  We present full-track high-resolution radio observations of the jet of the
galaxy M87 at 8 and 15 GHz. These observations were taken over three
consecutive days in May 2009 using the Very Long Baseline Array (VLBA), one
antenna of the Very Large Array (VLA), and the Effelsberg 100 m telescope. Our
produced images have dynamic ranges exceeding 20,000:1 and resolve linear
scales down to approximately 100 Schwarzschild radii, revealing a
limb-brightened jet and a faint, steep spectrum counter-jet. We performed
jet-to-counter-jet analysis, which helped estimate the physical parameters of
the flow. The rich internal structure of the jet is dominated by three helical
threads, likely produced by the Kelvin-Helmholtz (KH) instability developing in
a supersonic flow with a Mach number of approximately 20 and an enthalpy ratio
of around 0.3. We produce a CLEAN imaging bias-corrected 8-15GHz spectral index
image, which shows spectrum flattening in regions of helical thread
intersections. This further supports the KH origin of the observed internal
structure of the jet. We detect polarised emission in the jet at distances of
approximately 20 milliarcseconds from the core and find Faraday rotation which
follows a transverse gradient across the jet. We apply Faraday rotation
correction to the polarisation position angle and find that the position angle
changes as a function of distance from the jet axis, which suggests the
presence of a helical magnetic field.
