Estimation of Moho depth based on the inversion of gravity anomaly data from different sourcesCase study: Khorasan region

 Estimation of the Moho depth and thickness of the crust using the gravity anomaly datais one of the basic researches in the geophysics and geology sciences.

Based on many geophysical studies, the three-dimensional thickness determination of the density variationinterface using the gravity anomaly is a common method. One practical instanceis the modeling of the crustal discontinuity like Mohorovicic discontinuity using the gravity anomalies. To analyze the anomalies associated with these crustal discontinuities, many techniques are used. Among the common methods generallyused for estimating the Moho depth and studying thecrustal structure arethe analysis of surface and body wavesof the earthquakesrecorded at theseismological stations, the analysis of post-seismic waves, the gravity data inversion method and thermal analysis. In these cases, the inversion of the filtered gravity anomalies for determining the interface geometry of the density variations is one of the main goals. Different researches have proposed different methods for calculating the interface geometry of the density variationsbased on thegravity anomaly. Many of them approximate an irregular body with several cubic prismelements withconstant density. The overall gravity field of the bodyis calculated based on the sum of the gravity field effects of the prisms. Some methods such as Oldenburg (1974) have been developed based on the rewrite of Parker's forward method (Parker, 1973). Based on the Parker’s method,the Fourier transform of the gravity anomaly is consideredas an outcomeof thesum of theFourier transforms of the createddepth powersrelated tothe gravity anomaly. Oldenburg shows that theParker's formula can be rewritten to determine the geometry of the density interface from thegravity anomaly data. In this method, the Parker’s formula inversion is used to calculate the gravity anomaly created by an uneven layer of materials based on the Fourier series. Oldenburg rewrote this formula to calculate the interface depth of the density with undulating geometry using thegravity anomaly based on an iterative method (Parker-Oldenburg method). Therefore, the topography ofthe densityinterface is estimatedbased onan iterative inversion method, which is repeated until an acceptable solution is obtained. According to the method (Oldenburg, 1974), the process is convergedin casethe depth of the interface is greater than zero and is not removed from the topography. Moreover, the range of theinterface variations should be less than the average depth of the interface. When a specific number of iterations is performed or the difference between two successful approximations is less than a specific value, the iterative procedure ends. In general, this gravity anomaly modeled by the inversion method should be very similar to the input gravity anomaly in the first stage. This paper investigates the Moho depth behavior using gravity anomaly data based on the Parker-Oldenburg method. The formula rewritten by Oldenburg through integration with the Parker’smethod called the Parker-Oldenburg method is used here to obtain the results by the iterative inversion method oftheFourier transform of the gravity anomaly. Since this method is based on the Fast Fourier Transform(FFT), it has a very high speed which can be used to compute models with a very high number of points without spending too much time on computation. Good results can also be achieved by using a high-quality gravity field.  

In this study, the gravity anomalies derived from EGM08, EGM96 geopotential models and one of the GOCE-based global geopotential models (obtained only from the global satellite gravimetry data of GOCE), as well as those derived from terrestrial gravity data provided by the National Cartography Center (NCC) have been usedin Khorasan region. A  cell grid has been createdto generate the gravity field and estimate the Moho depth. Investigation of the results obtained from theMoho depth calculation in this region shows that the Moho depth model obtained from NCC data is very different from other models due to the limited number of observation points to reach the gravity field interpolation model. The difference of theMoho depth derived from the EGM08 model and the onederived from theEGM96 and GOCE models, gave 1.66 and 1.07 km for the RMS values, respectively. This accuracy improvement can be attributed to the quality and resolution of the geopotential models. Furthermore, comparing the results of the GOCE model with the EGM96 model, the RMS value is 0.85 km which is due to the close proximity of the two models’ qualities.  

In this paper, the Moho depth model has beenobtained based on the Parker-Oldenburg method using the gravity anomaly data forKhorasan region. In this method,the Fourier transform ofthe gravity anomalies accelerates themodeling for a large number of points. On the other hand, the high-quality of the models for the production of anomaly, results in the production of thehighly precise geometry of the density interface to a certain extent.