Jeofon dizinleri vasıtasıyla sismik yön tespiti
Seismic direction estimation via geophone arrays
Yazar: ARDA ARAS
Danışman: PROF. DR. TAYFUN AKGÜL
İstanbul Teknik Üniversitesi / Lisansüstü Eğitim Enstitüsü / Elektronik ve Haberleşme Mühendisliği Ana Bilim Dalı / Telekomünikasyon Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2024
Konu: Elektrik ve Elektronik Mühendisliği = Electrical and Electronics Engineering

Özet:
Bu çalışma, sismik sensör dizini kullanarak, kazazede tespitine yardımcı olabilecek
donanımsal ve yazılımsal bir sistem tanıtmaktadır. Kazazede tarafından oluştu-
rulabilecek bir darbe ile meydana gelen sismik sinyali hedef alarak, bu sismik
sinyalin yön kestirimi yapılmaya çalışılır. Sismik yön tahmini için varış zaman farkı
(VZF, Time Delay of Arrival-TDOA) yöntemi üzerine odaklanılmıştır. Bu yöntem,
sismik sensör olarak kullanılan jeofonlara ulaşan sismik sinyallerin arasındaki zaman
gecikmesinden yararlanarak yön tahmini gerçekleştirmeyi önermektedir. Çapraz ilinti
(Cross Correlation) algoritması kullanılarak zaman gecikmesi bilgisine ulaşılır. Bu
bilginin ışığında, kaynaktan gelen sismik sinyal ve jeofonların oluşturduğu geometri
düzeni ile sinyalin geliş açısı bulunur. Jeofonlar üzerinden, yüksek çözünürlükteki
harici analog sayısal çeviriciler (ASÇ, Analog Digital Converter-ADC) ile birlikte
mikroişlemci kontrolünde veriler parelel bir şekilde toplanır. Zaman gecikmesine
dayalı bir yöntem kullanıldığı için verinin parelel bir şekilde jeofonlar üzerinden
toplanabilmesi kritiktir. Toplanılan bu işlenmemiş veriler, direk sistem üzerinde
kullanabileceğini gibi sonrasında incelemeye olanak sağlayacak şekilde kayıt altına
alınabilmektedir. Mikroişlemci üzerinde bir kuyruk mekanizmasında tutulan veriler,
kablolu veya kablosuz ağlar aracılığıyla, verilerin işlendiği ve görselleştirildiği
operasyon bilgisayarına aktarılır. Buraya aktarılan veriler, öncelikle çeşitli süzgeçleme
işlemlerden geçirilerek çevresel gürültülerden arındırılır. Bu şekilde sinyal gürültü
oranı artırılarak, daha uzak mesafelerdeki sismik kaynağın varlığının tespitine imkan
sağlanır. Operasyon bilgisayarına ulaşan veri yığını, bu şekilde sürekli olarak
gözetlenilerek hedef sismik sinyalin varlığı incelenir. Eğer ki bir anomali tespit
edilirse, o andaki veri yığını sinyal işleme algoritmasına sokularak yön kestirimi
yapılır ve arayüz üzerinde bulunan açı değeri yansıtılır. Buradaki yönü dolayısıyla
sinyalin varış açısını hesaplamak için, örnekleme frekansı, sismik sinyalin ilgili zemin
üzerindeki hızı ve jeofonlar arasındaki mesafe bilgisi gereklidir. Bulunan zemindeki
sismik sinyal hızını hesaplamak için jeofonlar ile bir kalibrasyon işlemi gerçekleştirilir.
İki jeofon hizasında bir darbe oluşturularak, darbe ile oluşan sismik sinyalin jeofonlara
ulaşmasındaki zaman gecikmesi bulunur. Jeofonlar arasındaki mesafe bilindiği için
sismik sinyal hızı elde edilir. Farklı zeminlerde farklı sismik sinyal hızları ile
karşılaşmaktayız. Dolayısıyla öncelikle sistemin bulunduğu zemindeki sismik hızı
tespit etmek gerekmektedir. Sistem içerisindeki zaman gecikmesi çözünürlüğünü
artırabilmek için sismik hıza göre jeofonlar arası mesafe ayarlanmalıdır. Yapılan
deneylerde kum içerikli fayans gibi zeminlerde, jeofonlar arası mesafenin iki ve
dört metre olduğu durumlarda belli hata payı ile hedef sinyalin açısı bulunmuştur.
Ancak beton gibi sismik sinyalin görece daha hızlı olduğu zeminlerde jeofonlar arası
mesafenin daha fazla artırılması gerektiği sonucuna varılmıştır.

Abstract:
The purpose of this research is to present a software and hardware solution that has the
potential to aid in the identification of earthquake victims by utilizing a seismic sensor
array. An effort is made to determine the direction of a seismic signal by focusing on
the impact-generated seismic signal that may be caused by a victim.
The process of figuring out the direction from which seismic waves produced by any
source arrive at seismic sensors on the ground's surface is known as seismic direction
estimate. Because the physical characteristics of the ground directly affect the behavior
and speed of seismic waves, it is imperative to take these properties into account.
Seismic waves travel at varying speeds depending on the type of ground. The focus
is on TDOA method for seismic direction estimation, which involves analyzing the
arrival time difference of the seismic signal. The time delay information is acquired by
the utilization of the cross correlation technique. Based on the geometric arrangement
of the seismic signal from the source and the geophones, the arrival angle of the seismic
signal is computed. The aim of the study is to predict the direction of the target signal
using at least two geophone arrays arranged in a uniform linear based on time delay
of arrival. In this method, the time difference in the arrival of the seismic signal to
the geophones is determined as a result of the generated impacts. This time difference
depends on the speed of the seismic signal and the distance between the geophones.
If the speed of the seismic signal, the distance between the geophones, and the time
difference in the arrival of the seismic signal to the receivers are known, the direction
of the seismic source can be estimated.
The system created to implement the proposed method consists of three main units.
A geophone array, a data gathering and transfer device, and a system interface. First
unit consists of a vertical geophone array operating at a frequency of 4.5Hz, which
is employed for the acquisition of seismic signals. Geophones are non-active sensors
and do not expend energy. The geophone contains a suspended mass positioned above
a magnet, which is encircled by a wire coil. Seismic waves cause the mass to move,
which in turn generates an electromotive force by moving electrons along the wire coil.
This process is based on the idea of electromagnetic induction.
A microprocessor module and external ADCs make up the second unit. The system is
tasked with converting the analog signals from the geophones into digital format using
the 24 bit ADS1220 analog-to-digital converter, gathering the data, and transmitting it
to the server. The digital data acquired from sampling conducted within a single second
is appended to the queue mechanism, and the system then advances to the subsequent
sampling sequence. Concurrently, during this continuing operation, the data in the queue is transmitted to the server. To read analog data in parallel from the geophones,
a switching method controlled by Serial Peripheral Interface (SPI) is employed. The
ADCs are serially connected, except for environmental device selection. This allows
all ADCs to be triggered simultaneously, performing the conversion process. After
each reading operation, the chip select pin of the ADC to be read is pulled to logic
0, while others are pulled to logic 1. This high-impedance state allows sequential
readings from output pins, enabling parallel digitization of data from geophones.
The third and last unit enables the handling and presentation of incoming digital
data. Every algorithm is implemented within this framework. In the second section,
sampling is conducted within a specific time frame. During this procedure, the data
from the preceding sequence is subjected to a series of signal processing algorithms.
As a consequence, visual outputs are generated.
As seismic sensors, geophones are employed, and high-resolution external
analog-to-digital converters managed by microprocessors are used to capture data in
parallel. It is essential to gather data concurrently using geophones because the method
is time-delay based. The raw data gathered can be right away recorded for further
examination or directly processed by the system. After being collected and stored in
a queue mechanism on the microprocessor, the raw data is sent via wired or wireless
networks to an operations computer for processing and visualization.
The data transmitted here undergoes preliminary processing to reduce external noise.
This enhances the ratio between the desired signal and unwanted noise, enabling
the identification of seismic sources at larger distances. The operations computer
continuously monitors the incoming data to detect the presence of the desired seismic
signal. If an irregularity is identified, the data in this sequence is processed using
TDOA-based algorithms. The cross correlation method is primarily utilized to
determine the time delay in the arrival of the seismic signal to the geophones. Other
methods are also employed for comparative purposes. After acquiring the time delay
value, an estimation is made regarding the direction of the seismic signal that reaches
the geophones.
In order to determine the arrival angle of the signal, it is essential to have the sampling
frequency, the velocity of the seismic signal on the specific ground, and the distance
between geophones. The geophones undergo a calibration process to ascertain the
velocity of seismic signals in the underlying terrain. The time delay in the arrival of
the seismic signal to the geophones is determined by generating an impact between
two geophones. The speed of the seismic signal can be determined by calculating
the distance between the geophones. Various terrains display distinct seismic signal
velocities, hence it is imperative to initially ascertain the seismic velocity in the terrain
where the system is situated. In order to enhance the time frame precision of the
system, the spacing between geophones should be adjusted based on the seismic
velocity. Both adaptive techniques and physical approaches that can be incorporated
into the algorithm can be used to improve angular resolution. A specific technique that
can be used is cubic spline interpolation. In essence, it enhances the sampling rate
of the sampled signal by the specified factor. This significantly improves the angular
resolution of the system. However, it is important to evaluate the influence of signal
properties on this enhancement. 
During the experiments done on different surfaces, such as sand and tiles, the angular
position of the target signal was measured with an acceptable level of uncertainty when
the distance between geophones was either two or four meters. Nevertheless, in areas
where the seismic signal propagates at a significantly higher velocity, such as concrete,
it was determined that the spacing between geophones should be expanded even more.