觀音山位於淡水河的西南方、林口臺地的東北方，前人的地質調查指出在該地區內發現火成性的侵入岩體，因此觀音山被認為是一座古火山，而觀音坑則為受侵蝕的火山殘跡。為了揭露觀音山地區底下的構造，本研究在觀音山地區應用噪訊成像技術(ambient noise tomography)，於觀音山地區佈設了55個簡易地震儀（SmartSolo）組成的觀測陣列，平均測站間距約0.8公里，並結合Formosa Array的部分測站（9個）組成我們的觀測網，噪訊紀錄時間範圍為2022年9月18日至2022年10月17日以及2022年11月9日至2022年12月7日，使用了約兩個月長的噪訊紀錄進行分析，首先將兩兩測站對的連續波形紀錄進行互相關後並疊加，萃取各測站配對之間的基態雷利波格林函數（Green’s Function）；接著使用多重濾波法（Multiple Filter Technique， MFT）得到週期0.5至6秒間的群速度及相速度的頻散曲線；最後使用快速推進法（Fast Marching Method， FMM）計算測站配對之間的波線路徑以建構逆推矩陣，並逆推得出各週期的基態雷利波二維群、相速度模型，再將模型做第2次逆推得到三維的S波速度模型。在最終三維S波速度模型中，觀音山頂峰至觀音坑的區域，於深度0至0.6公里為相對高速異常，指示出該區域存在火成岩性的岩盤或岩盆；而到了深度1.4公里以下後，俯視的同一區域則變為低速異常，暗示這個範圍可能有岩脈入侵造成的裂隙或其脫水作用後產生之殘留流體。;Mt. Guanyin is located southwest of the Tamsui River and northeast of the Linkou Tableland. Previous geological surveys have identified igneous rock intrusions in this area， leading to the conclusion that Mt. Guanyin is an ancient volcano， while Guanyinkeng is considered a eroded volcanic edifice. To reveal the structure underneath Mt. Guanyin Area， this study applies ambient noise tomography (ANT) in the Mt. Guanyin area. A seismic array consisting of 55 portable seismometers (SmartSolo) was deployed， with an average station spacing of approximately 0.8 km. Additionally， data from nine stations of the Formosa Array were incorporated into our observation network. The ambient noise records were collected during two periods: from Sep. 18 to Oct. 17， 2022， and from Nov. 9 to Dec. 7， 2022， with approximately two months of data analyzed. First， cross-correlation and stacking were applied to continuous waveform recordings from each station pair to extract the fundamental-mode Rayleigh wave Green′s functions. Then， the multiple filter technique (MFT) was used to derive dispersion curves for group velocity and phase velocity within the 0.5–6 second period range. Finally， the fast marching method (FMM) was employed to compute ray paths between station pairs， constructing the inversion matrix. Through inversion， 2D group and phase velocity models were obtained for each period， which were subsequently used in a second inversion step to generate a 3D shear wave velocity model. The final 3D shear wave velocity model shows a relative high-velocity anomaly beneath the area from the peak of Mt. Guanyin to Guanyinkeng at depths of 0–0.6 km， suggesting the presence of an igneous laccolith or lopolith in this region. Below a depth of 1.4 km， however， the same area in map view changes into a relative low-velocity anomaly， implying that this region may contain fractures caused by dyke intrusion or residual fluids produced by magma dehydration.
