Principles of Geophysics (2024-2025) PDF

Principles of Geophysics (Geology, Geochemistry, Geophysics, Geology/Chemistry) 2024 – 2025 Prof. Dr. Adel A. A. Othman 1 Structural elements, layer-thickness, depth, dip, are considered as the geometrical parameters derived from...

Principles of Geophysics (Geology, Geochemistry, Geophysics, Geology/Chemistry) 2024 – 2025 Prof. Dr. Adel A. A. Othman 1 Structural elements, layer-thickness, depth, dip, are considered as the geometrical parameters derived from geophysical measurements. 2 The degree of cracking and weathering, contamination, radiological parameters are the spatial distribution of petrophysical/geophysical parameters. 3 Various geophysical methods are combined for enhance the reliability of interpretation. 4 Due to the ambiguity of interpretation, drilling is often necessary to confirm the results. 5 Physical properties of rocks in dams, tunnels, shafts, are one of the civil engineering problems. 6 The Centrifugal force is one of the Sources of gravitational Earth’s field. 7 Grid length (s) should be less than the depth (h) of the geologic feature. 8 Large-scale gravity surveys start from1 to n10 km for mapping regional geological structures. 9 Small-scale gravity surveys are from 10 to n100 m for detailing local geological features. 10 For reconnaissance site surveys, the grid length in microgravity surveys start from 15 to 30 m. 11 For investigating shallow geological features in High-resolution microgravity surveys, the grid length starts from 2 to 10 m. 12 Normal correction means removing the regional gravity effect by regression analysis. 13 Derivatives of gravity field are very sensitive to noise due to near-surface topographic irregularities. 14 Derivatives of gravity field, allow the enhancement of the gravity anomalies of small and shallow geological features. 15 After gravity data reduction some gravity anomalies as 10 µGals were detected, but after inversion near-surface density distribution associated with caves and voids was estimated. 16 Local magnetic anomalies are caused by subsurface bodies having different susceptibilities and magnetization. Page 1 of 3 17 Direction of anomalous magnetic field is compared to that of the ambient field. 18 The Magnetic Hysteresis loop, shows the behavior of a ferromagnetic core graphically as the relationship between B and H is non-linear. 19 By the Proton precession magnetometer, the observed quantity is the precession frequency from which the absolute value of magnetic field (B) is derived. 20 Absolute accuracy ~ 0.1 nT in Proton precession magnetometer. 21 Analytic signal enhances the edges of magnetized bodies (e.g. faults, structural contacts) relative to magnetic field T. 22 Magnetic Gradiometry confirmed automatic cancellation of diurnal effect. 23 Mud brick walls as an archeological feature applying magnetic method measure 10–50 nT 24 Asymmetric magnetic signatures may also be caused by a preferred alignment of the long axes of ellipsoidal UXO (Unexploded Ordnance) targets. 25 Aquifer transmissivity is T=kh in m2/d, where h is the aquifer thickness. 26 In induced polarization method, the apparent polarizability is derived from the measurement of potential difference. 27 Frequency domain EM (induction) method applied in shallow environmental investigation (0.320 kHz). 28 FDEM (frequency domain electromagnetic) methods used to locate buried tanks and pipes, pits and trenches containing metallic and/or nonmetallic debris. 29 In Very Low Frequency Method, we measure the EM field of a 10-30 kHz radio transmitter from a distance of n100-n1000km. 30 In Very Low Frequency Method, Maximal depth of investigation is approximately 100 m. 31 An object with high resistance is going to have a smaller phase shift in VLFM. 32 In Magnetotelluric Method, depth of investigation is controlled by changing frequency. 33 TDEM Methods are (relatively insensitive to ground structures such as fences, buildings, and vehicles) 34 In GPR, Transmitter emits high frequency (25 MHz−2.6 GHz) EM pulses into the ground, we record the amplitude and travel time of the energy reflected back to the surface. 35 Natural gamma-ray intensity is used to determine of lithology, depth of porous/permeable rocks, effective layer thickness, shale volume, classification of clay minerals. 36 Temperature log is used to measure Fluid inflow and outflow, abnormal radioactivity, oxidation regions. Page 2 of 3 37 NMR (Nuclear Magnetic Resonance) applied for Free-water porosity, irreducible water saturation, pore-size distribution, hydraulic conductivity. 38 Traditional variable-frequency IP (using two or more frequencies of< 10 Hz), is one of IP techniques. 39 IP is sensitive to dielectric rather than conductivity characteristics. 40 Apparent chargeability (Ma) increases with increasing duration of the pulses, (3-5 seconds). 41 Using the same array as in DC resistivity measurements but driving AC current at several frequencies. 42 Spectral (complex resistivity) IP, apply AC current at a range of frequencies from 30 to 4000 Hz. 43 One of the advantages of the electrical resistivity methods is its costs less than drilling. 44 The resolution is one of the disadvantages of the electrical resistivity methods. 45 The measured apparent resistivity, ρa, depends on the shape and size of anomalous regions, layering and relative values of resistivities in these regions. 46 Wave velocity values are related to the elastic moduli and density of the medium. 47 One of the applications of seismic methods is to monitor micro-earthquakes during mining or oil/gas production by fracking. 48 Refraction seismic detect high-velocity formations, Depths to beds, velocities and gradients. 49 Reflection seismics detect geometrical types of subsurface structures and sediment velocities. 50 Seismic velocities are generally increase with pressure, depth of burial, and decrease with temperature. 51 Rippability is estimated from seismic velocities obtained from short and shallow seismic refraction surveys. 52 Surface wave velocities are sensitive to all three parameters K, µ, and ρ. 53 Refraction only depends on wave velocities and is insensitive to density. 54 At oblique incidence, R (reflection coefficient) depends on the incidence angle (Φ) and is used to constrain the “Amplitude Variation with Offset” (AVO) analysis. 55 At higher frequencies, surface waves sample shallower depths, and their velocities are slower (plots below). This dependence V(f) is called velocity dispersion. Page 3 of 3