Simple linear regression (SLR) models for rapid estimation of true subsurface resistivity from apparent resistivity measurements are developed and assessed in this study. The objective is to minimize the processing time and computer memory required to carry out inversion with conventional algorithms. The arrays considered are Wenner, Wenner-Schlumberger and dipole-dipole. The parameters investigated are apparent resistivity (ρ a) and true resistivity (ρ t) as independent and dependent variables, respectively. For the fact that subsurface resistivity is nonlinear, the datasets were first transformed into logarithmic scale to satisfy the basic regression assumptions. Three models, one each for the three array types, are thus developed based on simple linear relationships between the dependent and independent variables. The generated SLR coefficients were used to estimate ρ t for different ρ a datasets for validation. Accuracy of the models was assessed using coefficient of determination (R 2), F-test, standard error (SE) and weighted mean absolute percentage error (wMAPE). The model calibration R 2 and F-value are obtained as 0.75 and 2286, 0.63 and 1097, and 0.47 and 446 for the Wenner, Wenner-Schlumberger and dipoledipole array models, respectively. The SE for calibration and validation are obtained as 0.12 and 0.13, 0.16 and 0.25, and 0.21 and 0.24 for the Wenner, Wenner-Schlumberger and dipole-dipole array models, respectively. Similarly, the wMAPE for calibration and validation are estimated as 3.27 and 3.49%, 3.88 and 5.72%, and 5.35 and 6.07% for the three array models, respectively. When compared with standard constraint least-squares (SCLS) inversion and Incomplete Gauss-Newton (IGN) algorithms, the SLR models were found to reduce about 80-96.5% of the processing time and memory space required to carry out the inversion with the SCLS algorithm. It is concluded that the SLR models can rapidly estimate ρ t for the various arrays accurately.

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Analyses and interpretation of magnetic and gravity fields data acquired at Bukit Bunuh, Lenggong, Perak, Malaysia have been carried out in this study. The objective is to identify the meteorite impact crater and possible rebound locations. This study was instigated by an archaeological investigation carried out by the Centre for Global Archaeological Research, Universiti Sains Malaysia. The investigation suggested evidences of shock metamorphisms (suevite breccia) and crater morphology at Bukit Bunuh in Lenggong area of northern Perak, Malaysia. To justify the occurrence of the impact crater, ground magnetic and gravity surveys were conducted first as regional study in the entire area, followed by detailed study at the suspected crater region. Data from both surveys were compiled and corrected. The residual magnetic data ranged between −272 nT and +134.2 nT, whereas the Bouguer gravity data were between −25 mGal and 120 mGal. Both the magnetic and gravity data were gridded and plotted for qualitative interpretation. It was found that low magnetic and gravity region, believed to be an impact crater which is now filled with sediments and surrounded by a highly magnetised shallow bedrock, exists at the mid-part of the study area. Other high magnetic and gravity key features located within the central regions are interpreted as rebounds. In conclusion, the impact structure was successfully modelled as a complex impact crater.

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