A GUIDE TO THE LANGUAGE OF GEOPHYSICAL INTERPRETATION
The following terms are intended for readers not versed in potential fields terminology or for those readers interested in a review of terms as used by IGC.
Analytic Signal Method
The analytic signal method, known also as the total gradient method, as defined here produces a particular type of calculated gravity or magnetic anomaly enhancement map used for defining in a map sense the edges (boundaries) of geologically anomalous density or magnetization distributions. In exploration potential field applications, the term analytic signal loosely refers to the calculated modulus of the gravity or magnetic anomaly field’s three mutually orthogonal spatial (x, y, z) derivative terms. Mapped maxima (ridges and peaks) in the calculated analytic signal of a gravity or magnetic anomaly map locate the anomalous source body edges and corners (e.g., basement fault block boundaries, basement lithology contacts, fault/shear zones, igneous and salt diapirs, etc.). Analytic signal maxima have the useful property that they occur directly over faults and contacts, regardless of structural dip which may be present, and independent of the direction of the induced and/or remanent body magnetizations. Various extensions to the analytic signal method (as defined here) exist. For example, some extensions to the method include as an additional solved parameter the anomalous source body depth(s).
Automated Depth Estimation
A variety of techniques, which include Werner deconvolution, the Euler method, Naudy’s method,Phillips’ method, and the analytic signal method, which analyze digital analytic signal method magnetic profiles or maps to obtain estimates of source body depth without specific user identification of key portions of anomalies. This contrasts with profile techniques such as Peters’ method (half-slope) or Vacquier’s method (straight slope) which may be implemented as computer programs but require interactive identification of special points on anomalies.
Bouguer Gravity Field
The gravity field obtained after latitude, elevation, Bouguer, and terrain corrections have been applied to the measured (observed or raw) gravity data. The Bouguer (named after Pierre Bouguer, a French geodesist) gravity field is often noted as simple Bouguer for the gravity field before applying terrain corrections or complete Bouguer for the gravity field after applying terrain (and sometimes curvature) corrections.
The gravity anomalies observed in the Bouguer field are caused by lateral density contrasts within the sedimentary section, crust and sub-crust of the earth.
A measured above sea level Bouguer gravity field and accurately corrected to sea level datum is not equivalent to gravity measured at sea level. Anomalies caused by mass inhomogeneities between station elevation and datum and which were measured at the original station elevations remain in the data unless special corrections are made.
Mass per unit volume, expressed in grams per cubic centimeter. Rock or formation densities are usually measured as either saturated bulk densities or grain densities. For gravity interpretation, the contrasts between rock bulk densities are of primary interest since these contrasts are responsible for the anomalous gravity field.
Rock bulk densities have been shown to vary as a function of geologic age, lithology and depth of burial. Rock densities typically range from 1.9 g/cm3 to 3.0 g/cm3.
The density of one rock unit relative to another. Density contrasts can be either positive or negative. For example, if Rock A = 2.30 g/cm3 and Rock B = 2.40 g/cm3 then the density contrast of Rock A relative to Rock B is -0.10 g/cm3. Conversely, the relative density contrast of Rock B relative to Rock A is +0.10 g/cm3.
Gravity anomalies caused by density contrasts within the earth’s sedimentary section, crust and sub-crust can be analyzed and interpreted as lithologic and/or structural anomalies.
The relationship between the change in density with a change in depth. In many areas of the world with thick clastic sections the increase in density with an increase in depth has been shown to be primarily a function of compaction. However, age, lithology and porosity may also influence the relationship. The relationship is important in gravity modeling because a gravity anomaly may be caused by a gradational change in density rather than a relatively abrupt density contrast, such as that which may occur at a fault, contact, or unconformity.
A model of the geology in which layers or bodies of given lithologies are replaced by equi-density layers or bodies. The equi-density layers or bodies may or may not correspond to specific geological formations.
Generically, the use of linear filters to isolate (based on wavelength criteria) anomaly contributions to a map derived from source bodies in a certain depth range. Numerous techniques are used to carry out the isolation.
See Prism. Dike model descriptions include wide, narrow, thin, vertical, and inclined.
The sum of the free-air and Bouguer corrections to observed or “raw” gravity. The Bouguer correction requires an estimation of bulk density to calculate and eliminate the gravitational effect of the subsurface mass between point of gravity measurement and a datum.
A profile-based or map-based depth estimation method based on the concept that the magnetic fields of localized structures are homogeneous functions of the source coordinates and therefore satisfy Euler’s equation. This equation can therefore be solved parametrically for the source locations. In recent years, use of this method has become more widespread because it has been automated to work with either grid or profile data.
A domain is where a mathematical function (the independent and dependent variables x and y and maybe z and perhaps more) exists. In the frequency domain, the independent variable has been transformed from a distance such as miles (seconds in the case of seismic) to frequency like cycles/mile (a spatial frequency versus a temporal frequency like cycles/second). The dependent variables then become the strength and phase of that frequency. See also, Space Domain.
Free-air Gravity Field
The gravity field after the free-air correction. This correction is applied to observed or “raw” gravity readings to correct for the change in gravity due to the difference in elevation of the gravity station relative to datum elevation (usually sea level). The change in gravity with elevation is inversely related to the change in distance between the meter’s center of mass (meter elevation) and the earth’s center of mass.
Simply, the units in which magnetic survey maps are often contoured. 1 gamma = 1 nanotesla. A unit for stating the magnitude of the magnetic field vector B represented by the number of lines of induction passing through a unit area perpendicular to the vector direction.
1 gamma = 10-5 gauss
= 10-5 lines/cm2
= 10-1 line/m2
= 10-9 weber/m2
= 10-9 tesla = 1 nanotesla
An empirically derived equation which describes the relationship between bulk densities and acoustic velocities of rocks: p = 0.23v0.25
The equation was derived by G.H.F. Gardner et al. from laboratory and field observations of brine-saturated (nonevaporite) rock types. Experience has shown that the equation (or some modification of it) is valid for many sedimentary basins of the world.
A unit of acceleration used with gravity measurements. Abbreviated as g.u. Measurements in gravity units were formerly widely used, but measurements in milligals are now more common. 1.0 milligal = 10 gravity units
A device or set of devices which measures the value of a field in at least two different points in space at the same time. The gradient is the difference in field values per unit of distance between the sensors. By measuring a field’s gradient (that is, its first derivative or rate of change with distance), the total field itself may be computed with varying degrees of accuracy. For potential fields, the direction of the measurement relative to the earth is critical. Is the gradient being measured horizontally, vertically, and in the case of magnetics, what is the orientation relative to the earth’s magnetic field? Even with these possible difficulties, measuring just the gradient has the advantage of removing non-geologic field signals, such as when measuring gravity, those introduced by the normal accelerations of the survey aircraft.
High Density Basement
The deepest significantly thick, high density unit(s) within the geologic section of an area, which provide a major positive density contrast. The rocks above the major density contrast are usually younger sediments and/or volcanics, typically having densities ranging from approximately 1.9 g/cm3 to 2.6 g/cm3. Those below the major density contrast are usually older sedimentary, volcanic and/or crystalline rocks, typically having densities ranging from 2.6 g/cm3 to 3.0 g/cm3. High density basement may or may not be equivalent to crystalline and/or magnetic basement.
High Resolution Aeromagnetics
This might more correctly be termed “high precision aeromagnetics” The term has gained wide acceptance in the industry to denote surveys flown at low terrain clearance (80 – 150 m), with close line spacings (100 – 500 m), recorded at high sample rates (0.1 – 0.25 sec.), and acquired with high-sensitivity magnetometers (0.001 – 0.005 nT).
International Geomagnetic Reference Field (IGRF)
The most widely used mathematical models for fitting the main magnetic field of the earth at a given time (e.g., 1965). The models consist of spherical harmonic coefficients derived from observatory and satellite data. They are used to objectively remove long wavelength components from survey data to obtain the anomalous magnetic field (TI), which contains the shorter wavelength components of exploration interest.
A technique whereby a 2D or 3D , density, susceptibility, or geometric (geologic) model is computed to satisfy (invert) a given observed gravity or magnetic field.
Magnetic basement is usually equated to crystalline (felsic and mafic) or sometimes, metamorphic basement. It is the unconformity upon which an essentially non-magnetic sedimentary section has been deposited. Large exposures of basement (e.g., the Canadian Shield) show it to be lithologically and magnetically heterogeneous. Very thick sequences of highly magnetic volcanics may sometimes be considered equivalent to a magnetic basement.
Magnetic Sedimentary Section
A surface or zone within the geologic column where magnetic susceptibility contrasts are significant enough to generate magnetic anomalies which could delineate sedimentary geology. Susceptibility variations within the sedimentary column are generally considered near zero except where relatively magnetic sediments (e.g., pyroclastics, arkoses, some shales) are present.
The unit of acceleration used with gravity measurements.
1 Gal = 1 cm/sec2
1 Gal = 1,000 milligals
1 mGal= 10 gravity units
The degree of homogeneity in Euler’s equation, interpreted physically as the fall-off rate with distance and geophysically as a structural index (SI). Values vary from 1 to 3 according to magnetic or gravity source body geometry.
Simply, the units in which magnetic survey maps are often contoured. 1 nanotesla = 1 gamma. A unit for stating the magnitude of the magnetic field vector B represented by the number of lines of induction passing through a unit area perpendicular to the vector direction.
1 nanotesla = 10-9 tesla
=10-5 gauss = 1 gamma
An automated profile-based depth estimation method wherein anomaly type and location are identified by cross-correlation of the observed magnetic profile with theoretical anomalies. The depth to a dike-like or plate-like source is then estimated from parameters relating source body half-width, depth, and data sampling interval.
A member of a class of software that is “trained” by presenting it examples of input and the corresponding desired output. For example, the input might be a magnetic anomaly and the required output the depth to the source of that anomaly. Training might be conducted using synthetic data, iterating on the examples until satisfactory depth estimates are obtained. Neural networks are general-purpose programs which have applications outside potential fields, including almost any problem that can be regarded as pattern recognition in some form.
Observed Gravity Field
The term “observed gravity” is also often used in lieu of “raw gravity” or “measured gravity”. Incorrectly, but often, the term “observed gravity map” may be posted on the following maps: Bouguer, free-air, regional or residual gravity field.
An automatic depth estimation method in which the source parameters are estimated from the autocorrelation function of the magnetic anomaly. Like Werner deconvolution, the method uses a dike or contact model.
A term used to describe a sheet-like magnetic source body with limited vertical dimension. That is, its thickness may range from 0.1 to 1.0 times its depth-to-top. Its anomaly character is similar to that of a set of dipoles.
A field which obeys a differential equation known as Laplace’s Equation. Gravity and magnetic fields are both vector potential fields. Most exploration gravity work utilizes the vertical component of the gravity field, while most exploration magnetic work utilizes the scalar total intensity of the magnetic field.
An approximation of a gravity field derived from a magnetic field measured at, or transformed to, the magnetic pole. The process requires conversion of susceptibility values to density values and a vertical integration of the magnetic data.
A term used to describe a magnetic source body which can be considered, for practical purposes, parallelepiped which is semi-infinite in vertical dimension. That is, its depth-to-bottom is at least four times its depth-to-top. Its anomaly character is similar to that of a monopole or line of poles. A two-dimensional prism (semi-infinite normal to the plane of section) is sometimes referred to as a dike model.
Also called measured gravity, or observed gravity. The gravity field measured at a gravity station before latitude, free-air, Bouguer or terrain corrections are applied.
A mathematical transformation of the total magnetic intensity (TI) field at its observed inclination (I) and declination (D) to that of the magnetic equator (i.e., I=0ï¿½).
A mathematical transformation of the total magnetic intensity (TI) field at its observed inclination (I) and declination (D) to that of the north magnetic pole (i.e., I=90ï¿½, D=0ï¿½).
Regional Gravity Field
The long wavelength component of the usually attributed to Bouguer gravity field density variations considered to be deeper than general exploration interest; (e.g., the gravity component due to crustal density variations or undulations of the crust/mantle interface). A subjective regional can often be designed to enhance residual anomalies of primary interest.
Residual Gravity Field
The shorter wavelength component of the attributed to density Bouguer gravity field contrasts withinhigh density basement and/or the lower density overburden. Anomalies in the residual field are usually of exploration interest.
A first residual is a difference field obtained by subtracting the regional gravity field from the Bouguer gravity field.
Second Vertical Derivative
A second vertical derivative map of a potential field may be calculated by application of a frequency domain or space domain filter to a potential field grid file. The result is an anomaly enhancement or residual map related to the “curvature” of the input field. Inflection points on the anomalies of the input field will be zero values on the derivative map and may have special interpretation significance.
A domain is where a mathematical function (the independent and dependent variables x and y and maybe z and perhaps more) exists. In the space domain, distance (1 if by profile, 2 if by map measured in perhaps feet, kilometers, degrees, seconds, etc.) is the independent variable and some quantity (milligals, gammas, density, seismic amplitude, etc.) is the dependent variable. See also, . Frequency Domain
Strike Filter (Pass or Reject)
A band-pass filter designed to pass or attenuate Fourier components of a potential field data set along a pre-determined angle (strike).
Structural Model (2D or 2ï¿½D)
A gravity or magnetic structural model is a 2D or 2ï¿½D density and/or susceptibility model of given or assumed geology. The geology of an area can be modeled by representing lithologic layers as equi-density and/or equi-susceptibility layers and/or blocks. The layers are formed by contrast boundaries which may or may not correspond to specific geologic formation boundaries. Where high density or susceptibility contrasts exist in nature, the model may correspond closely to those geologic formations.
For 2-dimensional modeling, the density and susceptibility models of the geology and theobserved gravity and magnetic anomalies for the model are assumed to be semi-infinite. For 2ï¿½-dimensional modeling, the third dimension y (in and out of the plane of the profile) is approximated by one or more given distances, thus providing a quasi-3D model.
A measure of the degree to which a substance may be magnetized. It is a ratio k of the intensity of magnetization I to the causative magnetic field H. It is typically expressed in micro cgs units for oil and gas exploration work. Susceptibility has been shown to be proportional to the volume percentage of magnetite contained in a rock. Susceptibility contrast is the susceptibility difference between two rocks or geologic bodies. See Density Contrast.
Three Dimensional (3D) Model
A network or grid of values which models a geologic surface represented as a surface of (gravity) or susceptibility contrast (magnetics). The output of a forward model is based on the calculated gravity or magnetic effect of a specified input surface. The output of an inverse model is the geometry of an appropriate (but non-unique) surface calculated by inverting the input gravity or magnetic field.
Total Magnetic Intensity Anomaly
The total magnetic intensity anomaly field (TI) is the resultant field after correcting TF, the total magnetic (observed) field for a regional gradient field, such as an IGRF.
An automated profile-based depth estimation method derived from S. Werner’s analysis of magnetic anomalies from sheet-like bodies. Polynomials representing a total field anomaly or its derivative (horizontal gradient) can be simultaneously solved to estimate the depth, dip, horizontal location, and susceptibility of the source body (thin sheet or interface).
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