2D Magnetic Resonance Tomography And 3D Forward Modeling For Groundwater Exploration

Surface nuclear magnetic resonance(SNMR)technique is the only one geophysical method which can be used to explore groundwater directly and quantitatively.And it has great potential for aquifer characterization,water resource management and pollution monitoring.During the last decade,SNMR has got substantial improvements in instrumentation,data processing,forward modeling,inversion,and measurement techniques.Now it has emerged as a promising method for groundwater system characterization,water resource management and near-surface geophysics studies.Since the first commercial available instrument was born in the early 1990 s,although extensive surveys and tests have been conducted in various hydrogeological environments and field conditions,particularly in sandy aquifers but also in clay-rich formations as well as in fractured hard rock.Currently,SNMR technique is in rapid development towards a common hydrogeophysical method.It still exists some shortages,such as limited exploration depth,weak abilities of anti-disturbance,not good enough 2D/3D groundwater exploration effects and so on.Futhermore,conventional SNMR surveys based on 1D inversion of amplitude data recorded only by coincident loops provide limited or distorted groundwater distribution information,especially in regions characterized by strong lateral heterogeneity and complicated hydrological environments.These simple strategies limit the application and efficiency of SNMR technique.In response to these issues,based on the most sophisticated signal response formulation,the elliptical polarization parameters of excitation magnetic field and 2-D sensitivity kernels(including real and imaginary parts)of three commonly used loop configurations are calculated first.After incorporating all individual complex signals of five series of simulated measurements along a profile,the data sets of three loop configurations are utilized to yield tomography of synthetic aquifer models separately.Through taking full advantages of differentiated sensitivity distributions offered by different loop configurations and high sensitivity of imaginary part signal to deep structures,the introduced 2D magnetic resonance tomography(MRT)complex inversion scheme implements high resolution tomography of synthetic models under the three loop configurations.Contrastive analyses of tomographic results show that complex inversion can significantly reduce model ambiguities and increase depth resolution,even in the case of adding artificially noise.Besides,COI loop measurements usually yield best vertical resolution and separated loops can provide higher lateral resolution.Additionally,to promote the development of 3D groundwater exploration and expand the application range of SNMR,we presents a vector finite-element(FE)method for its 3D forward modeling.The key of SNMR forward modeling is the calculation of excitation magnetic field,and the total field algorithm is an alternative scheme to numerically simulate the electromagnetic(EM)field.In this paper,we viewed the circular loop source as the combination of a certain number of horizontal electric dipoles(HEDs).The unstructured tetrahedral mesh and local refinement technology were combined to precisely delineate the distribution of circular loop source,which effectively reduce the adverse effects of field source singularity.Then,the vector FE solver based on the total electric field was used to calculate the magnetic field distribution.The accuracy of the calculated results was validated by the analytical solutions for a circular loop laid on the surface of a homogeneous half-space.After calculating the excitation field,SNMR responses of three typical synthetic 3D groundwater models were obtained with the basic signal response formula.We also studied the effects of some important factors which have significant influences on SNMR signal responses and have practical importance in groundwater issues.This study will help to expand the application range of SNMR technique,and promote SNMR successfully applied in the complex hydrogeological environment.