Cannot retrieve gather when offsets are negative

[James-Beckwith]

When the SEGY file is pre-stack data and contains negative values for the offset and specific lines/crosslines are requested then the code (gather.py, line 111) attempts to use these negative offsets to retrieve the indices to extract. This doesn't work as you can't have negative lengths:

segy.gather[3,:,:]
Traceback (most recent call last):
File "", line 1, in
File "C:\Users\james\Miniconda3\envs\tf\lib\site-packages\segyio\gather.py", line 111, in getitem
range(*offs.indices(self.offsets[-1]+1))))
ValueError: length should not be negative
segy.offsets
array([-3237, -3212, -3187, -3162, -3137, -3112, -3087, -3062, -3037,
-3012, -2987, -2962, -2937, -2912, -2887, -2862, -2837, -2812,
-2787, -2762, -2737, -2712, -2687, -2662, -2637, -2612, -2587,
-2562, -2537, -2512, -2487, -2462, -2437, -2412, -2387, -2362,
-2337, -2312, -2287, -2262, -2237, -2212, -2187, -2162, -2137,
-2112, -2087, -2062, -2037, -2012, -1987, -1962, -1937, -1912,
-1887, -1862, -1837, -1812, -1787, -1762, -1737, -1712, -1687,
-1662, -1637, -1612, -1587, -1562, -1537, -1512, -1487, -1462,
-1437, -1412, -1387, -1362, -1337, -1312, -1287, -1262, -1237,
-1212, -1187, -1162, -1137, -1112, -1087, -1062, -1037, -1012,
-987, -962, -937, -912, -887, -862, -837, -812, -787,
-762, -737, -712, -687, -662, -637, -612, -587, -562,
-537, -512, -487, -462, -437, -412, -387, -362, -337,
-312, -287, -262], dtype=int32)

[jokva]

You're right! We'll get a test case up and running and fix this.

[James-Beckwith]

@jokva FYI, I was using the Mobil Viking graben open source data set: https://wiki.seg.org/wiki/Mobil_AVO_viking_graben_line_12 when I encountered this issue