I have a sample.mpg
mpegts file with a video keyframe starting at 65738.512389
$ ffprobe -hide_banner -loglevel fatal sample.mpg -select_streams v -print_format json -show_packets -read_intervals "%+#1"
{
"packets": [
{
"codec_type": "video",
"stream_index": 0,
"pts": 5916466115,