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The nearby metal-poor giant HD122563 is an important astrophysical laboratoryfor which to test stellar atmospheric and interior physics. It is also abenchmark star for which to calibrate methods to apply to large scale surveys.Recently it has been remeasured using various methodologies given the new highprecision instruments at our disposal. However, inconsistencies in theobservations and models have been found. In order to better characterise thisstar we have been measuring its radial velocities since 2016 using theHertzsprung telescope (SONG network node). In this work we report the firstdetections of sun-like oscillations in this star, and to our knowledge, adetection in the most metal-poor giant to date. We apply the classical seismicscaling relation to derive a new surface gravity of $\log g_{\nu} = 1.39 \pm0.01$ dex. Constraints on the mass imposed by its PopII giant classificationthen yield a radius of $30.8 \pm 1.0$ R$_{\odot}$. By coupling this with recentinterferometric measurements we infer a distance to the star of 306 $\pm$ 9 pc.Data from the Gaia mission corroborates the distance hypothesis ($d_{\rm GDR2}$= 290 $\pm$ 5 pc), and thus the updated fundamental parameters. We confirm thevalidity of the seismic scaling relation without corrections for surfacegravity in metal-poor and evolved star regimes. The small discrepancy of 0.04dex reduces to 0.02 dex by applying corrections to the scaling relations. Thenew constraints on the HR diagram ($L_{\odot} = 381 \pm 26$) reduce thedisagreement between the stellar parameters and evolution models, however, adiscrepancy still exists. Fine-tuned stellar evolution calculations show thatthis can be reconciled by changing the mixing-length parameter by an amount(--0.35) that is in agreement with predictions from recent 3D simulations andempirical results.

First detection of oscillations in the Halo giant HD 122563: validation of seismic scaling relations and new fundamental parameters