Many-to-many tables tracking participation at multiple events
Ideally, your system could comprise:

a Locations table with location information and a location_id
a People or Observers table with personal details and a person_id
an Observations table just the way you have it
and an Observer_Observations table with person_id and obs_id (i.e. your many-to-many table)

The total_time column in your Observers table should be calculated from Observer_Observations rather than stored.