、钻孔中接收三分量感应场的观测方式进行地面-钻孔瞬变电磁探测。通过构建积水采空区地电模型,正演分析了异常的响应特征及异常场随异常体电阻率、规模和相对钻孔距离变化的规律。计算结果表明:异常场的3个分量均响应明显,X,Y分量过零点与Z分量极值点对应异常体深度;X,Y分量幅值相对较小,但为纯异常场,便于识别和提取;Z分量幅值相对较强,但异常场为背景场的千分之一,不易识别和提取;由于更接近异常体,地面-钻孔观测方式相比地面瞬变电磁采集到的Z分量异常信号更强;异常响应强度随异常体电阻率降低、规模增大、距离减小而增强。基于等效电流环理论,设计了最小二乘约束反演算法,通过反演拟合异常场能获得异常体的中心坐标、倾角、尺寸等参数,实现异常体的空间定位。在陕西省榆林市某煤矿使用地面-钻孔瞬变电磁法开展积水采空区探测实验,采集的X,Y分量数据在目标层附近表现明显的异常响应特征,通过对1.0 ms和1.3 ms的异常场数据进行反演拟合,发现并推断钻孔北侧存在小煤窑积水采空区,反演结果得到后期钻孔验证,与实际揭露情况吻合。理论分析与现场实验证明:地面钻孔瞬变电磁法能有效探测到钻孔旁侧的积水采空区,实现"一孔多用"并突破"一孔之见",为小煤窑积水采空区的精细探查提供了一种新的地球物理勘探手段。

Technology of detecting water filled goaf beside borehole using downhole transient electromagnetic method

Water filled goaf of abandoned small coal mine has always threatened the safety of coal mine in normal production.In order to effectively identify the hidden water filled goaf beside a borehole and obtain its spatial location,the downhole transient electromagnetic method (TME) that transmitting step magnetic field on the ground and receiving three component induction field in the borehole was studied.Geoelectricity models of water filled goaf were constructed and 3D forward modeling were carried out.The anomalous responses of water filled goaf with different parameters were calculated.The characteristics of anomalous response and the influence of parameters,such as resistivity,scale and relative borehole distance,were analyzed.The results show that all three components of anomalous field respond obviously,and the zero crossing points of X and Y components and the extreme points of Z components correspond to the depth of the water filled goaf.The amplitudes of X and Y components are relatively small,but they are pure anomalous fields,which are easy to be identified and extracted.The amplitude of Z component is relatively strong,but the anomalous field is one thousandth of the background field,which is difficult to be identified and extracted.Due to the closer proximity to the anomalous body,the Z component acquired by the downhole TEM is stronger than that collected by ground TEM.The amplitudes of three components anomalous response increase as the water filled goaf resistivity decreases,the scale increases,and the distance decreases.Based on the equivalent current filament theory,a least square constraint inversion algorithm was developed,which can obtain the key parameters such as central coordinates,azimuth,inclination and size of the anomaly by fitting the anomalous field.An experiment of detecting water filled goaf was carried out in a coal mine in Yulin city,Shaanxi province,China,using downhole TEM.The X and Y components of collected data show obvious anomalous response characteristics near the target layer.By inversion fitting the abnormal field data of 1.0 ms and 1.3 ms,it is found and inferred that there is an abandoned small coal mine water filled goaf on the north side of the borehole.The detection results are verified by the later drilling,which is consistent with the actual disclosure.Through theoretical analysis and field test,it is proved that the downhole TEM can effectively detect the water filled goaf beside the hole,realize “one hole for multi purpose”,overcome the “one hole view”,and provide a new geophysical exploration method for the fine exploration of the water filled goaf of abandoned small coal mine.