兰州市西部地热资源大地电磁探测

Magnetotelluric investigation of geothermal resource in western Lanzhou

甘肃与青海边界的民和盆地为中新生代盆地,其中含有油气和煤炭资源,随着开采资源枯竭,适合就近利用的地热资源的勘探评价具有重要的实际价值.通过4条大地电磁剖面(D5,D9,D13,D17)对民和盆地局部地壳电性结构、区域构造进行了探测,对本区地热资源进行初步评价.采用德国Geometrix公司GMS07e电磁系统,按64 Hz和4096 Hz采样率分别进行了2~3 h和5 min的采集,由仪器系统软件Procmt进行时频转换,得到各测点频率-电阻率和相位曲线,通过数值圆滑处理对曲线进行拟合,采用相似原理对曲线进行静校正,最后采用非线性共轭梯度法进行二维TM+TE联合反演得到电阻率剖面.结果显示在不同剖面隐伏的F18断层表现特征有所差别,在D5和D13表现为背斜相关断层,而在D9线表现为低角度断层,产状不稳定,说明F18断层属于压扭性.在D13和D17线探测到F19断层,从地层电阻率对比推测其性质为由西向东的逆冲断层.盆地基底埋藏深度从西南向东北逐渐变浅.在D5、D13线南部和D9线北部热储层深度分别在1000 m左右;而在D9线南部和D13北部热储层深度在1500~2000 m左右.

The Minhe Basin located on the border of Gansu and Qinghai is a Mesozoic and Cenozoic basin, which contains oil, gas and coal resources. With the thirst of mining resources, the exploration and evaluation of geothermal resources suitable for nearby use is of practical implication. The four magnetotelluric profiles were used to investigate the crustal electrical structure and regional tectonics of the Minhe Basin, and the geothermal resources in this area were initially evaluated. The GMS07 e electromagnetic system made by Germany Geometrix company was used to collect the data in 2~3 hours and 5 minutes using the 64 Hz and 4096 Hz sampling rate respectively. The data conversion between the time series and frequency was performed by the Procmt software to obtain the frequency-resistivity and phase curves in each station. The curve are smoothed by numerical method. The static correction of stations is carried out by the similar principle between the station and nearing station. Finally the non-linear conjugate gradient method is used to perform two-dimensional TM+TE joint inversion to obtain the resistivity profile. The resistivity profiles show that the fault F18 in different profiles are of different characteristics, shows anticline related high-angle faults in the D5 and D13 profiles, while shows low-angle faults in the D9 profile, and the altitude changes along the strike, and is indicated a compression shear fault. The fault F19 was detected on the D13 and D17 lines, and was a thrust from west to east based on the comparison of resistivity section. The basement depth of the basin gradually decreased from southwest to northeast. The depth of the thermal reservoir along D5 line, in the southern part of the D13 line and the northern part of the D9 line are about 1000 meters;while the depth of the thermal reservoir in the southern part of the D9 line and the northern part of D13 is about 1500~2000 m.