O

Objective Function  – a 3-D integral function which, being minimized, produces a 3-D model of the subsurface structure as a result of Minimizing Inversion. Mathematically, O.F. can be defined by a sum of integrals combining the initial (i.e., reference) model approximation parameters with spatially dependent weighting functions and their corresponding coefficients that affect the relative importance of different model components. O.F. has the flexibility of constructing various models and incorporating into Inversion process of additional information other then gravity or magnetic data. ^[144]. See also Parametric Inversion.

Observation Platform  – see Moving Platform.

Observed Gravity  – the Earth’s gravity field attraction (acceleration) value measured at any point on or above the Earth’s surface. O.G. is the sum of various components that contribute to the measured value (the name of a corresponding correction for each component is shown in parenthesis):

O.G. = attraction of Reference Ellipsoid (Theoretical Gravity Correction)

+ effect of elevation above or below sea level (Free-Air Correction)

+ effect of mass between sea level and level of observation (Bouguer Correction)

+ effect of mass above and/or below observation point (Terrain Correction)

+ time-dependent variation (Tidal Correction) + effect of moving

observation platform in marine and airborne surveys (Ëotvös Correction)

+ effect of masses that support topographic loads (Isostatic Correction)

+ effect of Crust density variations (i.e., effect of local and regional geological structures).

            The purpose of Gravity Corrections is to isolate the last component in this formula. The gravity acceleration due to the Earth’s mass is about 9.8 m/sec² (980 Gal), whereas the crustal density variations are about 10^–3 m/sec² (100 mGal) or less, i.e., the gravity anomalies of exploration interest are less than 0.01% of O.G. ^[25].

Observed Magnetics  – the sum or integrated effect of the following components, which contribute to the value measured at any point on or above the Earth’s surface: 1) Earth’s main (core) magnetic field; 2) Earth’s local (regional and residual) magnetic field; 3) irregular solar radiation energy pulsations (Magnetic Storms); 4) regular fluctuations of the Earth’s external magnetic field due to solar radiation activity (Diurnal Variations); 5) man-made effects (Cultural Noise). The purpose of Magnetic Corrections is to isolate the second component, i.e., local magnetic field. The Earth’s main magnetic field varies from 70,000 nT at magnetic Poles to 25,000 nT at magnetic Equator, whereas local magnetic field varies from 0.1 to 100 nT over magnetically quiet, deep sedimentary basins up to several thousand nT over highly magnetized crystalline basement outcrops, i.e., the magnetic anomalies of exploration interest, in average, are less than 2–10% of the Earth’s main (core) magnetic field, which is the dominant component of O.M. ^{[54, 238].} See Earth’s Magnetic Field Components.

Oceanic Crust  – see Crust.

Octave  – an interval between two Spatial Frequency or Wavelength values having a ratio of two (i.e., 1200 m and 2400 m in wavelength values). O. is used to define filter response parameters. ^[223]. See Cutoff Wavelength Width.

Offleveling Errors  – the airborne gravity measurement errors that result from high-amplitude horizontal accelerations of the survey aircraft due to course changes or turbulence, which drive the gravimeter gyro-stabilized platform offlevel. ^[37]. See also Aerogravity Corrections and Stabilized Platform.

Offset Zones  – see Interruption Zones.

One-Dimensional (1-D) Directional Filtering  – a space-domain Microleveling technique which is based on the use of 1-D directional filters applied to the standard leveled magnetic grid. At first, the original grid is low-pass filtered in the traverse-line direction with a wavelength Cutoff value greater than the tie-line spacing. Then, this grid is high-pass filtered with a wavelength (cut-off value) 2 to 4 times the traverse-line spacing in the direction of the control lines. The obtained raw leveling error grid containing the elongated anomalies (“line corrugation”) is subtracted from the original grid to obtain the final 1-D directional filtered grid. ^[67]. See also Two-Dimensional (2-D) Directional Filtering.

One-Dimensional Fast Fourier Transform (1-D FFT) Filters  – a group of Fourier Domain filters that are applied to the line (i.e., one-dimensional) gravity or magnetic data. These filters are also referred to as Channel Filters or Line Filters. See also Two-Dimensional Fast Fourier Transform (2-D FFT) Filters.

One-Dimensional Fast Fourier Transform (1-D FFT ) – a mathematical procedure that converts the line (profile) gravity or magnetic data from their original Space Domain into the equivalent Frequency Domain. After 1-D FFT, gravity and magnetic profiles can be analyzed for their frequency (wavelength) content. See also Fourier Transform.

On-Site Magnetometer Calibrations  – a set of tests of the airborne magnetometer readings made prior to the commencement of routine survey flying and whenever the configuration of aircraft and/or magnetometer system was altered. The following tests are usually made: Compensation Test, Lag Test and Heading Error Test. These tests are also referred to as Pre-Survey Magnetometer Calibrations. ^[58]. See also Figure-Of-Merit (FOM).

Operating Range  – an instrument characteristic that defines a difference between the largest and the smallest possible measurement values.

Optically Pumped Magnetometer  – a cesium-vapor or rubidium-vapor Magnetometer that measures the Earth’s magnetic field using the nuclear magnetic resonance between the frequency of precessing atoms in vapor cells, excited by the monochromatic polarized light, and the transverse radio-frequency field at Larmor Frequency. The achievable accuracy is up to 0.001 gamma and far exceeds the requirements of the contemporary exploration efforts. ^{[57, 223].} See also Cesium Magnetometer, Fluxgate Magnetometer and Proton Precession Magnetometer.

Optimum Filtering  – a filter procedure which pre-supposes the knowledge of a desired output (for example, such as a level of irregular noise to be suppressed) and involves the application of individually designed filters for each processing problem. See Wiener Filter.

Outer Terrain Correction  – see Outer Zone Terrain Correction.

Outer Zone Terrain Correction  – a correction applied to the gravity data to compensate for the deviation of the surface topography from flat Bouguer Slab approximation by using a Zone Chart where the outermost radius equals 21,826 ft (7270 m) and the innermost radius equals 558 ft (186 m). See Terrain Correction and Inner Zone Terrain Correction.

Overhauser-Effect Magnetometer  – a high-sensitivity magnetometer of a proton-precession type.  The proton nuclei are aligned with and precess about the Earth’s magnetic field by interaction with free electrons which are brought to a resonance through the excitation by a VHF field.  The frequency of a proton nuclei precession (i.e. Larmor Frequency) is measured to determine the value of the Earth’s magnetic field. O.-E.M. is used in the marine magnetic surveys, often, combined with the seismic acquisition system.  In this case, O.-E.M. is towed behind a seismic tail buoy to provide operation in the magnetically quiet zone.  Also referred to as Marine Magnetometer and Tail-Buoy Magnetometer.  See also Proton Precession Magnetometer.

Overhead Point  – a Sun Angle Image parameter which defines the center of the image shown in the display window. See also Sun Declination and Sun Inclination.