Document Type : Research Article
Institute of Geophysics
In standard gravimetric correction methods, after the raw gravity data sets were corrected for drift, tide, latitude, and free-air effects to obtain free air anomalies, the effect of the mass between the reference surface and ground surface eliminates in two steps including Bouguer and terrain corrections. But this study removes this effect in one step through the forward modeling method. To do this, two things are necessary for finding more accurate answers. First, how the underground discretization, and to what extent a network of Digital Terrain Model (DTM) is available? Quad tree mesh accessible in Simulation and Parameter Estimation in Geophysics (SimPEG) is a very accurate and advanced meshing algorithm to discretize subsurface based on our requirements. This meshing system can choose the size of cells in the desired locations. Hence, using this flexible discretization it is possible to define the smaller cells in borders, near the topographic region, which helps to more precise answers. Having a dense DTM, the SRTM GeoTiff pictures are downloaded from USGS Earth explorer with 1 arc-second (90 m) resolution (https://doi.org/10.5066/F7PR7TFT), and then height information is extracted from these pictures through GeoToolkit (http://toolkit.geosci.xyz) script. Assuming a flat geoid for our study area, topography extracted from the SRTM pictures is interpolated to estimate the elevation at the gravity observation points.
The gravity effect of the model space (the space between the reference surface and topography) is computed via numerical forward modeling assuming a constant density (2.67 gr/cm3). This procedure is done by the Simulation module in SimPEG and is considered as the Bouguer and terrain corrections simultaneously. These corrections are subtracted from the free-air anomalies, which yields the complete Bouguer anomaly.
This method is powerful in contrast with other standard methods. In standard methods, Bouguer correction considers Bouguer slab approximation. Therefore, accuracy is lost. Also, in large-scale problems, curvature correction becomes necessary. Also, terrain correction for removing the effects of the mass between the lowlands and heights of the region is inevitable. Terrain correction considers two approximations. First, use average height. Hence the procedure does with low precision. Second, divide the surrounding area into three zones (near, middle, and far) and computes the effects of middle and far zones with lower precision. Therefore, decrease the accuracy of the results.
The mentioned method is tested on 399 ground gravity data with a grid spacing of about 5 km prepared by the National Cartographical Center of Iran (NCC) in an area of about 200 km in 200 km located in parts of Central Zagros and Central Iran. The results obtained from this one-stage correction method are more accurate in results and less complicated in running compared to the results of the usual procedure. Because in this method, we have no simplifying assumptions such as infinite Bouguer slab in Bouguer correction or using relative heights in terrain correction that there is in standard methods.