Fig. 9

Complexities associated with the helicorder geometry. a Illustration of a helicorder recording with only rotation. The black line is the trace-zero line that would be produced if the amplitude is always zero. b Same as (a) except that this case includes translation that results in the slope of the trace-zero line. If the amplitude is measured perpendicular to the trace-zero line, it will result in an underestimation of amplitude (dashed blue line) and a time offset (open blue circle). c Effect on curvature introduced by pen or needle recording. The arc is generated by instantaneous amplitude change from zero to a maximum value with the armature (yellow lines). The solid red arrow shows how this curvature should be corrected (open red circle) while the blue arrow shows correction if the correction is calculated parallel to the trace-zero line leading to overestimated amplitude (open blue circle)