Study On Semi-airborne Transient Electromagnetic Exploration And Transcranial Photoacoustic Imaging

Electromagnetic detection is a detection method that uses electromagnetic field to detect the internal condition of an object.Nowadays,electromagnetic detection technology has become an important method of exploration,which is widely used in petroleum,geological exploration,urban construction,military,biomedical and other fields.For low frequency electromagnetic detection,this paper presents the inverse algorithm of the Distorted Born Iterative Method(DBIM)base on the Grounded Electrical-source Air-borne Transient Electromagnetic(GREATEM).The Fréchet derivative is derived for the electromagnetic response at the receivers due to a small perturbation of the conductivity in a certain layer underground.The initial expression of the Fréchet derivative has an ex-pensive triple integral and contains the Bessel function in the integrand.It is simplified by partially eliminating the integration along the source line and deriving the analytical expression for the integration in the vertical direction inside the perturbed layer.In the inverse solution,we use the distorted Born iterative method(DBIM).Verification of in-version results under different noise and different stratification.Compared with the Occam method,the DBIM algorithm has advantages in inversion accuracy and inversion speed.This is the first time that the DBIM is applied to data measured by the GREATEM system.We propose a data processing method based on the GREATEM system,the data recon-struction results can be consistent with the existing geological data.In order to solve the jump phenomenon in the inversion results,we increase the horizontal and vertical con-straints.The reconstructed results are agreement with the results of other methods.In the high-frequency GPR detection,the Fréchet derivative formula for the formation permittivity and layer thickness perturbation is derived.The DBIM inversion formula based on multiple parameters is given.Single-parameter and multi-parameter results were reconstructed using simulation data.The results show that the algorithm can reconstruct the real value in single parameter.In the case of multiple parameters,the error between the deepest layer and the true value is large due to the increase of the unknown amount,but the trend of the parameters can be well reconstructed.In the optical wave band,photoacoustic imaging is the fastest growing detection method in recent years.However,non-invasive PAT of mouse brains with intact skulls has been a challenge due to the skull s strong acoustic attenuation,aberration,and reverberation,especially in the high-frequency range(>15 MHz).In this work,we systematically investi-gated the impacts of the murine skull on the photoacoustic wave propagation(i.e.,the for-ward model)and on the PAT image reconstruction(i.e.,the inverse model).In the forward model,for the first time,we simulated the detailed photoacoustic wave propagation from a point target through a digital mouse skull.We studied the wave aberration due to the a-coustic impedance mismatch at the skull boundaries and the mode conversion between the longitudinal wave and shear wave.The wave s reverberation within the skull was inves-tigated for both longitudinal and shear modes.In the inverse process,we reconstructed the transcranial photoacoustic computed tomography(PACT)and photoacoustic microscopy(PAM)images of a point target enclosed by the mouse skull,showing the skull s different impacts on both modalities.We experimentally validated the simulations by imaging an in vitro mouse skull phantom using representative transcranial PAM and PACT systems.The experimental results agreed well with the simulations and confirmed the accuracy of our forward and inverse models.We propose to combine the model structure and model parameter information to reduce the artifacts caused by the skull during the reconstruction process.Simulation and experimental results show that the reconstruction algorithm with model parameters can reduce the impact of the skull.Finally,the propagation of photoa-coustic signals in human skulls is studied.The results show that the influence of skulls on photoacoustic signals is more serious due to the thicker and stratified human skulls.The simulation results show that the reconstruction algorithm,which takes into account the structure and parameters of the skull,can effectively reduce the impact of the skull.