A method of identifying reflected signals, subsequent to their reflection within a medium, said method comprising: obtaining a plurality of return signals, resulting from a plurality of measurements being performed over a measurement period, said measurement period comprising plural sub-periods, said return signals comprising reflected signals and noise; characterised in that for each of said time sub-periods the method further comprises: partitioning the plurality of return signals into a first set and a second set of equal cardinality or as equal as possible such that their cardinality differs by no more than one; determining a first mean of all signals in the first set and a second mean of all the signals in second set; determining a stacked correlation value for said return signals by: partitioning said first and second sets randomly into plural pairs of first subsets and second subsets; determining a correlation value of said first mean and said second mean over the time sub-period for each partition into first subsets and second subsets, and determining a mean of the correlation values; for each partition into first subsets and second subsets, determining a stacked standard deviation value for each time sub-period, by determining the standard deviation of said stacked correlation values over the number of said partitions; and identifying peaks in said stacked correlation value over time having a value greater than said stacked standard deviation value at the corresponding time and attributing each of said peaks in said stacked correlation value over time to a reflected signal. obtaining a plurality of return signals, resulting from a plurality of measurements being performed over a measurement period, said measurement period comprising plural sub-periods, said return signals comprising reflected signals and noise; characterised in that for each of said time sub-periods the method further comprises: partitioning the plurality of return signals into a first set and a second set of equal cardinality or as equal as possible such that their cardinality differs by no more than one; determining a first mean of all signals in the first set and a second mean of all the signals in second set; determining a stacked correlation value for said return signals by: partitioning said first and second sets randomly into plural pairs of first subsets and second subsets; determining a correlation value of said first mean and said second mean over the time sub-period for each partition into first subsets and second subsets, and determining a mean of the correlation values; for each partition into first subsets and second subsets, determining a stacked standard deviation value for each time sub-period, by determining the standard deviation of said stacked correlation values over the number of said partitions; and partitioning the plurality of return signals into a first set and a second set of equal cardinality or as equal as possible such that their cardinality differs by no more than one; determining a first mean of all signals in the first set and a second mean of all the signals in second set; determining a stacked correlation value for said return signals by: partitioning said first and second sets randomly into plural pairs of first subsets and second subsets; determining a correlation value of said first mean and said second mean over the time sub-period for each partition into first subsets and second subsets, and determining a mean of the correlation values; partitioning said first and second sets randomly into plural pairs of first subsets and second subsets; determining a correlation value of said first mean and said second mean over the time sub-period for each partition into first subsets and second subsets, and determining a mean of the correlation values; for each partition into first subsets and second subsets, determining a stacked standard deviation value for each time sub-period, by determining the standard deviation of said stacked correlation values over the number of said partitions; and identifying peaks in said stacked correlation value over time having a value greater than said stacked standard deviation value at the corresponding time and attributing each of said peaks in said stacked correlation value over time to a reflected signal.