For air showers with energies in excess of 1015 eV, the shower maximum penetrates to half the vertical atmospheric depth or more. There is also sufficient number of particles in the cascade such that the remnant of the shower can be detected as a correlated event by an array of individual particle detectors on the ground. The threshold (the lowest energy detectable by an instrument) of such a "Ground Array" depends on the altitude of the array. Typically it is difficult to measure cosmic rays with energies below 1014 eV with ground arrays.
The figure to the left shows the schematic of a ground array. Each station of the array samples the density of particles in its neighborhood of the shower. The footprint of air showers typically extends hundreds of meters. The particles in the air shower arrive in the form of a thin pancake traveling at essentially the speed of light. By measuring the time of arrival of the shower front at the individual stations, the direction of the primary cosmic rays can be calculated to about one degree accuracy. Conventionally, the energy of the shower is deduced from r(600), the density measured at 600 meters from the core of the shower at ground level. This density was chosen because it was the quantity which showed least amount of variations between different shower models.
An early example of a ground array is the Haverah Park array operated by University of Leeds between 1967-1987. A more recent experiment is the Akeno Giant Air Shower Array (AGASA) operated by University of Tokyo. When coupled with an underground muon array, it is also possible to measure the composition of the primary particle with a ground array. An example of such a combination used to search for very-high energy gamma rays was the CASA-MIA array.
CASA-MIA, depicted in this photograph, was operated between 1989-1997 at the U.S. Army Dugway Proving Ground. It is commonly agreed in the astrophysics community that CASA the definitive search for very-high energy gamma ray sources with energies above ~1014 eV. Their results effectively put to rest the controversy in the 1980's over reports of gamma ray detections from Cygnus X-3 and Hercules X-1.