diff --git a/sfm/report_pdf.py b/sfm/report_pdf.py
index 2771bba..a793b08 100644
--- a/sfm/report_pdf.py
+++ b/sfm/report_pdf.py
@@ -930,10 +930,17 @@ def _draw_waveform_subplot(fig, gridspec_cell, rd: ReportData) -> None:
             sc_axes.append(scx)
             sc_vals = rd.sensor_check_waveforms.get(ch) or []
             if sc_vals:
-                scx.plot(range(len(sc_vals)), sc_vals,
-                         color=_channel_axis_color(ch), linewidth=0.5)
-                _amx = max((abs(v) for v in sc_vals), default=1.0) or 1.0
-                scx.set_ylim(-_amx * 1.15, _amx * 1.15)
+                _col = _channel_axis_color(ch)
+                # Faint zero baseline (BW draws the channel baseline through the
+                # strip) — reference for the one-sided geophone ring-downs.
+                scx.axhline(0.0, color=_col, linewidth=0.3, alpha=0.4)
+                scx.plot(range(len(sc_vals)), sc_vals, color=_col, linewidth=0.5)
+                # Fit the trace to the box (BW-style) rather than a symmetric
+                # scale: the geo self-checks are one-sided dips, so a symmetric
+                # scale would strand them in the bottom half with an empty top.
+                _lo, _hi = min(sc_vals), max(sc_vals)
+                _pad = 0.10 * ((_hi - _lo) or 1.0)
+                scx.set_ylim(_lo - _pad, _hi + _pad)
             scx.set_xticks([]); scx.set_yticks([])
             for _s in scx.spines.values():
                 _s.set_linewidth(0.4); _s.set_color("#999")