The Study On Joint Inversion Of Multi-geophysical Field Based On Wide-range Constraints

As shallow resources become increasingly depleted,the supply situation for energy and metallic mineral resources in China is becoming critical.Advancing into deeper regions is imperative,and geophysical exploration plays a key role in this process.By inverting and constructing models of subsurface resistivity,density,magnetic susceptibility,and thermal conductivity,we can provide significant technical support for deep mineral resource exploration and geothermal energy development.However,the limitations of individual geophysical methods and the non-uniqueness of inversion results have necessitated the development of joint inversion techniques for multiple geophysical fields.Joint inversion,as a significant quantitative interpretation tool in geophysics,can reduce the non-uniqueness of geophysical inversion and is currently a hot topic in the field.It is of great importance for studying geological structures,deep mineral exploration,and geological disaster prevention.With the improvement in computational capabilities,large-scale three-dimensional geophysical computations are no longer a constraint on inversion.In 3D model space modeling,compared to structured grids,unstructured tetrahedral grids are more flexible and better suited for simulating models with complex geometries and terrains.However,issues such as the indeterminate edge and surface directions and the uneven size of unstructured tetrahedral grids make structural constraint joint inversion difficult to implement.Although rock petrophysical constraints provide more significant coupling effects compared to structural constraints,they require stricter prior information.This has limited the development of rock petrophysical constraints,making them far less widely applied than structural constraints.This is especially true for joint inversions based on 3D unstructured grids with rock petrophysical constraints,where research is scarce.Currently,although research on joint inversion with unstructured grids is rapidly developing,there is still a lack of general joint inversion methods for multiple geophysical datasets,which invisibly hinders the development of joint inversion for multi-geophysical datasets.Furthermore,considering the complex geological and rock petrophysical characteristics in practical scenarios,it is imperative to develop rock petrophysical constraint joint inversion methods with strong applicability and fault tolerance.Additionally,geothermal data,as an important type of geophysical data,provides the potential for reliable estimation of the Earth’s internal thermo-chemical structure.However,there are currently few techniques in geophysics that directly utilize geothermal data.In the area of large-scale 3D thermal conductivity inversion,research on implementing inversion in unstructured grids and related details is also lacking.Firstly,to address the implementation of joint inversion for multiple geophysical methods,this paper proposes a co-reference model method.This method alternates the minimization of the joint inversion objective function,dividing the process into several stages and sequentially determining the order in which different geophysical methods conduct joint inversion within each stage.This approach not only promotes information complementarity among different geophysical methods but also fully considers the characteristics of each method.Using this scheme simplifies the objective function and,to a certain extent,overcomes the difficulties of implementing joint inversion for multiple geophysical methods.Additionally,this paper demonstrates the application of the co-reference model-based joint inversion with gravity,magnetic,and electrical data.Model tests and real data cases show that this method reduces the prior information requirements for rock petrophysical constraint inversion,allows the use of imprecise prior information,improves the imaging results of gravity and magnetic inversion,and increases the fault tolerance of rock petrophysical constraints,facilitating their promotion in practical inversion applications.Next,to address the problem of joint inversion in regions with complex rock petrophysical correlations,the previously mentioned method uses a global rock petrophysical correlation.However,in practice,strong correlations may only exist in specific regions.In areas with complex geological conditions,rock petrophysical correlations may exhibit multiple statistical relationships.This limitation is addressed by proposing a joint inversion method with regional wide-ranging petrophysical constraints.This method divides the inversion region into sub-regions with different rock petrophysical correlations and performs joint inversion for each sub-region separately.When dealing with complex rock petrophysical parameter correlations,this method allows flexible regional partitioning to enhance local correlation characteristics.The algorithm is further applied to magnetotelluric and gravity joint inversion in Anda City,Heilongjiang Province,China.The joint inversion results consistently reflect similar structural information,better delineating the top boundary of the Upper Paleozoic.Finally,to achieve large-scale thermal conductivity inversion,this paper first introduces boundary detection technology,using surface heat flow data to constrain the range of regional anomalies.Then,by combining borehole temperature data inversion results,an initial reference model for inversion is established.Using the co-reference model method,simultaneous joint inversion of borehole temperature field data and surface heat flow data is achieved.Model tests and real data cases verify the effectiveness and practicality of this method.In summary,this paper conducts research on wide-ranging petrophysical constrained joint inversion of multi-geophysical datasets based on 3D unstructured grids.It establishes a joint inversion framework for multiple geophysical methods,achieving a wide-ranging petrophysical constraint technique suitable for complex prior information and terrains.This technique not only improves the fault tolerance of rock petrophysical constraints but also facilitates their application and promotion in joint inversion,providing effective technical assurance for resource evaluation,oil and gas exploration,mineral exploration,and geological disaster prevention.