  Stellar magnetic field measurements obtained from spectropolarimetry offer
key data for activity and dynamo studies, and we present the results of a major
high-resolution spectropolarimetric Bcool project magnetic snapshot survey of
170 solar-type stars from observations with the Telescope Bernard Lyot and the
Canada-France-Hawaii Telescope. For each target star a high signal-to-noise
circularly polarised Stokes V profile has been obtained using Least-Squares
Deconvolution, and used to detect surface magnetic fields and measure the
corresponding mean surface longitudinal magnetic field ($B_{l}$). Chromospheric
activity indicators were also measured.
  Surface magnetic fields were detected for 67 stars, with 21 of these stars
classified as mature solar-type stars, a result that increases by a factor of
four the number of mature solar-type stars on which magnetic fields have been
observed. In addition, a magnetic field was detected for 3 out of 18 of the
subgiant stars surveyed. For the population of K-dwarfs the mean value of
$B_{l}$ ($|B_{l}|_{mean}$) was also found to be higher (5.7 G) than
$|B_{l}|_{mean}$ measured for the G-dwarfs (3.2 G) and the F-dwarfs (3.3 G).
For the sample as a whole $|B_{l}|_{mean}$ increases with rotation rate and
decreases with age, and the upper envelope for $|B_{l}|$ correlates well with
the observed chromospheric emission. Stars with a chromospheric S-index greater
than about 0.2 show a high magnetic field detection rate and so offer optimal
targets for future studies.
  This survey constitutes the most extensive spectropolarimetric survey of cool
stars undertaken to date, and suggests that it is feasible to pursue magnetic
mapping of a wide range of moderately active solar-type stars to improve
understanding of their surface fields and dynamos.
