By sorting the data into classes of offset, we can restrict the size of the spiral to the maximum offset of each class. The spiral data movement allows any azimuth distribution of the input traces. However, because a trace contributes to the stack only along its original source-geophone line, some processors are left idle during the spiral trip of the trace. Figure shows the working processors in grey and the idle processors in white. In this example, the computer load, defined as the ratio of the number of working processors to the total number of processors, is thirty percent. For larger offsets, the load balance is expected to be worse because the number of working processors increases in proportion to the offset length whereas the total number of processors increases in proportion to the square of the offset. The unoptimized use of processing power makes the algorithm inefficient for 3-D land data.