Project Overview
In order to verify the effect of comprehensive geophysical methods on geothermal resources exploration in igneous rock areas, a geothermal resource exploration with comprehensive geophysical methods was carried out in a place in Huairen County, Shanxi Province, China.
This area is located in the uplift belt on the west side of the central part of the Sanggan River New Rift in the Datong Basin. It straddles the Huairen Sag and the Huanghualiang Sag uplift from west to east. Martial, Lower Ordovician, Carboniferous, Permian, and Cenozoic (Q + N) strata, with Archean granite and Late Tertiary basalt intrusions locally, among which basalt and deep igneous rocks of the Wutai Group are underground thermal Good storage of water.
This area straddles two structural units, the Huairen Graben and the Huangliang Horst, which are secondary structures of the New Rift of Sanggan River. A series of NE-strike fault structures develop in the area. The development of fault structures provides a better connection channel for various underground aquifers.
As shown in Fig. 4, 4 CSAMT survey lines are arranged in the exploration area.
In the CSAMT detection, the instrument used is the GDP32Ⅱ multifunctional electrical method workstation. The transmitting pole distance AB = 1500m, the transmitting current is 14-16A, the transmission distance is 6km; the receiving point distance of line 59 and 64 is 50m, the receiving point distance of line 10 and line 60 is 100m, and the signal frequency range is 0.125- 8192Hz.
In the TEM detection, the instrument used is also the GDP32Ⅱ multifunctional electrical method workstation, and the center loop device is used for measurement. The transmitting wire frame is a 600m×600m single-turn loop, powered by a generator, the fundamental frequency of the transmitting source is 16Hz, and the transmitting current is 15A; the distance between the measuring points is 50m.
After the CSAMT measurement is completed, a TEM measurement line is arranged near the 60 line where the CSAMT resistivity is more obvious, which is used to more accurately delineate the low resistance fracture zone, and combined with the CSAMT and TEM results.
Verified situation
In the processing of the measured data, we first use the data preprocessing software of GDP32Ⅱ to sort the collected data and remove the dead pixels, and then use the CSAMT-2D software and TEM-1D software based on the OCCAM algorithm to invert the data. The inversion results of CSAMT and TEM resistivity profiles are as follows (Fig. 6):
In general, the morphology of the resistivity profile of CSAMT and TEM is basically the same, and both show that the resistivity is medium-low resistance in the medium and shallow layers, and the resistivity gradually increases as the depth increases, and the deep substrate shows high resistance.
Comparing the detection effects of CSAMT and TEM, the differences are:
(1) TEM is better than CSAMT in the detection effect at shallow depths on the surface. CSAMT is affected by topography, the low-resistance shielding interference of the Quaternary muddy sand and clay layer is relatively large, which shows abnormal medium resistance, and the resistivity curve presents characteristics such as distortion, which reduces the resolution of shallow layers to a certain extent. Although there is a blind zone with a depth of about 100m in TEM, the low-resistance shielding layer at the shallow surface is less interference, and the resistivity value shows obvious regularity at the shallow surface, which is consistent with the actual geological characteristics.
(2) CSAMT is better than TEM in the detection effect at large depths. In the 4500–10000 point section, CSAMT has better data stratification in the deep part, while the TEM resistivity curve in this section is slightly confused, and the TEM shows a circle of the resistivity curve in the 9000–9500 point section. Closed high-resistance abnormal value. This high-resistance abnormal value should be a false abnormality caused by interference. The secondary field potential of the late TEM measurement track also fluctuates greatly in this section, lacking regularity, indicating that CSAMT has better anti-interference ability TEM.
(3) TEM and CSAMT have their own advantages in the detection of water-conducting fault structures.
Since the pure TE field mode of TEM is especially sensitive to low-resistance targets (Xue et al. 2013), this is more obvious in the reflection of F1 fault. CSAMT hardly reflects the F1 fault in the low-resistance area in the shallow part, but there is obvious low-resistance anomaly on the TEM profile. It can also be seen from the TEM secondary field potential multi-channel map that there is an obvious abnormal high value of the secondary field potential in this area.
The advantage of CSAMT for fault detection is mainly reflected in the detection of deep high-resistance basement interruption layer. In the sections of 5000m-5700m and 9500m-10000m, it is inferred that the buried depth of the Quaternary and Tertiary loose deposits is about 500m. The lower part is the basement of igneous rock, and the resistivity reflects the characteristics of high resistance. In the CSAMT cross-section map, these two sections appear as steep gradient zones, which are inferred to be F2 and F3 faults, which can more intuitively distinguish the upper and lower walls of the fault.
(4) CSAMT is better than TEM for the detection effect of igneous rock basement. In the 5800m-9300m section, CSAMT has a very obvious high-resistance response to the igneous rock basement, while the high-resistance response of TEM is not obvious at this position, and it does not highlight the igneous rock basement.
After the completion of the geophysical prospecting construction, the drilling verification was carried out at 3350 point. The borehole encountered underground hot water at 1610m; then a pumping test was carried out. According to the results of the pumping test, the unit output of underground hot water in this area was 233m3/d. The water temperature is 58°C. This result confirmed the occurrence of geothermal water in faults and igneous rock formations of the Wutai Group.