Zhanxiang He

Application of EM Methods for the Investigation of Qiyueshan Tunnel, China

A successful case history of applying the high-frequency passive source electromagnetic (EM) method and controlled-source audiomagnetotellurics (CSAMT) to investigate the Qiyueshan (Q) Tunnel route is presented in this paper. The high-frequency EM system (EH-4, with frequency range from 90 KHz to 12.8 Hz) and the CSAMT system (V6-A Multipurpose Receiver, with frequency range from 8,192 Hz to 0.125 Hz) were used for the data acquisition. The orthogonal components of the electromagnetic field were measured in the high frequency EM method, while scalar measurements of the electrical and magnetic field components were used in the CSAMT method. The relevant electrical properties of the earth were extracted from the electromagnetic profiles. High frequency EM has high resolution in the shallow earth but a smaller depth of exploration, while the CSAMT method has a powerful signal but a lower resolution in the shallow earth. The integration of the two methods might be effective for the survey of the deep tunnel route. Q Tunnel, located in central south China, has a length of 10 km and a depth of up to 900 m. Half of the tunnel goes through karst terrain, where the geologic structures are very complex, due to cavities, underground rivers and faults. The EM mapping results distinguish the electrical resistivity of different rock formations. Five low-resistivity areas and four high-resistivity areas were found and nine faults were verified by the EM method. These findings were very useful for the later engineering design.

Two case studies from China of hydrocarbon detection with SIP

Spectral induced polarization (SIP) has been used in east China for nearly 20 years to evaluate hydrocarbon charge. In assessing 65 wells in six oil fields in east China, SIP made 47 successful positive (producer) and negative (dry hole) predictions for a success rate of 72.3%.

Mapping the concealed structure using high-resolution EM - A case history of the investigation of B Tunnel route

Summary A successful case history of mapping the concealed structure using high-resolution EM in the investigation of B Tunnel route is presented in this paper. The high frequency electromagnetic system named STRATAGEM EH4, with the frequency ranging from 90 KHz to 1Hz, is used for the data acquisition. The orthogonal components of the electromagnetic field are measured, and the relevant electromagnetic attributes are extracted on the basis of the characteristics of the electromagnetic profiles. Hybrid sources, including the natural source and the full tensor controlled-source, are utilized to collect high quality field data. B Tunnel lies in the western part of Hubei province, the south of central China. The tunnel is only buried at the depth of less than 200 meters, but the geologic structure is very complex because of the regional geological action. During the first time of the B Tunnel route investigation, the investigators were deceived by the outcrop of the rock fall, considering that it is from the bed rocks and the roof of the tunnel fell when tunneling was in operation. High-resolution EM and seismic refraction were used in the second time investigation. The “real” bed rock and the geologic structure were mapped this time and a hidden fault was also detected. The tunneling result fits closely with what we have predicted.

Geothermal exploration using MT and gravity techniques at Szentlőrinc area in Hungary

Summary 2-D AMT/MT and gravity surveys were completed in 12 su survey areas in Hungary during 2008. The main objective of this project was to locate potential geothermal targets for alternative energy development in Hungary. We selected here the Szentlőrinc (Szl) survey area because the geothermal drilling project just completed. The main geothermal reservoir systems found in Hungary are the Mesozoic carbonate–karstic basement rocks and the Pliocene-Upper Pannonian porous sedimentary formations (Arpasi, Lorberer, and Pap, 2000). The interpretation of 2-D AMT/MT and gravity focuses on locating potential geothermal areas of the geothermal reservoir system within Mesozoic fractured carbonate–karstic basement rock for drilling locations. We estimated that the faults within the north-south strike in the Szl survey area were developed in the deep basement. In addition, dense fractures have also been widely developed in the top basement (limestone) of the two survey areas. Thermal energy, which was transported up along the fault systems from the deep Earth, seems to be the heat source of geothermal formation. A set of thick tertiary deposits, are located above the formation. Fractured karst limestone and dolomite deeply buried in the Mesozoic system contain the targeted geothermal reservoirs. Based on the cooperative constrained inversion of magnetotelluric (MT) and gravity data, we surmise that the geothermal aquifer is characterized by a relatively low apparent resistivity and low density, while the higher porosity and permeability formations are unique for faults and fractured zones. The distribution characteristics of the fault zones with relatively low resistivity and with boundaries outlined by cooperative constrained inversion of MT and gravity data indicate that the prospective zones for potential geothermal reservoirs in the Szl survey area indicate that the midnorthern part of AMT/MT line 1 and the middle part of AMT/MT line 2 are potential areas for geothermal power plants or space heating.

Characterizing a Geothermal Reservoir Using Broadband 2-D MT Survey In Theistareykir, Iceland

Geothermal energy is playing a larger role as an alternative energy source for both electricity generation and for space heating. Our recent magnetotelluric (MT) surveys in Iceland have both characterized known geothermal reservoirs and identified new drilling opportunities. MT data confirmed the findings of a previous transient electromagnetic (TEM) survey in the Theistareykir field, outlined the boundaries of the geothermal reservoir and for the first time identified and mapped a deeper conductive layer. The success of these surveys has resulted in additional 2-D and 3-D data acquisition and will be incorporated into a drilling program to evaluate the identified geothermal potential.

Mapping deep targets based on integrated 3D MT-gravity interpretation: a case study

Mapping deep geological hydrocarbon targets is of significant importance in basin exploration. In areas lacking reliable seismic data, magnetotelluric (MT) and gravity explorations are helpful to delineate the distribution of potential deep geological hydrocarbon targets. Here we investigate the effectiveness of the integrated 3D MT and gravity explorations for mapping the potential deep hydrocarbon source rocks. The result based on the data from the W Basin (part of the Ordes Basin) of China demonstrates that the method is efficient and economical for basin exploration. The method is particularly useful in target areas which are of great interest for oil and gas exploration but lack high quality seismic data. In our method, we first use the high-precision 3D small-bin MT data acquisition to improve the data accuracy. Then we perform datum static correction method and apply 3D inversion to obtain the3D resistivity distribution. We also develop a layered resistivity model based on resistivity logging to assist the interpretation of the inverted 3D resistivity data so as to derive an initial 3D geological model. Starting from the initial model, we use 2D gravity data to update the model via 2D inversion line by line, and then pass the updated model for the next round of the 3D MT inversion. The integrated inversion is implemented iteratively so the model converges to satisfy the need of final geological analysis. The application to the W Basin shows that we could successfully delineate the geological distribution of the potential deep hydrocarbon source rocks within the basin and map the thickness of the upper Paleozoic.

Nonseismic methods guide seismology in steep structures

The Mashan structural belt, which extends several hundred km east-west and 8–12 km north-south, is in the east section of the southern faulted tectonic belt at Bachu uplift of Tarim Basin. It is mainly controlled by two large faults—one in the south and one in the north. Commercial gas has been discovered in the area and it is considered to have further exploration potential despite the huge obstacles its topography presents to seismic prospecting. Seismic data quality in the south Mashan is poor and the reflection from top Ordovician limestone, the exploration target, is not clear. Traditionally, the Mashan structural belt was regarded as a single anticline with a northern high as the principal feature (Figure 1). However, comprehensive nonseismic geophysical studies (gravity, magnetic, and electromagnetic methods) have resulted in a different view—one that has attracted oil explorationists. Figure 1. Seismic interpretation profile of the Mashan tectonic belt. The Mashan structural belt was formed during Himalayan orogeny. Drilling data show that the Quaternary and Tertiary consist of sandstone, shale, and lagoon sediments; the upper Permian mainly is sandstone-shale; the lower Permian has a basalt section and shale section; the Carboniferous is the continental-oceanic interaction sediment; and the Ordovician mainly consists of the marine sediment. Based on core density measurements and the P-wave interval velocity, we conclude that the interface between the Permian-Carboniferous and Ordovician (−0.15g/cm3) and the interface between the Cenozoic and Permian-Carboniferous (nearly −0.15g/cm3) are the principal density interfaces in this area. The lower Permian basalt has stronger magnetism but its thickness is no more than 200–350 m. Basement metamorphic rock has middle or weak …