Equations: Geophysics and seismic surveying

The equations of the “Geophysics and seismic surveying” lessons in “Petroleum geoscience”, with their conditions and units, and what each symbol means in them.

Gravity anomaly above a sphere (a point mass)

\Delta g(x) = \frac{G M z}{(x^2 + z^2)^{3/2}}, \qquad M = \tfrac{4}{3}\pi a^3\,\Delta\rho

SI; 1 mGal $= 10^{-5}$ m/s²

Holds when: The body is a uniform sphere or any spherically symmetric body: outside it, its field equals that of a point mass at the centre. The host rocks are uniform horizontally. The effect of everything else (deep sequences, basement relief) has already been subtracted as a regional background. Corrections for elevation and terrain have moved the observations to a horizontal surface (next section). We measure the vertical component; an ordinary gravimeter does not sense the horizontal components.

Geophysical exploration methods →

Half-width of the anomaly and the depth of the centre of the sphere

x_{1/2} = z\sqrt{2^{2/3} - 1} \approx 0.7664\,z, \qquad z \approx 1.305\,x_{1/2}

any consistent units of length

Holds when: The body is a sphere (a point mass), and the anomaly is isolated: the regional background has been subtracted correctly. For other shapes the factor is different. For a long horizontal cylinder (a body elongated along strike), $x_{1/2} = z$ . For a thin vertical column extending to a great depth, $x_{1/2} = \sqrt3\,z$ , where $z$ is the depth of its top.

Geophysical exploration methods →

Normal gravity on the ellipsoid

g_\text{n}(\varphi) = 978{,}032.7\,\bigl(1 + 0.0053024\sin^2\varphi - 0.0000058\sin^2 2\varphi\bigr)

mGal

Holds when: An approximate form of the normal field for the ellipsoid (the smooth, slightly flattened model of the Earth's shape) of the Geodetic Reference System 1980 (GRS80) adopted in geodesy. We calculate the correction from the latitude of each station.

Geophysical exploration methods →

Bouguer anomaly

\Delta g_\text{B} = g - g_\text{n}(\varphi) + 0.3086\,h - 0.04194\,\rho\,h + \delta g_\text{T}

mGal; $h$ in m, $\rho$ in g/cm³

Holds when: 0.3086 mGal/m is the mean normal vertical gradient: gravity decreases by this amount for every metre of rise. This is the free-air correction (also called the Faye correction). $0.04194\,\rho h$ is the attraction of an infinite flat slab of thickness $h$ , that is, $2\pi G\rho h$ in milligals for a density in g/cm³. This is the correction for the intermediate layer (also called the Bouguer correction). The terrain correction accounts for how the ground around the station differs from a flat slab. On land it is positive both for hills above the station and for hollows below it. For a station below the reference level, $h$ is negative. At sea and in the air the corrections are different, and on a moving ship or aircraft we add the Eötvös correction for the motion.

Geophysical exploration methods →

Anomaly of a magnetic pole — the top of a long vertical body

\Delta T(x) = \frac{C\,z}{(x^2 + z^2)^{3/2}}, \qquad C = \frac{\kappa\,B_0\,A}{4\pi}

$x$ , $z$ in m; $A$ in m²; $B_0$ and $\Delta T$ in nT

Holds when: The body is a thin vertical column, magnetised along its axis, that extends far deeper than its top. The field of the lower end is negligible, so only the field of the upper “pole” remains. The magnetisation is induced and the field is vertical, or else the map has been reduced to the pole. The susceptibility is small ( $\kappa \ll 1$ ), and we do not take the demagnetisation of the body into account (the weakening of its magnetisation by its own field).

Geophysical exploration methods →

Depth from two-way time

z = \sum_{i=1}^{n} \frac{V_i\,t_i}{2}, \qquad t = \sum_{i=1}^{n} t_i = \sum_{i=1}^{n} \frac{2 h_i}{V_i}

SI: m, s, m/s

Holds when: The rays are vertical. After processing, the time section is reduced to the case where the source and the receiver are at the same point. The layers are horizontal, and the velocity in each layer is constant. The velocities are known: from check-shot surveys and vertical seismic profiling (VSP) in wells (measurements of wave travel time from the surface down the hole), from sonic logs (travel time from the sonic log) or, less accurately, from the seismic data themselves.

Geophysical exploration methods →

Total anomalous mass from the integral of the anomaly

\iint \Delta g\,\dd A = 2\pi G M

Holds when: The integral is over the whole plane: the anomaly is measured far beyond the body and is completely separated from the regional background. The observation surface is flat, and all the anomalous masses lie below it.

Geophysical exploration methods →

Wavelength

\lambda = \frac{V}{f}

SI: m, m/s, Hz

Holds when: A wave of one frequency. A real wavelet has many frequencies, and we calculate the wavelength at a characteristic frequency. We must say which one (see the section on the wavelet). At a boundary the frequency stays the same, and the wavelength grows where the speed grows.

Seismic waves, acoustic impedance and reflections →

Acoustic impedance

Z = \rho\,V_p

SI: kg/(m²·s)

Holds when: The rock is uniform on the scale of the wavelength. A package of layers much thinner than the wavelength is “seen” by the wave through its averaged properties.

Seismic waves, acoustic impedance and reflections →

Reflection coefficient at normal incidence

R = \frac{Z_2 - Z_1}{Z_2 + Z_1}

dimensionless; impedances in any equal units

Holds when: Normal incidence of a plane P-wave. At oblique incidence, part of the energy goes into S-waves, and $R$ depends on the angle (see the section on AVO). The boundary is flat and extended, much wider than the zone from which the reflection comes. The rocks on both sides of the boundary are elastic and uniform.

Seismic waves, acoustic impedance and reflections →

Ricker wavelet

w(t) = \left(1 - 2\pi^2 f_p^2 t^2\right)e^{-\pi^2 f_p^2 t^2}

SI: s, Hz

Holds when: A model of a zero-phase wavelet. A real wavelet is found from seismic data near a well, and its shape differs from the Ricker shape. The shape of the wavelet does not change within the time interval under consideration.

Seismic waves, acoustic impedance and reflections →

Convolution model of a seismic trace

s(t) = \sum_k R_k\,w(t - \tau_k) + n(t)

time in s; amplitudes as fractions of the amplitude of the incident wavelet

Holds when: The layers are horizontal and the wave falls at normal incidence: a one-dimensional model. Only single reflections are included. Multiples (waves reflected many times between boundaries), transmission losses (energy lost on passing through boundaries) and spherical divergence (weakening as the wavefront spreads out) are left out, or compensated by processing. The wavelet is the same over the whole interval.

Seismic waves, acoustic impedance and reflections →

Tuning thickness for a Ricker wavelet

\Delta t_\text{tun} = \frac{\sqrt6}{2\pi f_p} \approx \frac{0.39}{f_p}, \qquad h_\text{tun} = \frac{V\,\Delta t_\text{tun}}{2}

SI: s, Hz, m, m/s

Holds when: A zero-phase Ricker wavelet. For other wavelets the factor is different. The reflections from the top and base are equal in size and opposite in sign, and the neighbouring reflections are far away.

Seismic waves, acoustic impedance and reflections →

Absorption at constant quality factor

\frac{A}{A_0} = e^{-\pi f t/Q}

SI: Hz, s; $Q$ is dimensionless

Holds when: The quality factor does not depend on frequency in the seismic range and is the same along the whole path (otherwise we take its average along the path, weighted by time). Spherical divergence and boundary losses are accounted for separately.

Seismic waves, acoustic impedance and reflections →

Two-term approximation of the reflection coefficient (Shuey)

R(\theta) \approx A + B\sin^2\theta,\quad A = \tfrac12\!\left(\frac{\Delta V_p}{V_p} + \frac{\Delta\rho}{\rho}\right),\quad B = \tfrac12\frac{\Delta V_p}{V_p} - 2\frac{V_s^2}{V_p^2}\!\left(\frac{\Delta\rho}{\rho} + 2\frac{\Delta V_s}{V_s}\right)

dimensionless $R$ , $A$ , $B$ ; speeds and densities in any consistent units

Holds when: Small relative contrasts of properties at the boundary. Angles of incidence less than about 30°. At larger angles a third term or the exact formulas are needed. A flat boundary between uniform isotropic rocks.

Seismic waves, acoustic impedance and reflections →

Weakening on passing a boundary down and back

T_\downarrow T_\uparrow = \frac{2Z_2}{Z_1 + Z_2}\cdot\frac{2Z_1}{Z_1 + Z_2} = 1 - R^2

dimensionless

Holds when: Normal incidence, and thick layers, much thicker than the wavelength. In a package of thin layers, multiple reflections inside the layers partly return energy to the transmitted wave, and a simple product of factors overstates the losses.

Seismic waves, acoustic impedance and reflections →

Fold of a 2D line

n = \frac{N\,\Delta x_\text{rec}}{2\,\Delta x_\text{shot}}

the intervals are in the same units of length

Holds when: A regular layout: all channels work, the shots follow one another at a constant interval, and the spread moves with them. The formula applies in the middle of the line. At the ends, the fold rises from one to its full value (figure below). The fold is counted for the nominal layout. Shots skipped because of obstacles, and rejected traces, reduce it.

Seismic acquisition and processing →

Fold of a 3D survey with an orthogonal layout

n = \frac{N_\text{ch}\,\Delta x_\text{rec}}{2\,D_\text{SL}}\cdot\frac{N_\text{RL}}{2}

lengths in the same units

Holds when: An orthogonal layout: each source point records a patch that is symmetric about it along and across the lines. Far from the edges of the survey. Near the edges the fold builds up from low values, as at the ends of a 2D line.

Seismic acquisition and processing →

Static correction of a station to the datum

t_\text{st} = \frac{h_\text{LVZ}}{V_\text{LVZ}} + \frac{E - h_\text{LVZ} - E_0}{V_0}

SI: m, m/s, s

Holds when: Rays near the surface are vertical. In the slow LVZ the wave really is refracted to nearly vertical. The datum is below the base of the LVZ, and the LVZ is uniform beneath the station. The correction of a trace is the sum of the corrections of its source point and its receiver point (the corrections are surface-consistent: a point has the same correction for every trace). If the charge is fired below the LVZ, only the second term remains at the source point, and it is worked out from the depth of the charge.

Seismic acquisition and processing →

Traveltime curve of a reflection and the normal moveout correction

t(x) = \sqrt{t_0^2 + \frac{x^2}{V_\text{stack}^2}}, \qquad \Delta t(x) = t(x) - t_0

SI: s, m, m/s

Holds when: A horizontal boundary below a uniform layer. Then the formula is exact, and $V_\text{stack}$ equals the velocity of the layer. For a layered overburden, the hyperbola is an approximation at offsets that are not larger than the depth of the boundary, and $V_\text{stack}$ is close to the root-mean-square velocity (below). For a boundary that dips at an angle $\theta$ in the plane of the line, $V_\text{stack} = V/\cos\theta$ , which is larger than the velocity of the layer.

Seismic acquisition and processing →

Root-mean-square velocity

V_\text{rms}^2 = \frac{\sum_i V_i^2\,\Delta\tau_i}{\sum_i \Delta\tau_i}

SI: m/s, s

Holds when: Horizontal, uniform layers. Offsets that are not larger than the depth of the boundary. At larger offsets the traveltime curve departs from a hyperbola, and the velocity that we pick depends on the range of offsets. Isotropic rocks. In shales the velocity depends on direction (anisotropy), and the stacking velocity usually comes out higher than the vertical velocity.

Seismic acquisition and processing →

Displacement of a reflection from a dipping boundary before migration

d = \frac{V t_0}{2}\sin\theta, \qquad z = \frac{V t_0}{2}\cos\theta, \qquad \mathrm{tan}\,\beta = \sin\theta

SI: m, s, m/s; angles in degrees

Holds when: A constant velocity above the boundary, and a plane boundary. The line runs in the dip direction of the boundary. On an oblique line, part of the displacement is across the line, and 2D migration will not fix it.

Seismic acquisition and processing →

Traveltime curve of a diffracted wave on a CMP section

t(x) = \sqrt{t_0^2 + \frac{4x^2}{V^2}}

SI: s, m, m/s

Holds when: A constant velocity. A CMP section, with the source and receiver at one point. The scatterer lies in the plane of the line.

Seismic acquisition and processing →

Radius of the first Fresnel zone

r_\text{F} = \frac{V}{2}\sqrt{\frac{t_0}{f}} \approx \sqrt{\frac{z\lambda}{2}}

SI: m, m/s, s, Hz

Holds when: A constant (average) velocity, zero offset, a horizontal boundary, and a depth much larger than the wavelength. If the velocity grows with depth, the rays bend, and the zone is a little wider.

Seismic acquisition and processing →

The largest bin without spatial aliasing

b \le \frac{V_\text{int}}{4 f_{max}\sin\theta}

SI: m, m/s, Hz

Holds when: A CMP section (zero offsets). The velocity above the target is constant. The estimate is made for the highest frequency that we still want to keep.

Seismic acquisition and processing →

The Dix formula

V_\text{int}^2 = \frac{V_n^2\,t_n - V_{n-1}^2\,t_{n-1}}{t_n - t_{n-1}}

SI: m/s, s

Holds when: Horizontal uniform layers, and the stacking velocity equals the root-mean-square velocity. Both reflections are on one vertical line, and the stacking velocities are picked without errors. In practice their errors grow many times over when they are subtracted.

Seismic interpretation →

Converting time to depth with an average velocity, and layer by layer

z = \frac{V_\text{avg}\,t}{2}, \qquad z = \frac{V_1 t_1}{2} + \frac{V_2\,(t - t_1)}{2}

SI: m, s, m/s

Holds when: Rays are vertical: the time is reduced to the vertical, and dips are moderate. The average velocity works while the thickness and velocity of the layers above the horizon change little sideways. The layer-by-layer method needs followed intermediate horizons and the interval velocity of each layer (from wells or by the Dix formula). For more layers the layer-by-layer formula continues as a sum (depth from two-way time).

Seismic interpretation →

Depth when velocity grows linearly with depth

z = \frac{V_0}{k}\left(e^{k t/2} - 1\right)

SI: m, m/s, s⁻¹, s

Holds when: Velocity grows with depth linearly (a compacting unit of uniform composition), and the instantaneous velocity does not change sideways. Vertical rays. For several units, each is described by its own $V_0$ and $k$ .

Seismic interpretation →

Symbols

Symbol	Meaning and context
$\Delta g$	gravity anomaly, m/s²Geophysical exploration methods
$\dd A$	element of area on the plane of observation, m²Geophysical exploration methods
$\Delta g(x)$	anomaly at a distance $x$ from the point above the centre of the sphere; the value in m/s² multiplied by $10^5$ gives milligals, m/s²Geophysical exploration methods
$\Delta T(x)$	anomaly at a distance $x$ from the point above the body (the map is reduced to the pole), nTGeophysical exploration methods
$\Delta t(x)$	normal moveout correction, the time that is subtracted at offset $x$ , sSeismic acquisition and processing
$a$	radius of the sphere, mGeophysical exploration methods
$A$	area of the horizontal cross-section of the body, m²Geophysical exploration methods intercept: the reflection coefficient at normal incidence in the linear approximation, —Seismic waves, acoustic impedance and reflections
$A/A_0$	share of the amplitude of the component of frequency $f$ left after absorption, —Seismic waves, acoustic impedance and reflections
$b$	bin size (distance between CMPs) in the dip direction, mSeismic acquisition and processing
$\Delta g_\text{B}$	Bouguer anomaly at the station, mGalGeophysical exploration methods
$B$	gradient: the rate of change of $R$ with $\sin^2\theta$ , —Seismic waves, acoustic impedance and reflections
$B_0$	induction of the Earth's magnetic field, nTGeophysical exploration methods
$C$	strength of the “pole”, nT·m²Geophysical exploration methods
$d$	horizontal distance from the CMP to the reflection point, up-dip, mSeismic acquisition and processing
$D_\text{SL}$	distance between the lines of source points, mSeismic acquisition and processing
$E$	elevation of the station (height above sea level), mSeismic acquisition and processing
$E_0$	elevation of the datum, mSeismic acquisition and processing
$f$	frequency of oscillation, HzSeismic waves, acoustic impedance and reflections frequency, HzSeismic waves, acoustic impedance and reflections dominant frequency of the reflection, HzSeismic acquisition and processing
$f_{max}$	highest useful frequency, HzSeismic acquisition and processing
$f_p$	peak frequency: the maximum of the amplitude spectrum, HzSeismic waves, acoustic impedance and reflections peak frequency of the Ricker wavelet, HzSeismic waves, acoustic impedance and reflections
$g$	observed gravity, corrected for tides and instrument drift, mGalGeophysical exploration methods
$G$	gravitational constant, $6.6743\cdot10^{-11}$ , m³/(kg·s²)Geophysical exploration methods gravitational constant, m³/(kg·s²)Geophysical exploration methods
$g_\text{n}$	normal gravity on the surface of the Earth's ellipsoid, mGalGeophysical exploration methods
$g_\text{n}(\varphi)$	normal gravity at the latitude of the station, mGalGeophysical exploration methods
$h$	elevation of the station above the reference level (usually above sea level), mGeophysical exploration methods
$h_i$	vertical thickness of the $i$ -th layer, mGeophysical exploration methods
$h_\text{LVZ}$	thickness of the low-velocity zone beneath the station, mSeismic acquisition and processing
$h_\text{tun}$	tuning thickness in metres, mSeismic waves, acoustic impedance and reflections
$k$	increase of velocity per metre of depth, s⁻¹Seismic interpretation
$M$	anomalous mass of the body — excess (positive) or deficit (negative), kgGeophysical exploration methods total anomalous mass of all bodies below the plane, kgGeophysical exploration methods
$n$	number of layers above the boundary, —Geophysical exploration methods fold, the number of traces added at one CMP, —Seismic acquisition and processing
$N$	number of active channels of the spread, —Seismic acquisition and processing
$N_\text{ch}$	number of active channels on one receiver line of the patch, —Seismic acquisition and processing
$N_\text{RL}$	number of active receiver lines in the patch, —Seismic acquisition and processing
$n(t)$	noise, —Seismic waves, acoustic impedance and reflections
$\Delta V_p,\ \Delta V_s,\ \Delta\rho$	differences of P-wave speed, S-wave speed and density: lower rock minus upper rock, m/s; g/cm³Seismic waves, acoustic impedance and reflections
$Q$	quality factor of the rocks: the larger it is, the weaker the absorption; average values for a sedimentary section are tens to hundreds, —Seismic waves, acoustic impedance and reflections
$R$	reflection coefficient: the ratio of the pressure amplitude in the reflected wave to that in the incident wave, —Seismic waves, acoustic impedance and reflections reflection coefficient of the boundary for a wave going down, —Seismic waves, acoustic impedance and reflections
$r_\text{F}$	radius of the first Fresnel zone on the boundary, mSeismic acquisition and processing
$R_k$	reflection coefficient of boundary $k$ , —Seismic waves, acoustic impedance and reflections
$\Delta x_\text{rec}$	interval between receiver points (receiver groups); the CMPs are $\Delta x_\text{rec}/2$ apart, mSeismic acquisition and processing interval between receiver points along the line, mSeismic acquisition and processing
$s(t)$	seismic trace: amplitude against time, —Seismic waves, acoustic impedance and reflections
$\Delta x_\text{shot}$	interval between source points, mSeismic acquisition and processing
$t$	two-way time to it, sGeophysical exploration methods time from the centre of the wavelet, sSeismic waves, acoustic impedance and reflections travel time along the whole path; for a reflected wave, the two-way time, sSeismic waves, acoustic impedance and reflections two-way time of the horizon, sSeismic interpretation two-way time from the top of the unit, sSeismic interpretation
$\delta g_\text{T}$	terrain correction (the effect of the hills and hollows around the station), mGalGeophysical exploration methods
$T_\downarrow,\ T_\uparrow$	ratios of the pressures of the transmitted and incident waves when passing the boundary downwards and upwards, —Seismic waves, acoustic impedance and reflections
$t_0$	two-way time at zero offset, the time $t_0$ below the CMP, sSeismic acquisition and processing two-way time of the reflection on the CMP section, sSeismic acquisition and processing time of the apex, $2z/V$ , sSeismic acquisition and processing two-way time of the reflection, sSeismic acquisition and processing
$t_1$	two-way time of the intermediate horizon, the base of the upper layer, sSeismic interpretation
$t_i$	two-way time in the $i$ -th layer, sGeophysical exploration methods
$t_n,\ t_{n-1}$	two-way times of these reflections, sSeismic interpretation
$t_\text{st}$	static correction of the station: the time that is subtracted from all traces of this station, sSeismic acquisition and processing
$t(x)$	arrival time of the reflection at offset $x$ , sSeismic acquisition and processing diffraction time on the section at a point that is $x$ away from the apex, sSeismic acquisition and processing
$\Delta t_\text{tun}$	tuning thickness in time: the difference of the two-way times of the top and base, sSeismic waves, acoustic impedance and reflections
$V$	speed of the wave in the rock, m/sSeismic waves, acoustic impedance and reflections P-wave speed in the layer, m/sSeismic waves, acoustic impedance and reflections velocity in the overburden above the boundary (constant), m/sSeismic acquisition and processing velocity (constant), m/sSeismic acquisition and processing average velocity down to the boundary, m/sSeismic acquisition and processing
$V_0$	velocity of the rocks below the LVZ (the replacement velocity), m/sSeismic acquisition and processing velocity at the top of the unit, m/sSeismic interpretation
$V_1,\ V_2$	interval velocities of the upper and the lower layer, m/sSeismic interpretation
$V_\text{avg}$	average velocity down to the horizon: $2z/t$ at wells, and from a map between them, m/sSeismic interpretation
$V_i$	interval velocity of the compressional wave in the $i$ -th layer, m/sGeophysical exploration methods interval velocity of the $i$ -th layer, m/sSeismic acquisition and processing
$V_\text{int}$	interval velocity above the target, m/sSeismic acquisition and processing interval velocity between two reflections, m/sSeismic interpretation
$V_\text{LVZ}$	velocity of P-waves in the LVZ, m/sSeismic acquisition and processing
$V_n,\ V_{n-1}$	root-mean-square velocities (in practice, stacking velocities) down to the lower and the upper reflection, m/sSeismic interpretation
$V_p$	P-wave speed, m/sSeismic waves, acoustic impedance and reflections
$V_p,\ V_s,\ \rho$	averages over the two rocks, m/s; g/cm³Seismic waves, acoustic impedance and reflections
$V_\text{rms}$	root-mean-square velocity down to the base of the $n$ -th layer, m/sSeismic acquisition and processing
$V_\text{stack}$	stacking velocity (the CMP velocity), m/sSeismic acquisition and processing
$w(t)$	amplitude of the wavelet, normalised to its value at the centre, —Seismic waves, acoustic impedance and reflections seismic wavelet, —Seismic waves, acoustic impedance and reflections
$x$	horizontal distance from the point above the centre, mGeophysical exploration methods offset between source and receiver, mSeismic acquisition and processing distance along the line from the point above the scatterer, mSeismic acquisition and processing
$x_{1/2}$	half-width of the anomaly: the distance from the extremum (the highest or lowest value) to the point where the anomaly is half as large, mGeophysical exploration methods
$z$	depth of the centre of the sphere below the line of observation, mGeophysical exploration methods depth of the centre of the sphere, mGeophysical exploration methods depth of the top of the body, mGeophysical exploration methods depth of the reflecting boundary, mGeophysical exploration methods true depth of the reflection point, mSeismic acquisition and processing depth of the horizon below the datum, mSeismic interpretation depth below the top of the unit (below the datum), mSeismic interpretation
$Z$	acoustic impedance of the rock, kg/(m²·s)Seismic waves, acoustic impedance and reflections
$Z_1$	impedance of the rock the wave comes from (above the boundary), kg/(m²·s)Seismic waves, acoustic impedance and reflections
$Z_2$	impedance of the rock beyond the boundary (below it), kg/(m²·s)Seismic waves, acoustic impedance and reflections
$z,\ \lambda$	depth of the boundary ( $V t_0/2$ ) and wavelength ( $V/f$ ), mSeismic acquisition and processing
$\beta$	dip of its image on the unmigrated section when the section is redrawn in depth by $V t_0/2$ , °Seismic acquisition and processing
$\Delta\rho$	density contrast between the body and the host rocks; for salt among compacted sediments it is negative, kg/m³Geophysical exploration methods
$\Delta\tau_i$	vertical two-way time in the $i$ -th layer, sSeismic acquisition and processing
$\theta$	angle of incidence of the P-wave on the boundary, °Seismic waves, acoustic impedance and reflections true dip of the boundary, the dip angle of the layer (in the lessons on structural geology it is $\delta$ ). Do not mix it up with the angle $\theta$ between the ray and the normal to the boundary in the topic on AVO, °Seismic acquisition and processing largest dip that must be imaged, °Seismic acquisition and processing
$\kappa$	magnetic susceptibility of the body relative to the host rocks, SI units (dimensionless)Geophysical exploration methods
$\lambda$	wavelength: the distance between neighbouring crests (compressions), mSeismic waves, acoustic impedance and reflections
$\rho$	density of the intermediate layer — the rocks between the station and the reference level, g/cm³Geophysical exploration methods bulk density of the rock, kg/m³Seismic waves, acoustic impedance and reflections
$\tau_k$	two-way travel time to boundary $k$ , sSeismic waves, acoustic impedance and reflections
$\varphi$	geographic latitude of the station, °Geophysical exploration methods