Detecting subsurface voids using the microgravity method - A case study from Kuala Lipis, Pahang


Author : Ahmad Tajuddin Hj. Ibrahim & Che Noorliza LatPublication : Bulletin of the Geological Society of MalaysiaVolume : 48Page : 31-35Year : 2004


Description

Bulletin of the Geological Society of Malaysia, Volume 48, June 2004, pp. 31-35

 

Detecting subsurface voids using the microgravity method - A case study from Kuala Lipis, Pahang

AHMAD TAJUDDIN HJ. IBRAHIM & CHE NOORLIZA LAT

Geology Department, University of Malaya, 50603 Kuala Lumpur

 

Abstract: The gravity method is one of the more popular geophysical methods to detect subsurface voids. The voids, whether they are filled with air or water, will give anomalous gravity lows above their localities. The gravity pattern is also related to subsurface topography of the bedrock. A borehole drilled earlier indicated a possibility of a cavity. This study was conducted to see if the suspected cavity could also be detected by gravity method. Based on the borehole information, the lithology of the area comprises sandy clayey silt with limestone bedrock. A total of 42 observations were made in four profiles bearing east-west and north-south along the bunds (batas) of a paddy field. The survey area measured 50 m by 40 m. Readings were made at 5 m spacing, using a LaCoste-Romberg Model D gravimeter. This instrument was chosen because of its capability for detecting minute changes in the earth gravity field caused by local anomaly. The results indicate the possibility of cavities at four locations at depths varying from 3.77 to 6.50 m. The depth to the center of each anomalous body was calculated using the half-width method. The depth of the cavity interpreted from the borehole turned out to be shallower when the field gravity data is inverted using that method. The cavities were probably filled with wet sediments of densities between 1.85 to 2.05 g/cm3.

 

Abstrak: Kaedah graviti merupakan salah satu kaedah geofizik yang popular untuk mengesan lohong bawah tanah. Lohong, samada diisi oleh udara atau air, akan memberikan bacaan rendah berbanding sekeliling sekiranya ketumpatan lohong kurang dari batuan sekeliling. Corak anomali juga boleh dikaitkan dengan topografi subpermukaan. Penggerudian yang dibuat terdahulu menunjukkan kemungkinan wujudnya lohong. Kajian ini dijalankan untuk menentukan samada lohong ini boleh dikesan dengan menggunakan kaedah graviti. Maklumat lubang gerudi menunjukkan geologi kawasan ini terdiri daripada lodak berpasir dan berlempung dengan batukapur sebagai batuan dasar. Sebanyak 42 bacaan dicerap sepanjang beberapa garis timur-barat dan utara-selatan mengikut batas yang ada. Luas kawasan adalah 50 m x 40 m. Cerapan dibuat menggunakan gravimeter LaCoste-Romberg Model D. Peralatan ini dipilih kerana keupayaannya mengukur perubahan mikro dalam medan graviti bumi yang disebabkan oleh anomali setempat. Keputusan yang diperolehi menunjukkan kemungkinan wujudnya lohong di empat lokasi, pada julat kedalaman antara 3.77 hingga 6.50 meter. Kedalaman ke titik pusat anomali dikira menggunakan kaedah separa lebar (half-width). Kedalaman lohong pada lokasi penggerudian didapati adalah lebih cetek daripada kedalaman yang ditafsirkan daripada data graviti. Lohong ini berkemungkinan dipenuhi oleh sedimen basah yang berketumpatan antara 1.85 ke 2.05 g/sm3.

https://doi.org/10.7186/bgsm48200407