Instead of running an Update Query every time when you find a new atrifact, it will be a better method if you design two forms for both Site details and Artifacts Table and link them together. Do this as follows;
  1. Design the Main Form for the Site Table
  2. Design a Sub-Form in Datasheet View or Continuous Form for Artifacts Table
  3. Insert the Artifacts form as Sub-form into the Main Site Form
  4. Select the Sub-form and display its property Sheet (View- ->Properties)
  5. Enter the SiteID in the Link Master Field Property
  6. Enter the SiteID of the Artifacts Table into the Link Child Field Property and save the Form.
  7. Before you attempt to enter an Artifact record find the Site Name in the Main Form.
  8. then enter the Artificats details in the sub-form.
  9. The SiteId will be automatically recorded into the Artiifacts table.