Geologists: Why can't we read geomagnetic excursions?

All I know is that it's speculated they occur more frequently than reversals, but how can we even speculate if they can't be detected in the record? Do you know anything about excursions or their possible effects? I've read that during excursions the geomagnetic field drops to between 0-20% & this lasts from a few thousand years to tens of thousands - but how can we know this if we can't read excursions in the record? Can you speculate about why they occur & also reason any effects such a drop would cause for our planet? It seems to me that our geomagnetic field is perpetually dipping to zero & rising back to 'normal' levels like a big rollercoaster, but that occasionally during a dip the geomagnetic poles reverse (like hitting the coaster loop) - is that a fair statement? Thank you.