  We present a portable optically pumped magnetometer instrument for
ultra-sensitive measurements within the Earth's magnetic field. The central
part of the system is a sensor head operating a MEMS-based Cs vapor cell in the
light-shift dispersed Mz mode. It is connected to a compact, battery-driven
electronics module by a flexible cable. We briefly review the working
principles of the device and detail on the realization of both, sensor head and
electronics. We show shielded and unshielded measurements within a static
magnetic field amplitude of 50 uT demonstrating a noise level of the sensor
system down to 140 fT/\sqrt{Hz} and a sensor bandwidth of several 100 Hz. In a
detailed analysis of sensor noise we reveal the system to be limited by
technical sources with straightforward strategies for further improvement
towards its fundamental noise limit of 12 fT/\sqrt{Hz}. We assess the
parameters defining the sensor bandwidth by theoretical modeling based on the
Bloch equations. Finally, we compare our sensors' performance to a commercial
SQUID system in a measurement environment typical for geomagnetic observatory
practice and geomagnetic prospection.
