Geophysics Study Guide

📖 Core Concepts Geophysics – the physical science that uses quantitative measurements to study Earth’s shape, gravity, magnetic & electromagnetic fields, internal structure, and surface processes. Primary study areas – Earth’s shape (ellipsoid), gravitation, magnetism, seismic vibrations, electrical & electromagnetic phenomena, fluid dynamics, heat flow, and mineral physics. Gravity – measured as variations in acceleration $g$ and the geopotential; anomalies reveal dense ore bodies or subsurface structures. Seismic vibrations – waves (P, S, surface) that travel through the interior; their speeds and paths map Earth’s layers. Magnetism – a dipole‑like field generated by fluid motion in the liquid outer core; records of past reversals are used in magnetostratigraphy. Radioactivity & Heat – decay of $^{40}$K, $^{238}$U, $^{235}$U, $^{232}$Th supplies 80 % of internal heat, driving mantle convection and the geodynamo. Fluid dynamics – mantle flow (high‑viscosity fluid over geological time) and core convection create plate motions and magnetic fields. Earth’s interior – crust, mantle (upper, transition, lower, D″), liquid outer core, solid inner core; discontinuities (e.g., Moho) are seismic velocity jumps. Geodesy & Remote Sensing – GPS, VLBI, gravimeters, and satellite missions (GRACE, magnetometers) provide precise positions, gravity, and magnetic field maps. --- 📌 Must Remember Earth is an oblate ellipsoid due to centrifugal force. Geoid ≈ mean sea level if oceans were in equilibrium. S‑waves do not travel through the outer core ⇒ outer core is liquid. Gravity anomalies → dense (positive) or low‑density (negative) subsurface features. Magnetic field is a tilted dipole; reverses every 0.44–1 Myr (last full reversal ≈ 41 ka). Radiogenic heat ≈ 80 % of Earth’s internal heat budget. Global surface heat flow ≈ $4.2 \times 10^{13}\,\text{W}$. Magnetosphere extends 10 Earth radii sun‑ward; contains Van Allen belts. PREM is the standard 1‑D Earth model (density, velocity, $Q$). Seismic reflection images structures to several km depth; refraction probes deeper velocity structure. --- 🔄 Key Processes Gravity anomaly survey Measure $g$ at many stations → compute Bouguer anomaly → map density contrasts. Seismic reflection workflow Generate controlled source → record travel times → apply normal‑move‑out → migrate to produce subsurface image. Magnetotelluric (MT) sounding Record natural electric & magnetic fields → calculate impedance → invert for conductivity vs depth. Radiometric dating (e.g., U‑Pb) Measure parent/daughter isotope ratios → apply decay law $N = N0 e^{-\lambda t}$ → solve for age $t$. Mantle convection cycle Heat → thermal expansion → buoyant upwelling → plate spreading → slab subduction → cooling. GPS positioning Receive signals from ≥ 4 satellites → trilaterate receiver coordinates → apply differential corrections for cm‑scale accuracy. --- 🔍 Key Comparisons Gravity vs. Magnetic surveys – gravity detects density contrasts; magnetics detect susceptibility (ferrous) contrasts. Seismic reflection vs. Refraction – reflection resolves sharp interfaces; refraction maps velocity gradients at depth. Inner core vs. Outer core – solid inner core (supports S‑waves) vs. liquid outer core (no S‑waves, conducts electricity). Radiogenic vs. Primordial heat – radiogenic is produced continuously by decay; primordial is leftover from formation. GPS vs. VLBI – GPS provides rapid, global positions; VLBI offers the highest long‑term stability for reference frames. --- ⚠️ Common Misunderstandings “Earth is a perfect sphere.” → It is an oblate ellipsoid with equatorial bulge. “All rocks have the same specific gravity.” → Surface rocks 2.7–3.3; deep material ≈ 5.5. “Magnetic field is static.” → It undergoes secular variation and reversals. “Seismographs only record earthquakes.” – They record any ground motion, including cultural noise. “Higher temperature always means lower viscosity.” – Viscosity also strongly depends on pressure and mineral phase. --- 🧠 Mental Models / Intuition Earth as an onion – each layer (crust → mantle → core) has distinct seismic speeds, density, and physical state. Mantle flow = slow‑moving honey – high viscosity but flows over millions of years, dragging plates. Magnetic field like a dynamo – moving conductive fluid (liquid iron) in the outer core acts like a giant electric generator. Gravity anomaly = hidden hills and valleys – positive anomalies = “mass hills,” negative = “mass valleys.” --- 🚩 Exceptions & Edge Cases S‑wave shadow zone – only absent in the outer core; the inner core still transmits S‑waves at high frequencies. Mantle plumes – not all hot spots correlate with deep‑mantle plumes (some are lithospheric). Magnetic reversals – intervals are irregular; the Laschamp event (≈ 41 ka) was a brief reversal, not a full polarity flip. GPS errors – ionospheric delays and multipath can degrade accuracy if not corrected. --- 📍 When to Use Which Gravity survey → locate dense ore bodies, basins, or salt domes. Magnetic survey → map ferrous mineralizations, volcanic intrusions, and seafloor spreading stripes. Seismic reflection → detailed imaging for hydrocarbon exploration or fault mapping. MT or EM methods → investigate deep conductivity (e.g., fluid-filled fractures, molten zones). Radiometric dating → obtain absolute ages of rocks/minerals when suitable isotope systems are present. GPS → high‑precision positioning for deformation studies; VLBI for defining the celestial reference frame. --- 👀 Patterns to Recognize Linear magnetic anomalies → mid‑ocean ridge spreading direction. Positive gravity anomaly coincident with low‑velocity seismic zone → hot, less dense mantle plume. Regular spacing of seafloor magnetic stripes → symmetric age pattern about a ridge axis. Elevated heat flow → active volcanism or thin lithosphere. Cluster of shallow, high‑frequency earthquakes → brittle crustal deformation zone. --- 🗂️ Exam Traps Distractor: “All S‑waves are blocked by the mantle.” – False; only the liquid outer core blocks them. Distractor: “Magnetic reversals occur every 500 kyr.” – Intervals vary widely (0.44–1 Myr). Distractor: “Higher gravity always means higher elevation.” – Gravity anomalies are independent of topography; the geoid separates the two. Distractor: “Radiogenic heat is the only heat source.” – Primordial heat and phase‑transition heat also contribute. Distractor: “GPS provides absolute positions without correction.” – Requires differential processing and atmospheric corrections for high accuracy.