A device configured to assign to the unknown onset of a seismic signal in a noisy time series which is a seismogram a probability of that onset having a signed excursion away from the trace mean, the seismogram obtained from seismic testing using one or more hydrophones, geophones, accelerometers, or distributed acoustic sensing, and the device including one or more means arranged to: define a polarity at a given time sample by reference to the sign of the amplitude difference between the extrema immediately before and immediately after the given time sample in the time series; define a positive polarity probability density function by combining the polarity at a given time sample with a statistical weight based on the amplitude difference between the extrema immediately before and immediately after the given time sample, marginalized over the estimated noise in the time series; define a negative polarity probability density function consistent with the positive polarity probability density function and a requirement that the instantaneous positive and negative probability values sum to unity; take the respective products of the positive and negative polarity probability density functions with an onset probability density function defining the probability that the onset of the signal occurs at a given time sample; marginalize the respective products of the positive and negative polarity probability density functions with the onset probability density function over time to estimate final probabilities that the onset of a seismic signal has a positive or a negative polarity representing positive or negative polarity of ground motion which propagated the seismic signal; and using the estimated final probabilities as a constraint when performing source inversion on the time series and determine a focal point source type of an earthquake. define a polarity at a given time sample by reference to the sign of the amplitude difference between the extrema immediately before and immediately after the given time sample in the time series; define a positive polarity probability density function by combining the polarity at a given time sample with a statistical weight based on the amplitude difference between the extrema immediately before and immediately after the given time sample, marginalized over the estimated noise in the time series; define a negative polarity probability density function consistent with the positive polarity probability density function and a requirement that the instantaneous positive and negative probability values sum to unity; take the respective products of the positive and negative polarity probability density functions with an onset probability density function defining the probability that the onset of the signal occurs at a given time sample; marginalize the respective products of the positive and negative polarity probability density functions with the onset probability density function over time to estimate final probabilities that the onset of a seismic signal has a positive or a negative polarity representing positive or negative polarity of ground motion which propagated the seismic signal; and using the estimated final probabilities as a constraint when performing source inversion on the time series and determine a focal point source type of an earthquake.