波線と鉛直方向とのなす角がangleになる深さ。
なお速度構造が単調増加でない場合は解が複数存在しうるが、
最初にangleになる深さを返す。
The depth where the ray angle from the vertical becomes
the value angle.
In cases where the velocity structure is not a monotonic increase function,
there may be two or more solutions.
In such cases, the first (i.e., shallowest) depth
where the ray angle reaches the value angle is returned.