the location of the fractures the flow of fracturing fluid to each of the fractures is an indicator of the relative size of the fracture compared to the other fractures into which fracturing fluid is flowing for example twenty to sixty sensors could be released at intervals of for example ten to sixty seconds so that flow back of the sensors would represent flow back of a number of sensors from each fracture that is proportional to the flow of fracturing fluid into each fracture during this operation after a fracturing operation fluids are typically permitted to flow back from the formation these fluids are initially the fluid it typically similar to the fluids initially injected possibly with higher levels of dissolved solids because of the exposure to divalent cations in the formation sensors returning to the surface for example with flow back after a fracturing operation may be either captured or integrated as they flow past antennas around return fluid piping it is preferable to capture the sensors both for reuse capturing the sensors also permits recharging of the sensor so that data may be transmitted from the sensor with greater power or downloading data via a hard wired connection rather than through a wireless transmission an.