# The unit tangent vector

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Series Geophysical References Series Digital Imaging and Deconvolution: The ABCs of Seismic Exploration and Processing Enders A. Robinson and Sven Treitel 2 http://dx.doi.org/10.1190/1.9781560801610 9781560801481 SEG Online Store

The problem now is to find an expression for the unit tangent vector of the seismic ray. Let the vector ${\displaystyle \mathbf {r} =\left(x,y\right)}$ represent a point on a given ray (Figure 7). Let s denote arc length along the ray. Let ${\displaystyle \mathbf {r+} d\mathbf {r} =\left(x{\rm {+}}dx{\rm {,\ }}y{\rm {+}}dy\right)}$ give an adjacent point on the same ray. The vector ${\displaystyle d\mathbf {r} =\left(dx{\rm {,\ }}dy\right)}$ is (approximately) a tangent vector to the ray. The length of this vector is ${\displaystyle {\sqrt {{\rm {(}}dx^{\rm {2}}{\rm {+}}dy^{\rm {2}}}}{\rm {)}}}$ which is approximately equal to the increment ds of the arc length on the ray. As a result, the unit vector tangent to the ray is

 {\displaystyle {\begin{aligned}\mathbf {u=} &{\frac {d\mathbf {r} }{ds}}{\rm {=}}{\frac {dx}{ds}}\mathbf {i} +{\frac {dy}{ds}}\mathbf {j} =\left({\frac {dx}{ds}},{\frac {dy}{ds}}\right).\end{aligned}}} (2)

Here, i and j are the unit vectors in the horizontal and vertical directions, respectively. In other words, these unit vectors are defined as the vectors that have magnitude equal to one and have directions lying along the x- and y- axes, respectively.

Figure 7.  The unit tangent vector to a ray. The ray is specified by the radius vector (i.e., position vector) from the origin to a point on the ray. The wavefronts are orthogonal to the ray.

At this juncture, we want to clear up one confusing aspect of the mathematics of rays. The usual convention in the case of a raypath is to measure the angle of the tangent line from the vertical. This usage goes right back to the original seventeenth-century statement of Snell’s law. However, in mathematics, the convention is to measure the angle of the tangent line from the horizontal. It is better to stick with conventional mathematics and then convert to raypath usage when the results are displayed. A raypath is a curve along which the seismic energy travels. The unit tangent vector can be written as

 {\displaystyle {\begin{aligned}\mathbf {u=} &\left({\rm {\ cos\ }}\theta {\rm {,\ \ sin\ }}\theta \right),\end{aligned}}} (3)

where ${\displaystyle \theta }$ is the angle that the ray makes with the horizontal.

Let us review the results. Let the vector r be a point on the raypath. A closely spaced point on the raypath can be represented by the vector ${\displaystyle \mathbf {r} +d\mathbf {r} }$. Their difference is the vector ${\displaystyle d\mathbf {r} }$, which is the vector that connects the two points in question. Let the path length between the two points be ds. The unit tangent to the raypath is the limit ${\displaystyle d\mathbf {r} /ds}$ as the points approach each other. The length of the vector ${\displaystyle d\mathbf {r} }$ is approximately equal to the path-length difference ds. As a result, the vector ${\displaystyle d\mathbf {r} /ds}$ is the unit vector. Let ${\displaystyle \theta }$ be the angle that the tangent makes with the horizontal axis. The unit tangent vector is then ${\displaystyle \mathbf {u=} \left({\rm {\ cos\ }}\theta {\rm {,\ \ sin\ }}\theta \right)}$. The vector u is directed along the tangent to the curve in the direction of increasing values of the arc length s.